Halogen Derivatives of Alkanes and Arenes: Comprehensive Study Guide

Introduction to Halogen Derivatives

• Parent Family: Hydrocarbons are the parent family of organic compounds.

• Definition: The replacement of hydrogen atom/s in aliphatic or aromatic hydrocarbons by halogen atom/s results in the formation of halogen derivatives of hydrocarbons.

• General Examples:

  • Haloalkane: $CH_3 - CH_2 - X$

  • Haloalkene: $CH_2 = CH - X$

  • Haloalkyne: $HC \equiv C - X$

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Classification of Halogen Derivatives

Halogen derivatives are mainly classified in two ways:

• On the Basis of Hydrocarbon Skeleton:

  • Haloalkanes: Halogen attached to an alkane chain.

  • Haloalkenes: Halogen attached to an alkene chain.

  • Haloalkynes: Halogen attached to an alkyne chain.

  • Haloarenes: Halogen attached to an aromatic ring ($X$ bonded to benzene ring).

• On the Basis of Number of Halogen Atoms:

  • Monohalogen compounds: One halogen atom. Example: $CH_3 - CH_2 - X$.

  • Dihalogen compounds: Two halogen atoms. Examples: $CH_2(X) - CH_2(X)$ or $CH_3 - CH(X)_2$.

  • Trihalogen compounds: Three halogen atoms. Example: $CH_2(X) - CH(X) - CH_2(X)$.

  • Polyhalogen compounds: Multiple halogen atoms. Example: Benzene hexachloride or Hexachlorocyclohexane ($C_6H_6Cl_6$).

Classification of Monohalogen Compounds

These are classified based on the position of the halogen and the hybridization of the carbon atom.

• Alkyl Halides or Haloalkanes: Halogen is bonded to an $sp^3$ hybridized carbon which is part of a saturated skeleton. Classified further as:

  • Primary halide ($1^o$): Carbon attached to halogen is bonded to only one other carbon ($R - CH_2 - X$).

  • Secondary halide ($2^o$): Carbon attached to halogen is bonded to two other carbons ($R - CH(X) - R$).

  • Tertiary halide ($3^o$): Carbon attached to halogen is bonded to three other carbons ($R - C(X)(R) - R$).

• Allylic Halides: Halogen atom is bonded to an $sp^3$ hybridized carbon atom next to a carbon-carbon double bond ($CH_2 = CH - CH_2 - X$).

• Benzylic Halides: Halogen atom is bonded to an $sp^3$ hybridized carbon atom which is further bonded to an aromatic ring ($Ar - CH_2 - X$).

• Vinylic Halides: Halogen atom is bonded to an $sp^2$ hybridized carbon atom of an aliphatic chain ($CH_2 = CH - X$).

• Haloalkyne: Halogen atom is bonded to an $sp$ hybridized carbon atom ($CH \equiv C - X$).

• Aryl Halides or Haloarenes: Halogen atom is directly bonded to an $sp^2$ hybridized carbon atom of an aromatic ring.

Nomenclature: Common and IUPAC Names (Table 10.1)

• Compounds and Names List:

  • $CH_2Cl_2$: Common: Methylene chloride; IUPAC: Dichloromethane.

  • $CH_3CH_2Br$: Common: Ethyl bromide; IUPAC: Bromoethane.

  • $CH_3CH(Cl)CH_3$: Common: Isopropyl chloride; IUPAC: 2-Chloropropane.

  • $(CH_3)_2CH-CH_2Br$: Common: Isobutyl bromide; IUPAC: 1-Bromo-2-methylpropane.

  • $(CH_3)_3CBr$: Common: Tert-butyl bromide; IUPAC: 2-Bromo-2-methylpropane.

  • $(CH_3)_3CCH_2Cl$: Common: Neopentyl chloride; IUPAC: 1-Chloro-2,2-dimethylpropane.

  • $CH_2 = CH - Cl$: Common: Vinyl chloride; IUPAC: 1-Chloroethene.

  • $CH_2 = CH - CH_2Br$: Common: Allyl bromide; IUPAC: 3-Bromopropene.

  • $CH \equiv C - Cl$: Common: Chloroacetylene; IUPAC: Chloroethyne.

  • $C_6H_5CH_2I$: Common: Benzyl iodide; IUPAC: Iodophenylmethane.

  • $p-Iodotoluene$ ($C_6H_4(I)(CH_3)$): IUPAC: 1-Iodo-4-methylbenzene or 4-Iodotoluene.

  • $m-dichlorobenzene$: IUPAC: 1,3-dichlorobenzene.

Methods of Preparation of Alkyl Halides

From Alcohols

This is the most widely used method, replacing the $-OH$ group with $-X$.

• Reactivity Order: The order of reactivity of alcohols with a given haloacid is 3^o > 2^o > 1^o.

• Using Hydrogen Halides ($HX$):

  • Grooves' process: Primary and secondary alcohols use $HCl$ with anhydrous $ZnCl_2$ (Lucas reagent). Tertiary alcohols react readily with concentrated $HCl$ alone.

  • Formation of Chlorides: $R - OH + HCl \xrightarrow{\text{anhydrous } ZnCl_2} R - Cl + H_2O$.

  • Formation of Bromides: Use constant boiling hydrobromic acid ($48\%$). Primary bromides can be made using $NaBr$ and $H_2SO_4$ to generate $HBr$ in situ.

  • Formation of Iodides: Heat alcohol with $NaI$ or $KI$ in $95\%$ phosphoric acid ($H_3PO_4$) to generate $HI$ in situ.

• Using Phosphorus Halides:

  • $3R - OH + PX_3 \rightarrow 3R - X + H_3PO_3$ ($PBr_3$ and $PI_3$ generated in situ using red phosphorus and $Br_2$/$I_2$).

  • $R - OH + PCl_5 \rightarrow R - Cl + HCl + POCl_3$.

• Using Thionyl Chloride ($SOCl_2$):

  • Reaction: $R - OH + SOCl_2 \xrightarrow{\Delta} R - Cl + SO_2\uparrow + HCl\uparrow$.

  • Advantage: Byproducts ($SO_2$ and $HCl$) are gases, leaving pure alkyl chloride. Preferred method for straight chain primary alcohols.

From Hydrocarbons

• Halogenation of Alkanes: Not ideal because it results in a mixture of mono and polyhalogen compounds.

• Addition to Alkenes:

  • Alkenes react with $HX$ to form alkyl halides (follows Markovnikov’s rule or peroxide effect/anti-Markovnikov for $HBr$).

  • Reaction with $X_2$: Alkenes react with $Cl_2$ or $Br_2$ in $CCl_4$ to form vicinal dihalides.

  • Allylic Substitution: At high temperatures, $Br_2$ or $Cl_2$ substitutes hydrogen at the allylic carbon.

Halogen Exchange

• Finkelstein Reaction: Preparation of alkyl iodides by treating alkyl chlorides/bromides with $NaI$ in methanol or acetone. $R - Cl + NaI \xrightarrow{\text{acetone}} R - I + NaCl\downarrow$.

• Swartz Reaction: Preparation of alkyl fluorides by heating alkyl chlorides/bromides with metal fluorides like $AgF$, $Hg_2F_2$, $AsF_3$, or $SbF_3$. $R - Cl + AgF \rightarrow R - F + AgCl\downarrow$.

Electrophilic Substitution (for Aryl Halides)

• Direct halogenation of benzene derivatives using Lewis acid catalysts ($Fe$, $FeCl_3$, or anhydrous $AlCl_3$) in the dark.

• Bromination of toluene yields a mixture of ortho and para bromo toluene.

• Iodination is reversible; shifted right by adding $HNO_3$ or $HIO_4$ to remove $HI$ via oxidation.

Sandmeyer’s Reaction

Aryl halides prepared by replacing nitrogen of a diazonium salt ($N_2^+ X^-$).

Physical Properties of Alkyl Halides

Nature of Intermolecular Forces

• The $C - X$ bond is a polar covalent bond due to the higher electronegativity of halogens compared to carbon ($C^{\delta+} - X^{\delta-}$).

• Size of halogen increases from $F$ to $I$, increasing $C - X$ bond length and decreasing bond strength ($p$-orbitals become more diffused, reducing overlap).

Table 10.2: Bond Parameters of $CH_3 - X$

• $CH_3 - F$: Length: $139\,pm$; Enthalpy: $452\,kJ\,mol^{-1}$; Dipole: $1.847\,D$.

• $CH_3 - Cl$: Length: $178\,pm$; Enthalpy: $351\,kJ\,mol^{-1}$; Dipole: $1.860\,D$.

• $CH_3 - Br$: Length: $193\,pm$; Enthalpy: $293\,kJ\,mol^{-1}$; Dipole: $1.830\,D$.

• $CH_3 - I$: Length: $214\,pm$; Enthalpy: $234\,kJ\,mol^{-1}$; Dipole: $1.636\,D$.

Boiling Points

• Boiling points are higher than corresponding alkanes due to polarity and mass.

• For same alkyl group: RI > RBr > RCl > RF (due to van der Waals forces).

• For same halogen, B.P. increases with carbon chain length.

• Isomeric Effect: B.P. decreases with increased branching because surface area decreases, lowering van der Waals forces. Example for $C_4H_9Br$:

  • $n$-butyl bromide: $375\,K$

  • $sec$-butyl bromide: $364\,K$

  • $tert$-butyl bromide: $346\,K$

Solubility

• Insoluble in water (cannot form hydrogen bonds; internal attraction is stronger than attraction to water).

• Soluble in non-polar organic solvents.

• Dihalobenzenes: Isomers have similar B.P., but para isomers have significantly higher melting points due to symmetry and better packing in the crystal lattice. Example ($Cl_2C_6H_4$):

  • ortho: M.P. $256\,K$

  • meta: M.P. $249\,K$

  • para: M.P. $323\,K$

Optical Isomerism in Halogen Derivatives

• Chiral Carbon Atom: A carbon atom bonded to four different groups or atoms. Marked with an asterisk ($*$). Example: $C-2$ in 2-chlorobutane ($CH_3 - ^*CHCl - CH_2 - CH_3$).

• Chiral Molecule: A molecule containing one chiral atom that is non-superimposable on its mirror image.

• Stereoisomers: Isomers with the same bond connectivity but different spatial arrangements.

• Plane Polarized Light: Light with oscillations in only one plane, produced by passing ordinary light through a Nicol's prism (polarizer made of calcite/$CaCO_3$).

• Optical Activity: The property of rotating the plane of polarized light.

  • Dextrorotatory ($d$ or $+$): Rotates plane right (clockwise).

  • Laevorotatory ($l$ or $-$): Rotates plane left (anticlockwise).

• Enantiomers: Non-superimposable mirror images with equal and opposite optical rotations. Identical physical properties (B.P., M.P., density) except for sign of rotation.

• Racemic Mixture ($dl$ or $\pm$): Equimolar mixture of enantiomers; optically inactive due to external compensation.

• Representations:

  • Fischer Projection (Cross Formula): Vertical and horizontal lines used to represent 3D on paper.

  • Wedge Formula: Normal lines (in plane), solid wedges (above plane), broken wedges (below plane).

Chemical Properties: Nucleophilic Substitution ($S_N$)

• Laboratory Test: Warming haloalkane with aqueous $NaOH$/$KOH$ converts covalently bonded halogen to halide ions. Acidifying with $HNO_3$ and adding $AgNO_3$ forms a silver halide ($AgX$) precipitate.

• General Reaction: $Nu^- + -C - X \rightarrow -C - Nu + X^-$.

• Reactivity Factors:

  • Substrate: 3^o > 2^o > 1^o (influences both mechanism types differently).

  • Halogen nature: R - I > R - Br > R - Cl.

Nucleophilic Reagents and Products (Table 10.3)

1. $NaOH$ (aq)/$KOH$ (aq) $\rightarrow$ Alcohol ($R - OH$).

2. $NaOR'$ (Sodium alkoxide) $\rightarrow$ Ether ($R - O - R'$).

3. $R'COOAg$ (Silver carboxylate) $\rightarrow$ Ester ($R'COOR$).

4. $NH_3$ (alc., excess) $\rightarrow$ Primary Amine ($R - NH_2$).

5. $KCN$ (alc.) $\rightarrow$ Alkyl Cyanide ($R - CN$) (Nitrile).

6. $AgCN$ (alc.) $\rightarrow$ Alkyl Isocyanide ($R - NC$).

7. $KNO_2$ $\rightarrow$ Alkyl Nitrite ($R - O - N = O$).

8. $AgNO_2$ $\rightarrow$ Nitroalkane ($R - NO_2$).

• Ambident Nucleophiles: Cyanide ($CN^-$) and Nitrite ($NO_2^-$). $KCN$ (ionic) attacks through $C$; $AgCN$ (covalent) attacks through $N$.

$S_N2$ Mechanism (Bimolecular)

• Example: Methyl bromide + $OH^-$.

• Kinetics: Second order ($Rate = k[CH_3Br][OH^-]$).

• Mechanism: Single step; simultaneous bond-breaking ($C-X$) and bond-forming ($C-Nu$).

• Salient Features:

  • Backside attack to avoid steric/electrostatic repulsion.

  • Transition state (T.S.) is pentacoordinate with partial bonds.

  • Walden Inversion: Configuration of product is opposite to substrate (like a flipped umbrella).

$S_N1$ Mechanism (Unimolecular)

• Example: $tert$-butyl bromide + $OH^-$.

• Kinetics: First order ($Rate = k[(CH_3)_3CBr]$).

• Mechanism: Two steps.

  • Step I: Slow heterolysis to form a planar carbocation intermediate.

  • Step II: Fast attack of nucleophile on either side of carbocation.

• Salient Features:

  • Produces almost racemic product (racemization) if chiral.

  • Favoured in tertiary halides ($3^o$) due to stability of carbocation ($+I$ effect and hyperconjugation).

Factors Influencing $S_N1$ and $S_N2$

• Substrate Structure:

  • $S_N2$ rate: CH_3X > 1^o > 2^o > 3^o (due to steric crowding in T.S.).

  • $S_N1$ rate: 3^o > 2^o > 1^o > CH_3X (due to carbocation stability).

  • Allylic and Benzylic halides show high $S_N1$ reactivity due to resonance stabilization of the intermediate carbocation.

• Solvent Polarity:

  • $S_N1$: Favoured by polar protic solvents (stabilize ions via solvation).

  • $S_N2$: Polar protic solvents decrease rate by stabilizing/deactivating the nucleophile; aprotic or low polarity solvents favour $S_N2$.

• Nucleophilicity: Strong nucleophiles favour $S_N2$. For $S_N1$, rate is independent of nucleophile strength.

Elimination Reactions

• Dehydrohalogenation ($\beta$-elimination): Heating alkyl halide with alcoholic $KOH$ removes a $\beta$-hydrogen and $\alpha$-halogen to form an alkene.

• Saytzeff’s Rule: In dehydrohalogenation, the preferred product is the more highly substituted alkene (the one with the greater number of alkyl groups attached to doubly bonded carbons).

  • Example: 2-bromobutane gives but-2-ene ($80\%$) as major product and but-1-ene ($20\%$) as minor.

• Elimination vs. Substitution Competition:

  • $3^o$ halides favour elimination; $1^o$ favour substitution.

  • Bulkier nucleophiles/bases favour elimination.

  • High temperature and less polar solvents favour elimination.

Reactions with Active Metals

• Grignard Reagent: $R - X + Mg \xrightarrow{\text{dry ether}} R - Mg - X$. Bond is polar covalent ($R^{\delta-} - Mg^{\delta+}$). Highly reactive with moisture; reacts with $H_2O$, $CH_3OH$, or $NH_3$ to form hydrocarbons ($R-H$).

• Wurtz Reaction: $2R - X + 2Na \xrightarrow{\text{dry ether}} R - R + 2NaX$. Forms higher alkanes with double the carbons.

• Wurtz-Fittig Reaction: Reaction of aryl halide, alkyl halide, and sodium to give substituted aromatic compounds.

• Fittig Reaction: Reaction of two aryl halides with sodium to give biphenyl.

Reactivity and Reactions of Haloarenes

• Low Reactivity towards $S_N$:

  1. Resonance Effect: Lone pair on halogen conjugates with ring, giving $C-X$ bond partial double bond character (shorter: $169\,pm$ vs $178\,pm$), making it harder to break.

  2. Hybridization: $sp^2$ carbon is more electronegative than $sp^3$, holding halogen tighter.

  3. Instability of Phenyl Cation: Rules out $S_N1$.

  4. Steric Hindrance: Aromatic ring blocks backside attack, ruling out $S_N2$.

• Nucleophilic Substitution (Aryl): Requires extreme conditions ($623\,K$, $300\,atm$ for chlorobenzene to phenol). Activated by electron-withdrawing groups ($-NO_2$) at ortho/para positions.

  • $p$-nitrochlorobenzene $\xrightarrow{433\,K}$ $p-nitrophenol.\n * 2,4-dinitrochlorobenzene \xrightarrow{403\,K} 2,4-dinitrophenol.\n * 2,4,6-trinitrochlorobenzene \xrightarrow{\text{warm } H_2O} 2,4,6-trinitrophenol (Picric acid).\n* **Electrophilic Substitution (S_E$):** Halogens are$o/p$directing but deactivating (due to$-I$effect being stronger than$+R effect).\n 1. **Halogenation:** X_2$+ Lewis acid$\rightarrow 1,4-dichlorobenzene (major) and 1,2-dichlorobenzene (minor).\n 2. **Nitration:** Conc. HNO_3 + H_2SO_4 \rightarrow 1-chloro-4-nitrobenzene (major).\n 3. **Sulfonation:** Fuming H_2SO_4 \rightarrow 4-chlorobenzene sulfonic acid (major).\n 4. **Friedel-Crafts Alkylation:** CH_3Cl + AlCl_3 \rightarrow 4-chlorotoluene (major).\n 5. **Friedel-Crafts Acylation:** CH_3COCl + AlCl_3 \rightarrow 4-chloroacetophenone (major).\n\n# Uses and Environmental Effects of Polyhalogen Compounds\n\n* **Dichloromethane (CH_2Cl_2):** Solvent, propellant in aerosols. Exposure: dizziness, nausea, eye damage.\n* **Chloroform (CHCl_3$):** Solvent for gums/fats; prepares refrigerant R-22. Forms poisonous **phosgene** ($COCl_2) when exposed to air/light. Toxic to central nervous system and liver.\n* **Carbon Tetrachloride (CCl_4):** Cleaning agent, solvent. Carcinogenic, causes liver damage, green house gas.\n* **Iodoform (CHI_3):** Antiseptic/healing agent for wounds. Disagreeable smell, causes skin/eye irritation.\n* **Freons (CFCs):** Refrigerants (e.g., Freon-12/CCl_2F_2$). Responsible for ozone layer depletion in the stratosphere. One chlorine atom can destroy$100,000 ozone molecules.\n* **DDT (Dichlorodiphenyltrichloroethane):** First chlorinated organic insecticide. Discovered by Paul Muller (1948$Nobel Prize). Persistent organic pollutant, accumulates in fat, toxic to fish, banned in the US in$1973 due to persistence/health risks.\n\n# Questions & Discussion\n\n* **Q:** Why is phosphoric acid preferred to H_2SO_4$to prepare$HI in situ?\n* **A:** Sulfuric acid is a strong oxidizing agent and would oxidize the produced HI$back to$I_2. $H_3PO_4$ is non-oxidizing.\n* **Q:** Does the functional group 'halogen' appear as a suffix or prefix in IUPAC?\n* **A:** Prefix (e.g., Chloromethane).\n* **Q:** What are trivial names for CHCl_3$and$CCl_4?\n* **A:** Chloroform and Carbon tetrachloride.\n* **Q:** Why does chlorobenzene to phenol require high temp (623\,K$) and pressure ($300\,atm)?\n* **A:** Due to low reactivity of aryl halides caused by resonance (partial double bond character) and sp^2$$ hybridization.