Haloalkanes and Haloarenes

Complete Notes for NEET & JEE Preparation

Unit Overview

Haloalkanes and haloarenes are hydrocarbons in which one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, I). These compounds have wide applications in industry, medicine, and daily life.

Learning Objectives

Name haloalkanes and haloarenes according to IUPAC system
Understand methods of preparation of haloalkanes and haloarenes
Study various reactions of haloalkanes and haloarenes
Correlate structures with reaction mechanisms
Understand stereochemistry in reaction mechanisms
Learn about organometallic compounds and their applications
Study environmental effects of polyhalogen compounds

1. Classification

1.1 Based on Number of Halogen Atoms

Monohalogen compounds: Contain one halogen atom
Dihalogen compounds: Contain two halogen atoms
Polyhalogen compounds: Contain three or more halogen atoms

1.2 Based on Hybridization

Compounds with sp³ C-X Bond

Alkyl halides (R-X): Halogen attached to alkyl group
- Primary (1°): Halogen attached to primary carbon
- Secondary (2°): Halogen attached to secondary carbon
- Tertiary (3°): Halogen attached to tertiary carbon
Allylic halides: Halogen attached to sp³ carbon adjacent to C=C
Benzylic halides: Halogen attached to sp³ carbon attached to aromatic ring

Compounds with sp² C-X Bond

Vinylic halides: Halogen attached to sp² carbon of C=C
Aryl halides: Halogen directly attached to aromatic ring

NEET/JEE Focus: Remember the classification based on hybridization as it determines reactivity. Alkyl halides are most reactive in nucleophilic substitution.

2. Nomenclature

2.1 IUPAC Naming Rules

Name as halosubstituted hydrocarbons
Number the chain to give the halogen the lowest possible number
Prefix with fluoro-, chloro-, bromo-, iodo-
For dihalides: use prefixes di-, tri-, etc.

Compound	Common Name	IUPAC Name
CH₃CH₂CH₂Br	n-Propyl bromide	1-Bromopropane
(CH₃)₂CHCl	Isopropyl chloride	2-Chloropropane
CH₂=CHCl	Vinyl chloride	Chloroethene
C₆H₅Br	Bromobenzene	Bromobenzene
CH₂Cl₂	Methylene chloride	Dichloromethane
CHCl₃	Chloroform	Trichloromethane

2.2 Special Cases

Geminal dihalides: Both halogens on same carbon atom (alkylidene halides)
Vicinal dihalides: Halogens on adjacent carbon atoms (alkylene dihalides)

Example: CH₃CHCl₂ is ethylidene chloride (common) or 1,1-dichloroethane (IUPAC)

CH₂ClCH₂Cl is ethylene dichloride (common) or 1,2-dichloroethane (IUPAC)

3. Nature of C-X Bond

The carbon-halogen bond is polar due to electronegativity difference:

C^δ+ — X^δ-

Bond	Bond Length (pm)	Bond Enthalpy (kJ/mol)	Dipole Moment (Debye)
CH₃-F	139	452	1.847
CH₃-Cl	178	351	1.860
CH₃-Br	193	293	1.830
CH₃-I	214	234	1.636

Key Points:

Bond length increases: C-F < C-Cl < C-Br < C-I
Bond strength decreases: C-F > C-Cl > C-Br > C-I
Reactivity order: RI > RBr > RCl > RF

4. Methods of Preparation

4.1 From Alcohols

R-OH + HX → R-X + H₂O (with ZnCl₂ for 1° & 2° alcohols)

3R-OH + PX₃ → 3R-X + H₃PO₃ (X = Cl, Br)

R-OH + PCl₅ → R-Cl + POCl₃ + HCl

R-OH + SOCl₂ → R-Cl + SO₂ + HCl (Best method - pure product)

Important: Thionyl chloride (SOCl₂) is preferred for preparing alkyl chlorides as byproducts are gases that escape.

4.2 From Hydrocarbons

From Alkanes

Free radical halogenation: R-H + X₂ → R-X + HX (with heat/UV light)

From Alkenes

Addition of HX: CH₃-CH=CH₂ + HBr → CH₃-CH₂-CH₂Br (minor) + CH₃-CHBr-CH₃ (major)

Markovnikov's rule: H adds to carbon with more H atoms

Anti-Markovnikov with peroxide: CH₃-CH=CH₂ + HBr → CH₃-CH₂-CH₂Br (major)

Addition of X₂: CH₂=CH₂ + Br₂ → BrCH₂-CH₂Br (vicinal dihalide)

4.3 Halogen Exchange

Finkelstein reaction: R-X + NaI → R-I + NaX (X = Cl, Br; in dry acetone)

Swarts reaction: R-X + MF → R-F + MX (M = Ag, Hg, etc.)

4.4 Preparation of Haloarenes

From Hydrocarbons

Electrophilic substitution: C₆H₆ + Cl₂ → C₆H₅Cl + HCl (with Fe/FeCl₃)

From Amines

Sandmeyer reaction: ArN₂⁺ + CuX → Ar-X + N₂ (X = Cl, Br)

ArN₂⁺ + KI → Ar-I + N₂ (no catalyst needed)

5. Physical Properties

5.1 Boiling Points

Higher than corresponding hydrocarbons due to polarity
For same alkyl group: RI > RBr > RCl > RF
Decrease with branching in isomeric haloalkanes
For dihalobenzenes: para-isomers have higher melting points

5.2 Density

Bromo, iodo, and polychloro derivatives are heavier than water
Density increases with carbon atoms, halogen atoms, and atomic mass of halogen

5.3 Solubility

Slightly soluble in water due to inability to form H-bonds
Completely soluble in organic solvents

6. Chemical Reactions of Haloalkanes

6.1 Nucleophilic Substitution Reactions

Reagent	Nucleophile	Product
NaOH/KOH	OH⁻	Alcohol
H₂O	H₂O	Alcohol
NaOR'	RO⁻	Ether
NaI	I⁻	Alkyl iodide
NH₃	NH₃	Primary amine
KCN	CN⁻	Alkyl cyanide
AgCN	Ag-CN	Alkyl isocyanide
AgNO₂	Ag-O-N=O	Nitroalkane
KNO₂	O=N-O⁻	Alkyl nitrite

Ambident Nucleophiles: CN⁻ and NO₂⁻ have two nucleophilic centers and can form different products with different reagents.

6.2 Reaction Mechanisms

S_N2 Mechanism (Bimolecular)

Single step mechanism
Rate = k[RX][Nu]
Inversion of configuration (Walden inversion)
Reactivity order: 1° > 2° > 3°
Preferred for primary halides

S_N1 Mechanism (Unimolecular)

Two-step mechanism: Ionization → Nucleophilic attack
Rate = k[RX]
Racemization (formation of both enantiomers)
Reactivity order: 3° > 2° > 1°
Preferred for tertiary halides

NEET/JEE Focus: S_N2 gives inversion, S_N1 gives racemization. Remember the factors affecting these mechanisms.

6.3 Elimination Reactions

R-CH₂-CH₂-X + KOH(alc) → R-CH=CH₂ + KX + H₂O

Saytzeff Rule: In dehydrohalogenation, the preferred product is the alkene with more alkyl groups on doubly bonded carbons.

6.4 Reaction with Metals

Grignard Reagents

R-X + Mg → R-MgX (in dry ether)

R-MgX + H₂O → R-H + Mg(OH)X

Wurtz Reaction

2R-X + 2Na → R-R + 2NaX (in dry ether)

7. Chemical Reactions of Haloarenes

7.1 Nucleophilic Substitution

Haloarenes are less reactive towards nucleophilic substitution due to:

Resonance effect: C-X bond acquires partial double bond character
sp² hybridized carbon: More electronegative, holds electron pair tightly
Instability of phenyl cation
Repulsion between electron-rich nucleophile and arene

Chlorobenzene → Phenol: C₆H₅Cl + NaOH → C₆H₅OH + NaCl (623K, 300 atm)

Activating Effect of -NO₂: Presence of -NO₂ at ortho and para positions increases reactivity of haloarenes towards nucleophilic substitution.

7.2 Electrophilic Substitution

Halogen is ortho-para directing but deactivating:

Halogenation: C₆H₅Cl + Cl₂ → o-C₆H₄Cl₂ + p-C₆H₄Cl₂ (with FeCl₃)

Nitration: C₆H₅Cl + HNO₃ → o-NO₂C₆H₄Cl + p-NO₂C₆H₄Cl

Sulphonation: C₆H₅Cl + H₂SO₄ → o-ClC₆H₄SO₃H + p-ClC₆H₄SO₃H

Friedel-Crafts: C₆H₅Cl + CH₃Cl → o-CH₃C₆H₄Cl + p-CH₃C₆H₄Cl (with AlCl₃)

7.3 Reaction with Metals

Wurtz-Fittig Reaction

Ar-X + R-X + 2Na → Ar-R + 2NaX (in dry ether)

Fittig Reaction

2Ar-X + 2Na → Ar-Ar + 2NaX (in dry ether)

8. Polyhalogen Compounds

Compound	Formula	Uses	Harmful Effects
Dichloromethane	CH₂Cl₂	Solvent, paint remover, aerosol propellant	Central nervous system damage
Chloroform	CHCl₃	Solvent, refrigerant production	Liver and kidney damage, forms phosgene
Carbon tetrachloride	CCl₄	Refrigerant, solvent, fire extinguisher	Liver cancer, nerve damage, ozone depletion
Freon-12	CCl₂F₂	Refrigerant, aerosol propellant	Ozone layer depletion
DDT	C₁₄H₉Cl₅	Insecticide	Environmental persistence, toxicity to fish
Iodoform	CHI₃	Antiseptic (formerly)	Objectionable smell

Practice Questions (NEET/JEE Level)

1. Which of the following undergoes S_N1 reaction faster and why?
(CH₃)₃CBr or CH₃CH₂CH₂Br

2. Arrange in order of increasing boiling points:
Bromomethane, Bromoform, Chloromethane, Dibromomethane

3. Why is chlorobenzene difficult to hydrolyze?

4. What happens when n-butyl chloride is treated with alcoholic KOH?

5. Explain why Grignard reagents should be prepared under anhydrous conditions.

6. Why does p-dichlorobenzene have higher melting point than o- and m-isomers?

7. Write the mechanism of S_N2 reaction.

8. What are ambident nucleophiles? Give examples.

Quick Revision Points

Reactivity order: RI > RBr > RCl > RF
S_N2: Inversion, 1° > 2° > 3°, bimolecular
S_N1: Racemization, 3° > 2° > 1°, unimolecular
Haloarenes: Less reactive towards nucleophilic substitution
Halogen: Ortho-para directing but deactivating
Grignard reagent: R-MgX, prepared in dry ether
Saytzeff rule: More substituted alkene is major product
Ambident nucleophiles: CN⁻, NO₂⁻ (two nucleophilic sites)
Best method for R-Cl: SOCl₂ (byproducts are gases)
Finkelstein reaction: R-X → R-I using NaI in dry acetone