Comprehensive Study Notes on Pharmaceutical Organic Chemistry: Aromatic Acids, Aromatic Amines, Polynuclear Hydrocarbons, Cycloalkanes, and Lipids

Aromatic Carboxylic Acids and Benzoic Acid Chemistry

• Classification and Structure of Aromatic Acids:

  • Aromatic acids consist of an aromatic ring covalently linked to an organic acid functional group.

  • Phenolic acids: Compounds containing an aromatic ring and an organic carboxylic acid functionality ($C_6-C_1$ carbon skeleton).

    • Monohydroxybenzoic acid aglycones: Salicylic acid (2-hydroxybenzoic acid).

    • Monohydroxybenzoic acid glycosides: $p$-Hydroxybenzoic acid glucoside.

    • Dihydroxybenzoic acids: 2,3-Dihydroxybenzoic acid (hypogallic acid); Gentisic acid (2,5-dihydroxybenzoic acid).

    • Trihydroxybenzoic acids: Gallic acid (3,4,5-trihydroxybenzoic acid).

    • Other phenolic acids and derivatives: Vanillin (4-hydroxy-3-methoxybenzaldehyde); Ellagic acid.

  • Hydroxycinnamic acids ($C_6-C_3$ skeleton): Caffeic acid (3,4-dihydroxycinnamic acid); Cinnamic acid.

  • Aromatic amino acids: Phenylalanine; Tryptophan.

• Physical and Chemical Properties of Benzoic Acid:

  • Preferred IUPAC name: Benzoic acid.

  • Systematic IUPAC name: Benzenecarboxylic acid.

  • Alternative nomenclature: Carboxybenzene, Dracylic acid, Phenylmethanoic acid.

  • Chemical formula: $C_7H_6O_2$.

  • Molar mass: $122.123\,g\cdot mol^{-1}$.

  • Appearance: Colorless crystalline solid.

  • Density: $1.2659\,g/cm^3$ at $15\,^\circ C$.

  • Melting point: $122\,^\circ C$.

  • Aqueous solubility by temperature:

    • $0\,^\circ C$: $1.7\,g/L$

    • $18\,^\circ C$: $2.7\,g/L$

    • $25\,^\circ C$: $3.44\,g/L$

    • $40\,^\circ C$: $5.51\,g/L$

    • $75\,^\circ C$: $21.45\,g/L$

    • $100\,^\circ C$: $56.31\,g/L$

  • Organic solubility: Freely soluble in acetone, benzene, carbon tetrachloride ($CCl_4$), chloroform ($CHCl_3$), ethanol, ethyl ether, hexane, phenyls, liquid ammonia, and acetates.

• Aqueous Solubility Profile and pH Dependence:

  • Benzoic acid exists in aqueous medium as a weak acid in partial dissociation equilibrium:

$C_6H_5COOH + H_2O \rightleftharpoons C_6H_5COO^- + H_3O^+ आत्मनिर्भर$

*   *Neutral medium:* Benzoic acid is nearly insoluble in cold water due to the large hydrophobic, non-polar benzene ring that interacts with water solely through weak van der Waals dispersion forces, which cannot overcome water-water hydrogen bonding networks. Because benzoic acid is a weak acid, spontaneous dissociation is insufficient to drive dissolution. In hot water, solubility increases markedly because thermal energy disrupts water-water hydrogen bonds, permits solute-solvent van der Waals contacts, and drives the endothermic acid dissociation equilibrium forward.
*   *Alkaline medium:* In the presence of aqueous bases containing hydroxide ions ($OH^-$), a quantitative acid-base neutralization occurs to yield the soluble benzoate anion ($C_6H_5COO^-$). Dissolution is driven by strong ion-dipole hydration forces between water molecules and the charged carboxylate group.
*   *Acidic medium:* In high concentrations of hydronium ions ($H^+$), Le Chatelier's principle shifts the dissociation equilibrium toward the un-ionized, non-polar acid form, suppressing solubility and precipitating benzoic acid.

• History and Natural Occurrence:

  • Discovered in the sixteenth century; dry distillation of gum benzoin was first recorded by Nostradamus in 1556, followed by descriptions from Alexius Pedemontanus (1560) and Blaise de Vigenère (1596).

  • In 1875, Ernst Leopold Salkowski identified the antifungal properties of benzoic acid, which led to its widespread application in preserving benzoate-containing cloudberry fruits.

  • Occurs naturally in castoreum, an animal secretion derived from ingested beaver plant forage.

• Commercial and Medicinal Applications:

  • Industrial chemical precursor in the production of plasticizers.

  • Antimicrobial food preservative, used as free acid or sodium/potassium benzoate salts.

  • Active antifungal component of Whitfield's ointment (combined with salicylic acid) indicated for topical dermatophyte infections including tinea pedis (athlete's foot) and tinea corporis (ringworm).

  • Primary constituent of gum benzoin resin, formulating topical antiseptic preparations, tincture of benzoin, and Friar's balsam inhalant decongestants.

  • Synthetic precursor to benzoyl chloride ($C_6H_5COCl$) via chlorination with thionyl chloride ($SOCl_2$), phosgene ($COCl_2$), or phosphorus chlorides ($PCl_3$, $PCl_5$). Benzoyl chloride is subsequently converted to benzyl benzoate for artificial flavorings and acaricidal insect repellents.

• Theoretical Basis of Acidity:

  • The carboxylic acid hydroxyl proton is labile because the conjugate base (carboxylate anion) shares negative charge equally across two electronegative oxygen atoms.

  • The negative charge on the carboxylate group is not effectively delocalized into the aromatic ring because the p-orbitals of the carboxylate oxygens are orthogonal or separated from direct ring conjugation, making inductive and field effects the primary mechanisms of transmission.

  • Comparative acidity (Carboxylic acids vs. Phenols): Carboxylic acids ($pK_a \sim 3-7$) are significantly stronger acids than phenols ($pK_a \sim 7-10$). Although the phenoxide anion delocalizes negative charge across the ortho and para positions of the aromatic ring, the resonance contributors place the charge on carbon atoms. Carboxylate resonance delocalizes negative charge entirely over two highly electronegative oxygen atoms, conferring greater thermodynamic stability upon the conjugate base.

  • Comparative acidity (Formic, Benzoic, and Acetic acids): The acidity ranking follows the order:

$\text{CH}_3\text{COOH } (pK_a = 4.756) < \text{C}_6\text{H}_5\text{COOH } (pK_a = 4.204) < \text{HCOOH } (pK_a = 3.751)$

*   Acetic acid is the weakest due to the electron-donating inductive effect ($+I$) of the methyl group, which destabilizes the conjugate base.
*   In benzoic acid, the $sp^2$-hybridized phenyl group acts as an electron-withdrawing group ($-I$) relative to an alkyl group, stabilizing the anion and increasing acidity relative to acetic acid.
*   Formic acid is the strongest because it possesses no electron-donating alkyl substituent, confining the negative charge solely across the electronegative oxygen framework without inductive destabilization.

• Substituent Effects on Benzoic Acid Dissociation:

  • Electron-Withdrawing Groups (EWGs): Groups such as $-NO_2$, $-CN$, $-CHO$, $-COR$, $-COOH$, and halogens pull electron density out of the ring through $-I$ and $-M$ (mesomeric) effects, stabilizing the carboxylate anion, increasing acid strength, and decreasing $pK_a$.

  • Electron-Donating Groups (EDGs): Groups such as $-NH_2$, $-NHR$, $-NR_2$, $-OH$, $-OCH_3$, and alkyl moieties donate electron density via $+I$ or $+M$ effects, destabilizing the carboxylate anion, decreasing acid strength, and elevating $pK_a$.

  • Dissociation constants ($pK_a$) of para-substituted benzoic acids ($p\text{-X-C}_6\text{H}_4\text{COOH}$):

    • $X = -N(CH_3)_2$: $pK_a = 6.03$

    • $X = -NHCH_3$: $pK_a = 5.04$

    • $X = -OH$: $pK_a = 4.57$

    • $X = -OCH_3$: $pK_a = 4.50$

    • $X = -C(CH_3)_3$: $pK_a = 4.38$

    • $X = -H$ (unsubstituted): $pK_a = 4.20$

    • $X = -Cl$: $pK_a = 4.00$

    • $X = -Br$: $pK_a = 3.96$

    • $X = -CHO$: $pK_a = 3.77$

    • $X = -CN$: $pK_a = 3.55$

    • $X = -NO_2$: $pK_a = 3.43$

• The Ortho Effect and Position Isomerism:

  • Ortho-substituted benzoic acids display substantially enhanced acidities relative to their meta- and para-substituted isomers, regardless of whether the substituent is inherently electron-donating or electron-withdrawing.

  • Mechanism: Steric hindrance from an ortho group forces the planar carboxyl group ($-COOH$) out of coplanarity with the benzene ring. This loss of coplanarity inhibits resonance overlap between the ring and the carboxyl group in the un-ionized acid, destabilizing the neutral acid relative to the solvated carboxylate anion, thereby driving proton loss.

  • Experimental acidic character hierarchies ($pK_a$ in brackets):

    • o-Bromobenzoic acid ($3.1$) > m-bromobenzoic acid ($3.93$) > p-bromobenzoic acid ($4.1$) > benzoic acid ($4.17$)

    • o-Chlorobenzoic acid ($2.89$) > m-chlorobenzoic acid ($3.82$) > p-chlorobenzoic acid ($3.98$) > benzoic acid ($4.17$)

    • o-Fluorobenzoic acid ($3.27$) > m-fluorobenzoic acid ($3.87$) > p-fluorobenzoic acid ($4.14$) > benzoic acid ($4.17$)

    • o-Toluic acid ($3.89$) > benzoic acid ($4.17$) > m-toluic acid ($4.28$) > p-toluic acid ($4.35$)

  • Hydroxybenzoic acid isomerism: Acidity rank: ortho-hydroxybenzoic acid ($pK_a = 2.98$) > meta-hydroxybenzoic acid ($pK_a = 4.08$) > benzoic acid ($pK_a = 4.19$) > para-hydroxybenzoic acid ($pK_a = 4.58$).

    • In salicylic acid (the ortho isomer), the phenolic $-OH$ forms an intramolecular hydrogen bridge with the adjacent carboxylate oxygen, stabilizing the conjugate base.

    • In the para isomer, strong $+M$ electron donation opposes ionization and elevates the $pK_a$.

    • In the meta isomer, resonance donation to the carboxyl group is geometrically impossible; only the weak $-I$ effect of oxygen operates, rendering it slightly more acidic than benzoic acid.

• Directing Influence in Electrophilic Aromatic Substitution:

  • The carboxyl group ($-COOH$) acts as a strong deactivating, meta-directing substituent.

  • Resonance withdraws electron density from the conjugated ring, placing formal positive charges at the ortho and para positions.

  • Incoming electrophiles ($E^+$) experience severe electrostatic repulsion at the ortho and para positions, directing substitution toward the less electron-deficient meta carbon.

• Synthetic Chemical Reactions of Benzoic Acid:

  • Salt formation: Neutralization with strong bases or carbonates:

$C_6H_5COOH + NaOH \rightarrow C_6H_5COONa + H_2O$

$2\,C_6H_5COOH + Na_2CO_3 \rightarrow 2\,C_6H_5COONa + CO_2 + H_2O$

*   *Fischer esterification:* Acid-catalyzed condensation of benzoic acid with alcohols (e.g., methanol) using sulfuric acid ($H_2SO_4$), phosphoric acid ($H_3PO_4$), or $p$-toluenesulfonic acid ($\text{TsOH}$) as catalysts. The reaction is reversible and reaches equilibrium:

$C_6H_5COOH + CH_3OH \xrightleftharpoons{H_2SO_4} C_6H_5COOCH_3 + H_2O$

    *   *Step 1:* Reversible protonation of the carbonyl oxygen by $H^+$ enhances electrophilicity of the carbonyl carbon.
    *   *Step 2:* Nucleophilic addition of methanol generates a tetrahedral oxonium intermediate. Intramolecular proton transfer to a hydroxyl group converts it into a good leaving group ($-OH_2^+$).
    *   *Step 3:* Elimination of $H_2O$ restores the carbonyl double bond, followed by proton loss to regenerate the acid catalyst and yield methyl benzoate.
*   *Acyl halide synthesis:* Substitution of the carboxylic hydroxyl group with phosphorus pentachloride ($PCl_5$) or thionyl chloride ($SOCl_2$):

$C_6H_5COOH + PCl_5 \rightarrow C_6H_5COCl + POCl_3 + HCl$

$C_6H_5COOH + SOCl_2 \rightarrow C_6H_5COCl + SO_2 + HCl$

*   *Lithium aluminium hydride reduction:* Benzoic acid is reduced to benzyl alcohol ($C_6H_5CH_2OH$) using lithium aluminium hydride ($LiAlH_4$) in anhydrous ether followed by aqueous acidic workup:
    *   *Step I (Deprotonation):* Hydride abstracts the acidic proton, releasing dihydrogen gas ($H_2$) and producing lithium benzoate and aluminium hydride ($AlH_3$).
    *   *Step II (Hydride transfer):* Hydride attacks the carbonyl carbon, forming a tetrahedral dialkoxyaluminate complex.
    *   *Step III (Leaving group expulsion):* The oxygen-aluminate moiety leaves, generating benzaldehyde intermediate and lithium aluminate oxide ($Li^+ [O-AlH_2]^-$).
    *   *Step IV (Second hydride addition):* Hydride attacks benzaldehyde to yield a lithium benzyl alkoxide complex.
    *   *Step V (Protonation):* Aqueous workup protonates the alkoxide to furnish benzyl alcohol and lithium hydroxide ($LiOH$).
*   *Decarboxylation:* Cleavage of the carboxyl group to yield benzene and carbon dioxide ($CO_2$). Industrial and laboratory variants include heating with alkali/soda lime, copper(I) oxide ($Cu_2O$) catalysts in quinoline under hydrothermal regimes, or catalytic radical decarboxylation via hydroxyl radicals generated by $Cu^{2+}$-catalyzed reduction of dioxygen ($O_2$) by ascorbic acid.
*   *Electrophilic ring nitration:* Nitration with concentrated nitric acid ($HNO_3$) and concentrated sulfuric acid ($H_2SO_4$) at $100\,^\circ C$ yields 3-nitrobenzoic acid ($80\%$), 2-nitrobenzoic acid ($18\%$), and 4-nitrobenzoic acid ($2\%$). Treatment with fuming nitric acid produces 3,5-dinitrobenzoic acid via secondary nitration.
*   *Electrophilic ring bromination:* Reaction of benzoic acid with elemental bromine ($Br_2$) in the presence of iron(III) bromide ($FeBr_3$) yields 3-bromobenzoic acid ($80\%$), 2-bromobenzoic acid ($18\%$), and 4-bromobenzoic acid ($2\%$).

Basicity and Reactions of Aromatic Amines (Arylamines)

• Classification and Nomenclature of Amines:

  • Amines are organic nitrogenous derivatives of ammonia where one or more hydrogen atoms are substituted by alkyl or aryl groups.

  • Primary aromatic amines (arylamines) contain an $-NH_2$ group bound directly to an aromatic ring. The parent compound is phenylamine, commonly designated aniline.

  • Representative derivatives: Aniline (benzenamine); 2-Methylaniline ($o$-toluidine / 2-methylphenylamine); 4-Nitroaniline ($p$-nitroaniline / 4-nitrophenylamine); Diphenylamine; $N$-Methylaniline; $N,N$-Diethylaniline; $N$-Ethyl-$N$-methylaniline; Phenylmethylamine (benzylamine, an arylalkylamine).

  • Diamines: Named with the suffix -diamine appended to the parent alkane or arene name: Hexane-1,6-diamine ($1,6$-hexanediamine); Benzene-1,4-diamine ($1,4$-benzenediamine).

• Physical and Chemical Characteristics:

  • Pure lower arylamines are colorless liquids or crystalline solids possessing distinctive amine odors.

  • Undergo aerial oxidation over time to form dark brown colored quinonoid degradation products.

  • Boiling points are higher than corresponding alkanes due to intermolecular hydrogen bonding ($N-H\cdots N$), but lower than carboxylic acids and alcohols due to nitrogen's lower electronegativity relative to oxygen.

  • Sparingly soluble in water; freely soluble in organic solvents including diethyl ether, ethanol, and benzene.

  • Exhibit high physiological toxicity; readily absorbed through skin membranes.

  • Aromatic amines are steam-volatile and can be purified by steam distillation.

• Chirality and Nitrogen Pyramidal Inversion:

  • Tertiary amines containing three distinct substituents plus the non-bonding electron pair display approximately tetrahedral geometry around nitrogen.

  • Although formally chiral, acyclic amines with three different groups (e.g., $(R)$- and $(S)$-ethylmethylamine) cannot be resolved into individual enantiomers at room temperature because they undergo rapid pyramidal inversion.

  • Inversion mechanism: The nitrogen atom passes through a planar, achiral $sp^2$-hybridized transition state with the unshared pair occupying an unhybridized $2p$ orbital, interconverting enantiomeric geometries.

  • Quaternary ammonium salts: Inversion is blocked in quaternary ammonium ions ($R_1R_2R_3R_4N^+ X^-$) because no unshared pair remains. Chiral quaternary ammonium salts can be separated and resolved into stable enantiomers.

• Mechanism of Basicity and Thermodynamic Relationships:

  • Amines act as weak bases in water, establishing a partial dissociation equilibrium:

$R-NH_2 + H_2O \rightleftharpoons R-NH_3^+ + OH^-$

$K_b = \frac{[RNH_3^+][OH^-]}{[RNH_2]}$

$pK_b = -\log_{10}(K_b)$

$pK_a + pK_b = 14$

*   *Arylamine vs. Aliphatic Amine Basicity:* Aromatic amines are weaker bases than aliphatic counterparts and ammonia by orders of magnitude (e.g., cyclohexylamine $pK_b = 3.34$, $pK_a = 10.66$; aniline $pK_b = 9.41$, conjugate acid $pK_a = 4.63$).
*   *Factor 1 (Resonance stabilization of the free base):* In aniline, the unshared electron pair on nitrogen is delocalized into the aromatic $\pi$-system through four resonance contributors (three of which place negative charges on the ortho and para positions). Protonation forces nitrogen into an $sp^3$ ammonium configuration ($C_6H_5NH_3^+$), completely abolishing resonance stabilization. Protonation is therefore thermodynamically disfavored.

*   *Factor 2 (Inductive effect of hybridization):* The nitrogen in an arylamine is attached to an $sp^2$-hybridized aromatic ring carbon, which possesses greater $s$-character ($33\%$) and higher electronegativity than the $sp^3$-hybridized carbon ($25\%$ $s$-character) of an aliphatic amine, exerting an electron-withdrawing inductive effect that reduces nitrogen electron density.

• Quantitative Basicity Reference ($pK_a$ and $pK_b$ Values):

  • Inorganic reference: Ammonia ($NH_3$): $pK_a = 9.26$, $pK_b = 4.74$.

  • Primary aliphatic amines: Methylamine ($CH_3NH_2$): $pK_a = 10.64$, $pK_b = 3.36$; Ethylamine ($CH_3CH_2NH_2$): $pK_a = 10.81$, $pK_b = 3.19$; Propylamine ($C_3H_7NH_2$): $pK_b = 3.03$; Cyclohexylamine ($C_6H_{11}NH_2$): $pK_a = 10.66$, $pK_b = 3.34$.

  • Secondary aliphatic amines: Dimethylamine ($(CH_3)_2NH$): $pK_a = 10.73$, $pK_b = 3.27$; Diethylamine ($(CH_3CH_2)_2NH$): $pK_a = 10.98$, $pK_b = 3.02$; Ethylmethylamine ($CH_3CH_2NHCH_3$): $pK_b = 2.88$.

  • Tertiary aliphatic amines: Trimethylamine ($(CH_3)_3N$): $pK_a = 9.81$, $pK_b = 4.19$ (also reported at $2.64$ under varying aqueous hydration contexts); Triethylamine ($(CH_3CH_2)_3N$): $pK_a = 10.75$, $pK_b = 3.25$.

  • Aromatic amines: Aniline ($C_6H_5NH_2$): $pK_a = 4.63$, $pK_b = 9.41$; 4-Methylaniline: $pK_a = 5.08$; 4-Chloroaniline: $pK_a = 4.15$; 4-Nitroaniline: $pK_a = 1.0$, $pK_b = 9.58$; 3-Nitroaniline: $pK_a = 2.47$.

  • Heterocyclic aromatic amines: Pyridine: $pK_a = 5.25$; Imidazole: $pK_a = 6.95$.

• Substituent Effects on Arylamine Basicity:

  • Electron-releasing groups (ERGs): Alkyl groups ($-CH_3$), alkoxy ($-OCH_3$), and amino ($-NH_2$) groups increase basic strength by releasing electron density through inductive ($+I$) or resonance ($+M$) effects, stabilizing the positively charged ammonium conjugate acid.

  • Electron-withdrawing groups (EWGs): Nitro ($-NO_2$), cyano ($-CN$), halogens ($-Cl$, $-Br$, $-I$), and carbonyl groups decrease basic strength by pulling electron density from nitrogen.

  • Position effects (Toluidines): Basicity follows the order: $p$-toluidine ($pK_b = 8.93$) > $m$-toluidine ($pK_b = 9.31$) > aniline ($pK_b = 9.42$) > $o$-toluidine ($pK_b = 9.6$). Para and meta isomers are stronger bases than aniline, while the ortho isomer is significantly weaker due to steric inhibition of ammonium solvation and steric distortion (ortho effect).

  • Position effects (Nitroanilines): Aniline > $m$-nitroaniline ($pK_a = 2.47$) > $p$-nitroaniline ($pK_a = 1.0$). In 4-nitroaniline, the nitrogen lone pair delocalizes directly into the oxygen atoms of the nitro group via an extended quinonoid resonance structure. In 3-nitroaniline, cross-conjugation prevents this direct delocalization, so only inductive withdrawal operates, making the meta isomer a stronger base than the para isomer.

• Synthetic Preparation of Primary Aromatic Amines:

  • Reduction of aromatic nitro compounds:

$Ar-H \xrightarrow{HNO_3/H_2SO_4} Ar-NO_2 \xrightarrow{[H]} Ar-NH_2$

    *   Reduction reagents: Catalytic hydrogenation ($H_2, \text{catalyst}$) or chemical reduction using iron and hydrochloric acid: (1) $Fe, HCl$, (2) $OH^-$ ($97\%$ yield for aniline).
    *   *Zinin selective reduction:* One molar equivalent of hydrogen sulfide ($H_2S$) in ethanolic ammonia selectively reduces one nitro group of $m$-dinitrobenzene to yield $m$-nitroaniline ($70-80\%$ yield):

$m\text{-C}_6\text{H}_4(\text{NO}_2)_2 \xrightarrow{H_2S,\,NH_3,\,C_2H_5OH} m\text{-O}_2\text{N-C}_6\text{H}_4\text{-NH}_2$

*   *Reductive amination of carbonyl precursors:*
    *   Benzaldehyde + ammonia ($NH_3$) + $H_2$ over Raney nickel ($90\,atm$, $40-70\,^\circ C$) yields benzylamine ($89\%$).
    *   Benzaldehyde + ethylamine ($CH_3CH_2NH_2$) reduced with lithium cyanoborohydride ($LiBH_3CN$) yields $N$-benzylethanamine ($89\%$).
    *   Cyclohexanone + dimethylamine ($(CH_3)_2NH$) reduced with sodium cyanoborohydride ($NaBH_3CN$) yields $N,N$-dimethylcyclohexanamine ($52-54\%$).
*   *Reduction of nitrogenous functional groups:*
    *   Cyclohexanone oxime reduced with sodium in ethanol ($Na, C_2H_5OH$) yields cyclohexylamine ($50-60\%$).
    *   2-Phenylethanenitrile (phenylacetonitrile) reduced via catalytic hydrogenation ($2\,H_2$, Raney Ni, $140\,^\circ C$) yields 2-phenylethanamine ($71\%$).
    *   $N$-Methylacetanilide reduced with $LiAlH_4$ in diethyl ether followed by hydrolysis yields $N$-ethyl-$N$-methylaniline.
*   *Monoalkylation via acylation:* Primary amine is acylated to an amide (e.g., benzylamine + acetyl chloride, $CH_3COCl$), followed by reduction with $LiAlH_4$ in $Et_2O$ and hydrolysis to furnish benzylethylamine without overalkylation.

• Reactions of Aromatic Amines:

  • Salt formation: Neutralization by strong mineral acids yields water-soluble ammonium salts:

$C_6H_5NH_2 + HCl \rightarrow C_6H_5NH_3^+ Cl^- \quad (\text{benzenaminium chloride})$

$C_6H_5NH_2 + H_2SO_4 \rightarrow C_6H_5NH_3^+ HSO_4^- \quad (\text{anilinium hydrogen sulfate})$

$2\,C_6H_5NH_2 + H_2SO_4 \rightarrow [C_6H_5NH_3^+]_2 SO_4^{2-} \quad (\text{anilinium sulfate})$

$C_6H_5NH_2 + HNO_3 \rightarrow C_6H_5NH_3^+ NO_3^- \quad (\text{anilinium nitrate})$

    *   Treatment of these crystalline salts with aqueous sodium hydroxide liberates the free amine, sodium chloride, and water.
*   *Nucleophilic alkylation:* Aniline reacts with iodomethane ($CH_3I$) in ethanol to generate $N$-methylaniline and hydriodic acid ($HI$). Excess haloalkane drives substitution through $N,N$-dimethylaniline to trimethylphenylammonium iodide (a quaternary ammonium salt).
*   *Nucleophilic acylation:* Primary arylamines react with acetyl chloride ($CH_3COCl$) to form secondary amides (acetanilide) and $HCl$. Secondary arylamines react with acyl chlorides to generate tertiary amides.
*   *Carbylamine reaction (Isocyanide test):* Heating a primary aliphatic or aromatic amine with trichloromethane (chloroform, $CHCl_3$) and ethanolic potassium hydroxide ($KOH$) yields an intensely foul-smelling isocyanide (carbylamine, $R-\overset{+}{N}\equiv\bar{C}$):

$R-NH_2 + CHCl_3 + 3\,KOH \rightarrow R-N\equiv C + 3\,KCl + 3\,H_2O$

$CH_3CH_2NH_2 + CHCl_3 + 3\,KOH \rightarrow CH_3CH_2NC + 3\,KCl + 3\,H_2O$

$C_6H_5NH_2 + CHCl_3 + 3\,KOH \rightarrow C_6H_5NC + 3\,KCl + 3\,H_2O$

    *   Isocyanides are zwitterionic species wherein the terminal carbon carries a negative charge and the nitrogen carries a formal positive charge.
*   *Oxidation:* Oxidation of aniline with potassium dichromate ($K_2Cr_2O_7$) in sulfuric acid converts the aromatic nucleus to $p$-benzoquinone (cyclohexa-2,5-diene-1,4-dione).
*   *Hofmann-Martius rearrangement:* Heating $N$-alkylaniline hydrochlorides (such as $N$-methylaniline hydrochloride) to $300\,^\circ C$ initiates thermal migration of the alkyl radical from the nitrogen atom to the ortho and para positions of the aromatic ring, yielding 4-methylaniline and 2,4-dimethylaniline hydrochlorides.
*   *Electrophilic aromatic halogenation:* Amino groups activate the benzene ring strongly. Reaction of 3-aminobenzoic acid with bromine water results in exhaustive bromination at all available activated sites to yield 3-amino-2,4,6-tribromobenzoic acid.

• Qualitative Analytical Identification Schemes for Amines:

  • Salt test: $0.1\,g$ sample mixed with $5\,ml$ of $10\%$ aqueous $HCl$. Dissolution to a clear solution indicates the presence of basic amine functions.

  • Carbylamines test: $0.1\,g$ sample treated with 2 drops of $CHCl_3$ and $2\,ml$ of alcoholic $KOH$ with gentle heating. Generation of an offensive isocyanide odor confirms a primary aliphatic or aromatic amine.

  • Diazotization test: $0.1\,g$ sample treated with 10 drops of concentrated $HCl$, heated, cooled in ice water, treated with 4–5 drops of aqueous sodium nitrite ($NaNO_2$), and coupled with alkaline 2-naphthol. Formation of an orange-red precipitate indicates a primary aromatic amine; production of a yellow oily solid indicates a secondary amine; appearance of a soluble red coloration indicates a tertiary amine.

  • Hinsberg test: $0.1\,g$ sample shaken with $0.3\,g$ benzenesulfonyl chloride and $5-10\,ml$ ice-cold water:

    • Primary amine: Produces a sulfonamide precipitate that dissolves in $5\%$ aqueous $NaOH$ (due to acidic sulfonamide proton) and reprecipitates upon acidification.

    • Secondary amine: Produces a neutral sulfonamide precipitate that remains insoluble in $5\%$ aqueous $NaOH$.

    • Tertiary amine: Fails to react; remains insoluble in base, but dissolves upon addition of $10\%$ aqueous $HCl$.

Diazonium Salts, Sandmeyer Transformations, and Azo Chemistry

• Diazotization Mechanism of Primary Amines:

  • In situ nitrous acid generation: Nitrous acid ($HONO$) is unstable and must be generated in situ at low temperature ($0-5\,^\circ C$):

$HCl_{(aq)} + NaNO_{2(aq)} \rightarrow HONO_{(aq)} + NaCl_{(aq)}$

$H_2SO_{4(aq)} + 2\,NaNO_{2(aq)} \rightarrow 2\,HONO_{(aq)} + Na_2SO_{4(aq)}$

*   *Primary aliphatic amines:* React with nitrous acid to yield aliphatic diazonium salts ($R-N_2^+ X^-$). These decompose spontaneously via nitrogen gas ($N_2$) expulsion to generate unstable carbocations ($R^+$), which partition into complex product mixtures of alkenes, alcohols, and alkyl halides.
*   *Primary arylamines:* React at $0-5\,^\circ C$ to form crystalline arenediazonium salts ($Ar-N_2^+ X^-$), which are stabilized by resonance overlap with the aromatic ring:

$Ar-NH_2 + NaNO_2 + 2\,HX \xrightarrow{0-5\,^\circ C} Ar-N\equiv N^+ X^- + NaX + 2\,H_2O$

*   *Step-by-step mechanism:* Protonation of nitrous acid generates the nitrosonium electrophile ($N=O^+$). Nucleophilic attack by the arylamine nitrogen produces an $N$-nitrosoammonium ion, which loses a proton to yield an $N$-nitrosoamine ($Ar-NH-NO$). Tautomeric proton transfers convert the nitrosoamine into a diazohydroxide ($Ar-N=N-OH$). Protonation of the hydroxyl group followed by loss of water ($H_2O$) generates the resonance-stabilized arenediazonium cation ($Ar-N^+\equiv N \leftrightarrow Ar-N=\overset{+}{N}$).
*   *Thermal instability:* Aromatic diazonium salts decompose above $5\,^\circ C$. Thermal warming initiates nitrogen gas release, producing highly reactive phenyl cations or aryl radicals.

• Replacement Reactions (Sandmeyer and Related Syntheses):

  • Arenediazonium salts undergo nucleophilic substitution where $N_2$ acts as an exceptional leaving group:

    • Sandmeyer Chlorination: Reaction with copper(I) chloride ($CuCl$) in $HCl$ yields aryl chlorides ($Ar-Cl$).

    • Sandmeyer Bromination: Reaction with copper(I) bromide ($CuBr$) in $HBr$ yields aryl bromides ($Ar-Br$).

    • Sandmeyer Cyanation: Reaction with copper(I) cyanide ($CuCN$) in $KCN$ yields aryl nitriles ($Ar-CN$).

    • Aryl Iodide Synthesis: Direct treatment with potassium iodide ($KI$) without copper catalysts yields aryl iodides ($Ar-I$).

    • Schiemann Fluorination: Precipitation of aryl diazonium fluoroborates using fluoroboric acid ($HBF_4$), followed by isolated thermal pyrolysis, yields aryl fluorides ($Ar-F$).

    • Hydroxylation: Heating diazonium salts with copper(I) oxide ($Cu_2O$) and copper(II) nitrate in water ($Cu^{2+}, H_2O$) or hot dilute sulfuric acid yields phenols ($Ar-OH$).

    • Deamination (Reduction): Reduction with hypophosphorous acid ($H_3PO_2, H_2O$) or ethanol replaces the diazonium group with hydrogen, yielding the unsubstituted aromatic hydrocarbon ($Ar-H$).

    • Nitration: Treatment with sodium nitrite ($NaNO_2$) in the presence of copper salts yields nitroarenes ($Ar-NO_2$).

    • Hydrazine formation: Reduction with four hydrogen equivalents ($4[H]$) using sodium sulfite or tin(II) chloride yields arylhydrazines ($Ar-NHNH_2$).

    • Gomberg-Bachmann biaryl coupling: Reaction under basic conditions ($NaOH$) with benzene rings yields symmetrical or unsymmetrical biaryls ($Ar-Ar'$).

• Diazo Coupling and Azo Dye Chemistry:

  • Arenediazonium ions act as weak electrophiles that undergo electrophilic aromatic substitution with activated aromatic rings (phenols, naphthols, and tertiary arylamines) to yield intensely colored azo compounds containing the chromophoric diazenediyl bridge ($-N=N-$):

$Ar-N_2^+ + H-Ar'-G \rightarrow Ar-N=N-Ar'-G + HX \quad (G = -OH, -NR_2)$

*   *Reaction pH parameters:* Phenolic coupling requires a mildly alkaline pH ($pH\,8-10$) to convert phenol into the more nucleophilic phenoxide ion. Excessive alkalinity must be avoided, as hydroxide converts the diazonium cation into an unreactive diazohydroxide ($Ar-N=N-OH$) or diazotate salt ($Ar-N=N-O^-$). Coupling with amines is executed in mildly acidic to neutral conditions ($pH\,5-7$).
*   *Regioselectivity:* Electrophilic attack occurs exclusively para to the activating group; if the para site is blocked, coupling is directed to the ortho position (e.g., coupling benzenediazonium chloride with 4-methylphenol/$p$-cresol in $NaOH$ produces 4-methyl-2-(phenylazo)phenol).
*   *Applications of Azo Compounds:* Industrial and analytical applications include:
    *   Acid-base indicators (e.g., methyl orange, which displays distinct colors in its un-ionized acid and resonance-stabilized sulfonate salt forms).
    *   Artist pigments and coatings covering the yellow, orange, and red spectral bands.
    *   Synthetic food colorants: Tartrazine (E102), Yellow 2G (E107), Sunset Yellow FCF (E110), and Azorubine/Carmoisine (E122).
    *   Industrial polymers: Nylon condensation copolymers produced by reacting stoichiometric diamines and dicarboxylic acids, forming repeating amide bonds.
    *   Regulatory restrictions: Global bans exist on specific azo dyes in textiles and leather goods (such as polyester buttons, sequins, zipper fabrics, sewing threads, apparel fabrics, and upholstery) due to reductive cleavage of the azo linkage releasing carcinogenic aromatic amines.

Polynuclear Aromatic Hydrocarbons (PAHs)

• Structural Classification and Aromaticity:

  • Polycyclic aromatic hydrocarbons (PAHs) are hydrocarbon frameworks containing two or more fused benzene rings.

  • Isolated ring assemblies: Aromatic rings linked directly by a single carbon-carbon bond without shared atoms (e.g., biphenyl / diphenyl).

  • Ortho-fused PAHs: Adjacent aromatic rings sharing two contiguous ring carbons and one mutual carbon-carbon single/double bond. Rings can extend in linear arrangements (e.g., anthracene) or angular geometries (e.g., phenanthrene, chrysene).

  • Ortho- and peri-fused PAHs: Three or more rings share common internal carbon atoms and contiguous bonds, forming condensed disc structures (e.g., pyrene, acenaphthylene, coronene).

  • Non-benzenoid aromatics: Fused unsaturated rings lacking standard six-membered alternating arrangements (e.g., azulene, a fused cyclopentadiene-cycloheptatriene system).

  • Aromaticity and Resonance Energy: Unlike benzene, fused rings do not have identical carbon-carbon bond lengths, causing uneven electron delocalization. Anthracene and phenanthrene have lower average resonance energies per ring than benzene:

    • Benzene: $36\,kcal\cdot mol^{-1}$

    • Naphthalene (2 rings): $61\,kcal\cdot mol^{-1}$ (second ring adds $25\,kcal\cdot mol^{-1}$)

    • Anthracene (3 linear rings): $84\,kcal\cdot mol^{-1}$ (average of $28\,kcal\cdot mol^{-1}$ per ring)

    • Phenanthrene (3 angular rings): $92\,kcal\cdot mol^{-1}$

    • Angular fusion confers greater stability on phenanthrene than linear anthracene because phenanthrene can preserve two fully aromatic terminal benzene rings ($2 \times 36 = 72\,kcal\cdot mol^{-1}$), leaving only $20\,kcal\cdot mol^{-1}$ localized across the central ring.

• Environmental Sources, Physical Properties, and Toxicity:

  • PAHs are non-polar, lipophilic substances. Lower molecular weight members exhibit limited aqueous solubility, whereas larger members are virtually insoluble in water and poorly soluble in organic solvents and lipids.

  • Heavier members exhibit vivid colors (e.g., perylene is intensely fluorescent yellow-orange).

  • Formed primarily via incomplete pyrolysis and combustion of carbonaceous matter. Natural sources include volcanic eruptions, forest fires, and microbial biosynthesis. Primary anthropogenic sources include fossil-fuel electricity generation, industrial smelting, motor vehicle exhaust, aviation emissions, cigarette smoke, and charbroiled/grilled meats.

  • Toxicology: Metabolized by hepatic cytochrome P450 enzymes into reactive diol-epoxides that intercalate and alkylate cellular DNA. They act as potent chemical mutagens and carcinogens, inducing neoplastic tumors in the lungs, stomach, kidneys, and liver.

• Chemistry of Naphthalene ($C_{10}H_8$):

  • Structure: Fused bicyclic system obeying Hückel's rule ($4n+2 = 10$ $\pi$-electrons, where $n=2$). X-ray diffraction confirms unequal bond lengths: the $C_1-C_2$ bond is $1.36\,\text{Å}$ (higher double bond character), while the $C_2-C_3$ bond is $1.40\,\text{Å}$.

  • Structure Elucidation: Oxidation of naphthalene with potassium permanganate ($KMnO_4$) yields phthalic acid, establishing the presence of an intact ortho-disubstituted benzene ring. Nitration gives 1-nitronaphthalene; subsequent reduction yields 1-aminonaphthalene. Oxidation of 1-nitronaphthalene degrades the unsubstituted ring to produce 3-nitrophthalic acid, whereas oxidation of 1-aminonaphthalene degrades the substituted ring to yield phthalic acid. This confirms that naphthalene consists of two mutually fused benzene rings.

  • Synthetic routes:

    • From $\beta$-benzylidene-3-propenoic acid: Cyclization with concentrated $H_2SO_4$ with loss of water yields 1-naphthol, which undergoes zinc dust reduction (-$ZnO$) to yield naphthalene.

    • From 4-phenyl-1-butene: Passing the vapor over red-hot calcium oxide ($CaO$) induces oxidative cyclodehydrogenation to naphthalene and $H_2$.

    • Haworth synthesis (five steps):

      • Step I: Friedel-Crafts acylation of benzene with succinic anhydride in the presence of aluminium chloride ($AlCl_3$) produces 3-benzoylpropanoic acid (4-oxo-4-phenylbutanoic acid).

      • Step II: Clemmensen reduction of the keto group using amalgamated zinc and hydrochloric acid ($Zn(Hg), HCl$) produces 4-phenylbutanoic acid.

      • Step III: Intramolecular Friedel-Crafts cyclization heated with concentrated sulfuric acid ($H_2SO_4$) yields $\alpha$-tetralone (3,4-dihydro-2H-naphthalen-1-one).

      • Step IV: Second Clemmensen reduction ($Zn(Hg), HCl$) converts the carbonyl to a methylene group, yielding tetralin (1,2,3,4-tetrahydronaphthalene).

      • Step V: Catalytic dehydrogenation of tetralin with palladium and heat (or selenium) yields naphthalene.

    • 1-Alkylnaphthalene synthesis: Treatment of $\alpha$-tetralone with a Grignard reagent ($RMgX$), followed by dehydration and selenium aromatization, yields 1-alkylnaphthalenes.

*   *Chemical transformations:*
    *   *Reduction:* Sodium in ethanol reduces naphthalene to 1,4-dihydronaphthalene; sodium in boiling isoamyl alcohol produces 1,2,3,4-tetrahydronaphthalene (tetralin); catalytic hydrogenation ($H_2, Ni$) yields decahydronaphthalene (decalin).
    *   *Oxidation:* Chromium trioxide ($CrO_3$) in acetic acid yields 1,4-naphthoquinone; ozonolysis ($O_3$) followed by $Zn/H_2O$ cleavage yields phthalaldehyde; vapor-phase catalytic air oxidation over vanadium pentoxide ($V_2O_5$) yields phthalic anhydride; acidified $KMnO_4$ yields phthalic acid.
    *   *Addition:* Addition of $Cl_2$ gives 1,4-dichloro-1,4-dihydronaphthalene; excess $Cl_2$ produces 1,2,3,4-tetrachloro-1,2,3,4-tetrahydronaphthalene.
    *   *Regioselectivity of Electrophilic Substitution:* Electrophilic attack occurs preferentially at the $\alpha$-position (C-1) over the $\beta$-position (C-2). Attack at C-1 produces a carbocation intermediate with two resonance contributors that leave an intact, fully aromatic benzene ring. Attack at C-2 produces only one contributor with an intact benzene ring, raising the activation energy.
    *   *Nitration:* Nitric acid and sulfuric acid at ambient temperature yield 1-nitronaphthalene.
    *   *Sulfonation:* Thermodynamic vs. kinetic control: Sulfonation at $40\,^\circ C$ produces naphthalene-1-sulfonic acid (kinetic product). At $180\,^\circ C$, reversible desulfonation occurs, forming naphthalene-2-sulfonic acid (thermodynamic product), which avoids steric clashing with the peri-hydrogen at C-8.
    *   *Halogenation:* Reaction with $Cl_2$ catalyzed by $FeCl_3$ yields 1-chloronaphthalene.
    *   *Friedel-Crafts acylation:* Reaction with acetyl chloride ($CH_3COCl$) and $AlCl_3$ in carbon disulfide ($CS_2$) yields 1-acetylnaphthalene; in nitrobenzene ($PhNO_2$), it yields 2-acetylnaphthalene.
    *   *Orientation in substituted naphthalenes:* Rings bearing activating groups (EDGs: $-OH$, $-NH_2$, $-R$) direct substitution to the same ring (homonuclear substitution, primarily at the ortho/para positions relative to the EDG). Rings bearing deactivating groups (EWGs: $-NO_2$, $-COOH$, $-SO_3H$) direct electrophiles to the other ring (heteronuclear substitution, yielding 1,5- and 1,8-disubstituted isomers).
*   *Commercial and medicinal uses of naphthalene derivatives:* Moth repellent goods; illuminating enrichment of coal gas; precursor for phthalic anhydride, carbaryl insecticide, and 2-naphthol. Pharmaceutical derivatives include:
    *   Propranolol ($\beta$-adrenergic antagonist for hypertension).
    *   Tolnaftate (synthetic topical antifungal).
    *   Menadione (synthetic Vitamin K analog).
    *   Naphazoline (sympathomimetic vasoconstrictor for rhinitis and sinusitis).

• Chemistry of Anthracene ($C_{14}H_{10}$):

  • Structure: Linear tricyclic aromatic hydrocarbon. Bond lengths: $C_1-C_2 = 1.37\,\text{Å}$; $C_2-C_3 = 1.42\,\text{Å}$. Resonance energy is $84\,kcal\cdot mol^{-1}$.

  • Synthesis:

    • Friedel-Crafts alkylations: Intermolecular condensation of two benzyl chloride molecules with $AlCl_3$ with loss of two $HCl$ and two $H$ atoms; or condensation of benzene with acetylene tetrabromide ($CHBr_2-CHBr_2$) or dibromomethane ($CH_2Br_2$) in the presence of $AlCl_3$.

    • Haworth synthesis: Phthalic anhydride condenses with benzene in the presence of $AlCl_3$ to produce $o$-benzoylbenzoic acid. Ring closure heated with concentrated $H_2SO_4$ yields 9,10-anthraquinone, which is reduced by zinc dust distillation to anthracene.

    • Diels-Alder synthesis: Cycloaddition of 1,4-naphthoquinone with 1,3-butadiene yields a tetrahydroanthraquinone adduct. Oxidation with $CrO_3$ in acetic acid yields 9,10-anthraquinone, followed by zinc reduction to anthracene.

  • Reactivity at C-9 and C-10: Electrophilic attack occurs preferentially at the central meso carbons (C-9 and C-10). Attack at C-9 yields an intermediate preserving two isolated benzene rings (loss of $84 - 72 = 12\,kcal\cdot mol^{-1}$ resonance energy). Attack at C-1 or C-2 leaves only one intact naphthalene nucleus (loss of $84 - 61 = 23\,kcal\cdot mol^{-1}$ resonance energy).

  • Diels-Alder addition: Anthracene undergoes a [4+2] cycloaddition across the 9,10-positions with maleic anhydride, forming a bicyclic endo-anthracene-maleic anhydride adduct.

  • Addition reactions: Photooxidation in sunlight with $O_2$ yields anthracene 9,10-endoperoxide. Reduction with sodium in ethanol yields 9,10-dihydroanthracene. Chlorination in $CCl_4$ produces 9,10-dichloro-9,10-dihydroanthracene, which loses $HCl$ upon heating to give 9-chloroanthracene. Nitration with $HNO_3$ in acetic anhydride ($(CH_3CO)_2O$) yields 9-nitroanthracene (standard nitric/sulfuric acid mixtures oxidize it directly to 9,10-anthraquinone).

• Chemistry of Phenanthrene ($C_{14}H_{10}$):

  • Structure: Angular tricyclic aromatic hydrocarbon with a resonance energy of $92\,kcal\cdot mol^{-1}$. Numbering starts from an outer ring carbon and proceeds around the perimeter, designating the bridgehead ethylene carbons as C-9 and C-10. The $C_9-C_{10}$ bond has high double bond character, behaving similarly to an alkene linkage.

  • Haworth Synthesis: Friedel-Crafts acylation of naphthalene with succinic anhydride and $AlCl_3$ produces 4-(naphthalen-1-yl)-4-oxobutanoic acid. Clemmensen reduction ($Zn(Hg), HCl$) yields 4-(naphthalen-1-yl)butanoic acid. Acid-catalyzed ring closure with $H_2SO_4$ yields 3,4-dihydrophenanthren-1(2H)-one. A second Clemmensen reduction produces 1,2,3,4-tetrahydrophenanthrene, which is aromatized over selenium/palladium ($Se/Pd$) with heat to furnish phenanthrene.

  • Pschorr Synthesis: Condensation of $o$-nitrobenzaldehyde with sodium phenylacetate in acetic anhydride yields $\alpha$-phenyl-$o$-nitrocinnamic acid. Reduction of the nitro group to an amine, followed by diazotization ($NaNO_2, H_2SO_4$) and ring closure with copper powder, yields phenanthrene-9-carboxylic acid, which undergoes thermal decarboxylation to phenanthrene.

  • Chemical reactions at C-9 and C-10:

    • Reduction: Sodium and heat in isoamyl alcohol ($C_5H_{11}OH$) reduces the central bond to form 9,10-dihydrophenanthrene.

    • Oxidation: Oxidation with potassium dichromate ($K_2Cr_2O_7$) or chromic acid in acetic acid yields 9,10-phenanthraquinone; continued oxidation cleaves the central dione ring to yield diphenic acid (biphenyl-2,2'-dicarboxylic acid).

    • Ozonolysis: Cleavage with ozone followed by reductive workup yields biphenyl-2,2'-dicarbaldehyde.

    • Bromination: Electrophilic addition of elemental bromine ($Br_2$) in the dark yields 9,10-dibromo-9,10-dihydrophenanthrene. Heating drives elimination of $HBr$, yielding 9-bromophenanthrene.

• Diphenylmethane ($C_{13}H_{12}$):

  • Consists of a central methylene bridge linking two isolated benzene rings.

  • Preparation: Friedel-Crafts alkylation of benzene with benzyl chloride in the presence of $AlCl_3$; condensation of two equivalents of benzene with dichloromethane ($CH_2Cl_2$); or reduction of benzophenone using $HI$ and red phosphorus, Clemmensen conditions ($Zn(Hg), HCl$), or Wolff-Kishner conditions ($NH_2NH_2, NaOEt$).

  • Reactions: Nitration with concentrated $HNO_3/H_2SO_4$ yields 1-benzyl-4-nitrobenzene, followed by bis(4-nitrophenyl)methane. Radical bromination ($Br_2, h\nu$) selectively brominates the benzylic position, producing diphenylmethyl bromide. Oxidation with potassium dichromate in sulfuric acid produces benzophenone. Thermal pyrolytic passage through a red-hot tube cyclizes diphenylmethane to fluorene with loss of $H_2$.

  • Derivatives and Applications: Synthetic precursor to benzhydrol (diphenylmethanol) and benzophenone. Applied in materials chemistry as luminogens for aggregation-induced emission (AIE). Its conjugate base, diphenylmethyl potassium, is utilized as an initiator in anionic polymerization.

• Triphenylmethane ($(C_6H_5)_3CH$):

  • Hydrocarbon core (chromogen) for the triarylmethane dye family.

  • Preparation: Friedel-Crafts reaction between three equivalents of benzene and one equivalent of trichloromethane (chloroform, $CHCl_3$) catalyzed by aluminium chloride ($AlCl_3$).

  • Properties and Dyes: Produces brilliant, intensely colored dyes that possess low photolytic fastness and fade under prolonged light exposure. While largely superseded in modern apparel textiles, they are applied to colored paper, printing inks, hectograph duplication, and typewriter ribbons.

  • Important Derivatives: Gentian violet (antiseptic/histological stain); Aurin (rosolic acid indicator); Crystal violet, an intensely colored dye used in industrial detergents, fertilizers, and textile colorants.

Cycloalkanes: Strain Theories, Conformations, and Reactions

• Definition, Isomerism, and Physical Properties:

  • Cycloalkanes (historically designated naphthenes) are saturated carbocyclic hydrocarbons composed of carbon rings. They have the general molecular formula $C_nH_{2n}$.

  • Possess two fewer hydrogen atoms than acyclic alkanes of the same carbon count.

  • Naturally distributed in biological systems, including prostaglandins (e.g., cyclopentyl ring in Prostaglandin $E_1$) and steroidal frameworks (e.g., cyclopentanoperhydrophenanthrene core in Cortisone).

  • Cis-Trans Stereoisomerism: Because the closed ring structure restricts free rotation around carbon-carbon bonds, disubstituted cycloalkanes exhibit geometric isomerism:

    • Cis isomer: Substituents lie on the same face/side of the carbocyclic ring (e.g., cis-1-bromo-3-chlorocyclobutane, cis-1,4-dimethylcyclohexane).

    • Trans isomer: Substituents occupy opposite faces/sides of the carbocyclic ring (e.g., trans-1-bromo-3-chlorocyclobutane, trans-1,4-dimethylcyclohexane).

  • IUPAC Nomenclature Principles:

    • The parent ring is the largest ring in the molecule, designated with the prefix cyclo- appended to the alkane name.

    • Ring carbons are numbered sequentially to assign the lowest locants at first points of difference.

    • Substituents are cited in alphabetical order.

    • Larger rings take naming precedence over smaller rings (e.g., 1-cyclopentyl-2-cyclopropyl-4-(2'-methylbutyl)cyclohexane).

    • If an attached acyclic alkyl chain contains a greater number of carbons than the ring, the aliphatic chain becomes the parent alkane (e.g., 1-(4'-cyclopentyl-3'-cyclopropylcyclohexyl)-6-methylheptane).

    • Bicyclic nomenclature: bicyclo[a.b.c]alkane, where bracketed numbers describe the carbon atoms in each bridge between bridgeheads in decreasing order (e.g., bicyclo[2.2.1]heptane).

  • Physical Properties Table:

    • Cyclopropane ($C_3H_6$): Boiling point $-33\,^\circ C$, Melting point $-128\,^\circ C$.

    • Cyclobutane ($C_4H_8$): Boiling point $12.5\,^\circ C$, Melting point $-91\,^\circ C$, Liquid density $0.720\,g\cdot cm^{-3}$ at $20\,^\circ C$.

    • Cyclopentane ($C_5H_{10}$): Boiling point $49.2\,^\circ C$, Melting point $-93.9\,^\circ C$, Liquid density $0.751\,g\cdot cm^{-3}$ at $20\,^\circ C$.

    • Cyclohexane ($C_6H_{12}$): Boiling point $80.7\,^\circ C$, Melting point $6.5\,^\circ C$, Liquid density $0.778\,g\cdot cm^{-3}$ at $20\,^\circ C$.

    • Cycloheptane ($C_7H_{14}$): Boiling point $118.4\,^\circ C$, Melting point $-12\,^\circ C$, Liquid density $0.811\,g\cdot cm^{-3}$ at $20\,^\circ C$.

    • Cyclooctane ($C_8H_{16}$): Boiling point $149\,^\circ C$, Melting point $14.6\,^\circ C$, Liquid density $0.834\,g\cdot cm^{-3}$ at $20\,^\circ C$.

    • Cyclononane ($C_9H_{18}$): Boiling point $69\,^\circ C$, Melting point $10-11\,^\circ C$, Liquid density $0.8534\,g\cdot cm^{-3}$ at $20\,^\circ C$.

    • Cyclodecane ($C_{10}H_{20}$): Boiling point $201\,^\circ C$, Melting point $9-10\,^\circ C$, Liquid density $0.871\,g\cdot cm^{-3}$ at $20\,^\circ C$.

    • Cycloalkanes possess higher boiling points, melting points, and densities than corresponding acyclic alkanes due to rigid ring geometry that facilitates greater contact area for London dispersion forces.

• Total Ring Strain Components:

  • Angle strain (Baeyer strain): Potential energy increase resulting from compression or expansion of carbon-carbon bond angles away from the ideal tetrahedral angle ($109.5\,^\circ$).

  • Torsional strain (Pitzer strain): Potential energy increase caused by eclipsing interactions between carbon-hydrogen or carbon-carbon bonds on adjacent atoms.

  • Steric strain (van der Waals strain): Repulsive non-bonded potential energy increase when atoms or substituents are forced into closer proximity than their van der Waals radii permit.

• Baeyer Strain Theory (1885): Assumptions, Predictions, and Limitations:

  • Fundamental Assumptions: (1) All carbocyclic ring systems are strictly planar polygons; (2) Deviations from the regular tetrahedral bond angle ($109.5\,^\circ$) induce angle strain that destabilizes the ring; (3) Rings with negative strain (angles exceeding $109.5\,^\circ$) cannot exist.

  • Regular polygon internal angles ($\text{Angle} = \frac{(n-2) \times 180^\circ}{n}$):

    • Ring 3 (cyclopropane): $60^\circ$ (compression deviation of $49.5^\circ$)

    • Ring 4 (cyclobutane): $90^\circ$ (compression deviation of $19.5^\circ$)

    • Ring 5 (cyclopentane): $108^\circ$ (deviation of only $1.5^\circ$)

    • Ring 6 (cyclohexane): $120^\circ$ (expansion deviation of $-10.5^\circ$)

    • Ring 7 (cycloheptane): $129^\circ$

    • Ring 8 (cyclooctane): $135^\circ$

    • Ring 9 (cyclononane): $140^\circ$

    • Ring 10 (cyclodecane): $144^\circ$

  • Predictions vs. Thermochemical Facts: Baeyer predicted cyclopentane ($108^\circ$) to be the most stable cycloalkane, with cyclohexane and larger rings experiencing progressive strain and instability. Experimental heats of combustion per methylene ($-CH_2-$) group disprove this:

    • Unstrained open-chain alkane: $157.4\,kcal\cdot mol^{-1}$ ($658.6\,kJ\cdot mol^{-1}$)

    • Cyclopropane ($n=3$): $166.6\,kcal\cdot mol^{-1}$

    • Cyclobutane ($n=4$): $164.0\,kcal\cdot mol^{-1}$

    • Cyclopentane ($n=5$): $158.7\,kcal\cdot mol^{-1}$

    • Cyclohexane ($n=6$): $157.4\,kcal\cdot mol^{-1}$ (completely strain-free)

    • Cycloheptane ($n=7$): $158.3\,kcal\cdot mol^{-1}$

    • Cyclooctane ($n=8$): $158.6\,kcal\cdot mol^{-1}$

    • Cyclononane ($n=9$): $158.8\,kcal\cdot mol^{-1}$

    • Cyclodecane ($n=10$): $157.4\,kcal\cdot mol^{-1}$

  • Why Baeyer's theory failed: Rings larger than cyclopropane are not planar. They adopt puckered, three-dimensional conformations that relieve both angle strain and torsional strain by maintaining tetrahedral bond angles near $109.5\,^\circ$.

• Modern Bonding Models:

  • Coulson-Moffitt Model (Bent or Banana Bonds in Cyclopropane): Cyclopropane cannot achieve standard collinear orbital overlap along its internuclear axes without extreme destabilization. Instead, the carbon atoms employ hybrid orbitals ($\sim sp^5$ for C-C bonds, with increased $p$-character, and $sp^2$ for C-H bonds) that project outward at an angle of approximately $109^\circ$. Overlap occurs off-axis, creating curved, bent "banana bonds." This non-linear overlap is intermediate between $\sigma$ (head-on) and $\pi$ (sideways) bonding, resulting in weaker C-C bonds. Combined with complete eclipsing of all six C-H bonds (torsional strain), this makes cyclopropane highly strained and reactive.