Comprehensive Study Notes on Benzene, Arenes, and Phenol

Characteristics and Structure of the Benzene Ring

• The benzene ring is a fundamental functional group consisting of a hexagonal arrangement of six carbon atoms.

• Benzene rings are integral to diverse commercially significant compounds, including:

  • Medicines

  • Dyes

  • Plastics

• Definitions and Classifications:

  • Arenes: Organic hydrocarbons that contain one or more benzene rings.

  • Aryl Compounds (Aromatic Compounds): General terms for compounds containing benzene; an example provided is chlorobenzene ($C_6H_5Cl$), which belongs to the category of halogenoarenes.

• Evolution of Structural Theories:

  • Kekulé's Structure: Originally proposed as a hexagonal ring with alternating single ($C-C$) and double ($C=C$) bonds. This led to the "-ene" suffix in the name benzene, similar to alkenes.

  • Evidence against Kekulé's Structure:

    • Symmetry and Planarity: Modern analytical techniques show benzene is a perfectly symmetrical, planar molecule. Kekulé's alternating bonds would result in a distorted hexagon with three shorter double bonds and three longer single bonds.

    • Bond Lengths: Carbon-to-carbon bond lengths in benzene are intermediate between single and double bonds.

      • Table 25.1: Comparing Bond Lengths:

        • $C-C$ (single bond): $0.154\,nm$

        • $C=C$ (double bond): $0.134\,nm$

        • Carbon-to-carbon bond in benzene: $0.139\,nm$

    • Chemical Reactivity: If benzene contained literal double bonds, it would undergo addition reactions (e.g., decolorizing bromine water at room temperature). However, benzene requires significantly harsher conditions compared to alkenes like ethene.

• The True Bonding Model of Benzene:

  • Each of the six carbon atoms in the ring is $sp^2$ hybridised.

  • Each carbon shares one pair of electrons with its two neighbouring carbon atoms and one pair with a hydrogen atom, forming three $\sigma$ (sigma) bonds.

  • Distribution of $\sigma$ bonds: These electron pairs are found primarily between the nuclei of the bonded atoms.

  • The $\pi$ (pi) System: Each carbon has one spare electron in a $p$ orbital. These six electrons are not confined to specific pairs of carbons as in alkenes; instead, they are spread over all six carbon atoms.

  • Delocalisation: The six electrons in the $\pi$ bonds are described as delocalised. The $p$ orbitals overlap to form a ring of delocalised electrons situated above and below the plane of the carbon atoms.

  • Geometric Details: The molecule is planar to achieve maximum overlap of orbitals. All bond angles around the $sp^2$ hybridised carbon atoms are exactly $120^\circ$.

Nomenclature of Aryl Compounds

• Aryl compounds are named based on the substitution of hydrogen atoms on the benzene ring with various functional groups.

• The Phenyl Group: In substituted benzene, the $C_6H_5$ group is referred to as the phenyl group. For example, phenylamine has the structural formula $C_6H_5NH_2$.

• Table 25.2: Names of Aryl Compounds:

  • Chlorobenzene: Benzene ring with a $-Cl$ group.

  • Nitrobenzene: Benzene ring with a $-NO_2$ group.

  • Phenol: Benzene ring with an $-OH$ group.

  • 2,4,6-Tribromophenol: A phenol ring with bromine atoms substituted at carbon positions 2, 4, and 6.

  • Phenylamine: Benzene ring with an $-NH_2$ group.

• Numbering: Carbon atoms are numbered to specify the position of multiple substitutions (e.g., 1,3-dichloro-5-nitrobenzene).

General Reactions of Arenes

• Stability and Mechanism: Arenes are highly stable due to the delocalised $\pi$ electron ring (aromatic stabilisation). Most reactions involve electrophilic substitution, which replaces a hydrogen atom while keeping the delocalised ring intact. Addition reactions are avoided as they would disrupt this stability.

• Electrophilic Attack: The high electron density above and below the ring attracts electrophiles.

Electrophilic Substitution with Halogens

• Bromination:

  • Reagents/Conditions: Benzene reacts with bromine ($Br_2$) using an anhydrous aluminium bromide ($AlBr_3$) catalyst (a halogen carrier).

  • Reaction: $C_6H_6 + Br_2 \rightarrow C_6H_5Br + HBr$

  • Mechanism of Electrophile Generation: The catalyst polarises the bromine molecule. A dative bond forms between a bromine lone pair and an empty $3p$ orbital in aluminium. This creates a partial positive charge: $\delta+ Br-Br-AlBr_3$. The effective electrophile is the $Br^+$ cation.

  • Mechanism steps:

    • Stage 1: The $Br^+$ cation is attracted to the ring. A pair of electrons from the delocalised system forms a bond with $Br$, temporarily disrupting the ring and creating a positively charged intermediate.

    • Stage 2: An electron pair from a $C-H$ bond returns to the $\pi$ system, and $H^+$ is lost (reacting with $[AlBr_4]^-$ to regenerate $AlBr_3$ and produce $HBr$).

• Chlorination:

  • Similar to bromination, but uses chlorine gas and an anhydrous aluminium chloride ($AlCl_3$) catalyst at room temperature.

• Substituent Activation in Halogenation:

  • Groups like methyl ($-CH_3$), hydroxyl ($-OH$), and amine ($-NH_2$) activate the 2 and 4 positions of the ring.

  • When methylbenzene reacts with $Cl_2$ and $AlCl_3$, the products are 2-chloromethylbenzene and 4-chloromethylbenzene.

  • Excess chlorine can result in 2,4-dichloromethylbenzene, 2,6-dichloromethylbenzene, and 2,4,6-trichloromethylbenzene.

• Bond Strength: Carbon-halogen bonds in halogenoarenes are stronger and less reactive than in halogenoalkanes because a lone pair on the halogen overlaps with the benzene $\pi$ system, providing partial double bond character.

• Side-Chain Reaction (Free-Radical Substitution):

  • Conditions: Boiling methylbenzene with chlorine in the presence of ultraviolet (UV) light.

  • Result: Chlorine substitutes into the methyl group, not the ring. Further chlorination can replace all three side-chain hydrogens.

  • Example: Conversion of methylbenzene to chloromethylbenzene and eventually (trichloromethyl)benzene.

Nitration and Sulfonation of Benzene

• Nitration:

  • Introduction of the $-NO_2$ group using the nitronium ion (nitryl cation), $NO_{2}^{+}$.

  • The Nitrating Mixture: Concentrated nitric acid ($HNO_3$) and concentrated sulfuric acid ($H_2SO_4$).

  • Equation: $HNO_3 + 2H_2SO_4 \rightarrow NO_{2}^{+} + 2HSO_{4}^{-} + H_3O^{+}$

  • Conditions: Refluxed with benzene at temperatures ranging from $25\,^\circ C$ to $60\,^\circ C$.

  • Mechanism Details:

    • Stage 1: $NO_{2}^{+}$ accepts a pair of electrons from the ring. The ring now has 4 $\pi$ electrons and a positive charge spread over 5 carbons.

    • Stage 2: The $C-H$ bond breaks heterolytically. The two electrons return to the system, restoring the ring's stability and releasing $H^+$.

  • Directing Effects: The $-NO_2$ group is electron-withdrawing and deactivates the 2 and 4 positions. Further nitration is directed to the 3 and 5 positions, producing 1,3-dinitrobenzene and 1,3,5-trinitrobenzene.

• Sulfonation:

  • Conditions: Reflux with fuming sulfuric acid for several hours.

  • Electrophile: The $SO_3$ molecule.

  • Product: Benzenesulfonic acid ($C_6H_5SO_3H$).

Friedel-Crafts Reactions (Alkylation and Acylation)

• Used to introduce side-chains (alkyl or acyl groups) into the benzene ring. These are essential for manufacturing detergents and plastics (e.g., polystyrene).

• General Mechanism: Involves attack by a carbocation electrophile generated by the reaction of a halogenoalkane or acyl chloride with an $AlCl_3$ catalyst.

• Alkylation:

  • Reagents: Halogenoalkane (e.g., $CH_3CH_2Cl$) and $AlCl_3$.

  • Step 1 (Electrophile generation): $R-Cl + AlCl_3 \rightarrow R^+ + [AlCl_4]^-$ (via a dative covalent bond between $Cl$ and $Al$).

  • Step 2: Carbocation attacks the benzene ring.

  • Step 3: $AlCl_3$ is regenerated and $HCl$ is formed.

• Acylation:

  • Reagents: Acyl chloride (e.g., ethanoyl chloride, $CH_3COCl$) and $AlCl_3$.

  • Adds an acyl group ($R-C=O$) to form an acylbenzene (e.g., phenylethanone).

  • Process follows the same three steps as alkylation.

Oxidation and Hydrogenation of Arenes

• Oxidation of the Side-Chain:

  • Unlike alkanes, the alkyl side-chain of an arene is easily oxidised.

  • Reagents: Reflux with alkaline potassium manganate(VII) ($KMnO_4$), followed by acidification with dilute sulfuric acid ($H_2SO_4$).

  • Result: Methylbenzene (or any alkylarene like hexylbenzene) is oxidised to benzoic acid ($C_6H_5COOH$).

• Hydrogenation:

  • Benzene can behave like an unsaturated alkene under specific conditions.

  • Reagents: Hydrogen gas ($H_2$) with a nickel ($Ni$) or platinum ($Pt$) catalyst and heat.

  • Result: Benzene converts to cyclohexane ($C_6H_{12}$). Methylbenzene converts to methylcyclohexane ($C_6H_{11}CH_3$).

Physical and Chemical Properties of Phenol

• Physical Properties:

  • Formula: $C_6H_5OH$.

  • Appearance: Crystalline solid with a melting point of $43\,^\circ C$.

  • Solubility: Only slightly soluble in water because the large non-polar benzene ring disrupts hydrogen bonding, although the $-OH$ group itself can hydrogen bond.

• Preparation of Phenol:

  1. Form nitric(III) acid (nitrous acid, $HNO_2$): $NaNO_2 + HCl \rightarrow HNO_2 + NaCl$.

  2. React phenylamine with $HNO_2$ and $HCl$ below $10\,^\circ C$ to form benzenediazonium chloride ($C_6H_5N_{2}^{+}Cl^-$).

  3. Warm the diazonium salt in water to produce phenol and nitrogen gas ($N_2$).

• Acidity of Phenol:

  • Phenol is a weak acid, dissociating as follows: $C_6H_5OH(aq) \rightleftharpoons C_6H_5O^-(aq) + H^+(aq)$.

  • Comparing Acidity (pKa values): Phenol ($10.0$) > Water ($14.0$) > Ethanol ($16.0$).

  • Explanation for Acidity:

    • Stable Phenoxide Ion: One of the lone pairs on the oxygen atom overlaps with the delocalised $\pi$ system of the ring. This spreads the negative charge, reducing charge density.

    • Reduced Attraction: Because the charge is spread, $H^+$ ions are less strongly attracted back to the phenoxide ion compared to hydroxide ($OH^-$) or ethoxide ($C_2H_5O^-$) ions.

  • Ethanol Acidity: Ethanol is the weakest because the electron-donating ethyl group concentrates negative charge on the oxygen, making it more likely to accept an $H^+$ ion.

Reactions of Phenol

• Reactions of the Hydroxyl Group:

  • With Alkalis: Reacts with $NaOH$ to form water-soluble sodium phenoxide ($C_6H_5O^-Na^+$) and water.

  • With Sodium Metal: Molten phenol reacts vigorously with sodium to produce sodium phenoxide and hydrogen gas ($H_2$).

• Substitution into the Benzene Ring:

  • Phenol is much more reactive than benzene (activated ring).

  • Activation Mechanism: The overlap of the oxygen lone pair with the $\pi$ system increases the electron density of the ring, making it more susceptible to electrophilic attack.

  • Directing Effect: The $-OH$ group directs substitution to the 2, 4, and 6 positions.

• Bromination of Phenol:

  • Unlike benzene, which requires pure bromine and a catalyst, phenol reacts with bromine water ($Br_{2}(aq)$) at room temperature.

  • Observation: The orange bromine solution is decolorised, and a white precipitate of 2,4,6-tribromophenol is formed.

• Nitration of Phenol:

  • Reacts with dilute nitric acid at room temperature to form 2-nitrophenol or 4-nitrophenol.

  • Concentrated nitric acid produces 2,4,6-trinitrophenol.

• 1-Naphthol: A related compound where the $-OH$ group activates the attached ring and directs attack to the 2 and 4 positions.

Summary of Ring Substituents (Table 25.4)

• 2, 4, and 6 positions (Activators):

  • Typically electron-donating groups.

  • Includes: $-NH_2$, $-OH$, $-R$ (alkyl), $-Cl$ (Note: $-Cl$ is an exception—it directs to 2,4 but technically deactivates slightly compared to benzene).

• 3 and 5 positions (Deactivators):

  • Typically electron-withdrawing groups.

  • Includes: $-NO_2$, $-COR$, $-CHO$, $-COOH$.

Questions & Discussion

• Question 1:

  • a: 6 electrons are involved in the benzene $\pi$ bonding system.

  • b: These come from $p$ atomic orbitals.

  • c: Delocalised electrons are spread over more than two nuclei rather than being localized in a single bond between two atoms.

  • d: Compare $\pi$ bonding in benzene (delocalised over 6 carbons, planar, equal bond lengths) vs hex-3-ene (localized between carbons 3 and 4).

  • e: Requires drawing 1,3,5-tribromobenzene and 1,3-dichloro-5-nitrobenzene.

  • f: Identifying molecules like 2-methylphenol or 2-bromophenol (based on images).

• Question 2:

  • Equation for benzene + $Cl_2$: $C_6H_6 + Cl_2 \xrightarrow{AlCl_3} C_6H_5Cl + HCl$.

  • Mechanism: Electrophilic substitution.

  • Reaction of methylbenzene + excess $Br_2$: Forms 2,4,6-tribromomethylbenzene.

  • Boiling methylbenzene + $Br_2$ with UV: Side-chain substitution occurs instead of ring substitution via a free-radical mechanism.

• Question 3:

  • Nitration of methylbenzene creates 2-nitromethylbenzene and 4-nitromethylbenzene.

  • Sulfonation: The oxygen atom in $SO_3$ accepts the electron pair. Equation: $C_6H_6 + SO_3 \rightarrow C_6H_5SO_3H$.

• Question 4:

  • Alkylation of benzene with $CH_3CH_2CH_2Cl$ produces propylbenzene.

  • Hexylbenzene + alkaline $KMnO_4$: Forms benzoic acid. Hexane + alkaline $KMnO_4$: No reaction.

  • Reagents for benzene to cyclohexane: $H_2$, $Ni$ or $Pt$ catalyst, heat. Type: Addition.

• Question 5:

  • Acidity order: $HCl$ > $CH_3COOH$ > $C_6H_5OH$ > $H_2O$ > $C_3H_7OH$.

  • Methanol vs Phenol: Methanol is less acidic because it lacks the delocalising phenyl ring to stabilise the negative charge on the conjugate base.

  • Diazonium salt preparation: Phenylamine, sodium nitrate(III), and dilute hydrochloric acid at below $10\,^\circ C$.