JEE Chemistry Haloalkanes and Haloarenes Complete Guide

Haloalkanes and haloarenes (also called alkyl halides and aryl halides respectively) are an important part of organic chemistry in the JEE syllabus. These compounds contain halogen atoms (F, Cl, Br, I) bonded to sp³ carbon atoms in haloalkanes and sp² carbon atoms in haloarenes. Understanding their preparation, properties, and reaction mechanisms is essential to score well in both JEE Mains and Advanced.

1. Classification and Nomenclature

1.1 Classification of Haloalkanes

Haloalkanes are classified based on the carbon atom to which the halogen is attached:

• Primary (1°) haloalkanes: Halogen attached to a primary carbon (bonded to one other carbon).
• Secondary (2°) haloalkanes: Halogen attached to a secondary carbon (bonded to two other carbons).
• Tertiary (3°) haloalkanes: Halogen attached to a tertiary carbon (bonded to three other carbons).

1.2 Classification of Haloarenes

Haloarenes are aromatic compounds with halogen atoms directly attached to the benzene ring or other aromatic systems.

1.3 Nomenclature

The IUPAC naming of haloalkanes and haloarenes involves naming the parent hydrocarbon and prefixing the halogen substituents as fluoro-, chloro-, bromo-, iodo- etc., with appropriate locants.

Example: \( \mathrm{CH_3CH_2Cl} \) is named as chloroethane.

2. Preparation of Haloalkanes

2.1 From Alkanes (Free Radical Halogenation)

Haloalkanes can be prepared by free radical substitution of alkanes with halogens (Cl₂, Br₂) under UV light.

\[ \mathrm{CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl} \]

This method gives a mixture of products and is less selective.

2.2 From Alcohols

• Using HX (Hydrogen Halides): Primary, secondary, and tertiary alcohols react with HX to form haloalkanes.

\[ \mathrm{R-OH + HX \rightarrow R-X + H_2O} \]

• Using PCl₃, PCl₅, or SOCl₂: Alcohols are converted to alkyl chlorides with these reagents.

2.3 From Alkenes (Electrophilic Addition)

Addition of HX to alkenes follows Markovnikov's rule, giving haloalkanes.

\[ \mathrm{CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br} \]

3. Preparation of Haloarenes

3.1 Direct Halogenation

Aromatic rings react with halogens in presence of a Lewis acid catalyst (FeX₃, AlX₃) to form haloarenes.

\[ \mathrm{C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr} \]

3.2 From Diazonium Salts (Sandmeyer Reaction)

Aromatic amines (anilines) are converted to diazonium salts, which react with cuprous halides to yield haloarenes.

\[ \mathrm{C_6H_5N_2^+Cl^- + CuCl \rightarrow C_6H_5Cl + N_2} \]

4. Physical Properties

Haloalkanes and haloarenes are generally colorless or pale liquids with characteristic odors.

• They are denser than water due to heavy halogen atoms.
• Boiling points increase with molecular weight and polarity.
• Solubility in water is low but soluble in organic solvents like alcohol, ether.

5. Chemical Properties

5.1 Nucleophilic Substitution Reactions

Haloalkanes undergo nucleophilic substitution reactions where the halogen is replaced by a nucleophile. There are two main mechanisms:

S_N2 Mechanism

• Occurs mainly with primary haloalkanes.
• One-step reaction with backside attack and inversion of configuration (Walden inversion).
• Rate depends on both substrate and nucleophile concentration.
• Example:

\[ \mathrm{CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-} \]

S_N1 Mechanism

• Occurs mainly with tertiary haloalkanes.
• Two-step reaction involving carbocation intermediate.
• Rate depends only on substrate concentration.
• Can lead to racemization due to planar carbocation.

5.2 Elimination Reactions (dehydrohalogenation)

Haloalkanes undergo elimination in the presence of strong bases to form alkenes.

\[ \mathrm{CH_3CH_2Br + KOH \xrightarrow{alc} CH_2=CH_2 + KBr + H_2O} \]

This is an E2 mechanism typically with secondary or tertiary haloalkanes.

5.3 Reactions of Haloarenes

Haloarenes are less reactive in nucleophilic substitution due to resonance stabilization of the aromatic ring.

Electrophilic Substitution

Haloarenes undergo electrophilic substitution reactions where the halogen atom directs new substituents mostly to ortho and para positions.

Nucleophilic Aromatic Substitution

Occurs under harsh conditions or if the ring has electron-withdrawing groups (like NO₂).

6. Mechanisms of Important Reactions

6.1 S_N2 Reaction Mechanism

The nucleophile attacks the electrophilic carbon opposite to the leaving group causing simultaneous displacement. This results in inversion of stereochemistry.

6.2 S_N1 Reaction Mechanism

First step is slow formation of carbocation intermediate by loss of halide ion, followed by rapid nucleophilic attack.

6.3 Elimination (E2) Mechanism

Base abstracts a β-hydrogen while the halide leaves, forming a double bond in a single concerted step.

7. Uses and Applications

• Haloalkanes are used as solvents, refrigerants (like Freons), and intermediates in organic synthesis.
• Haloarenes are important in the manufacture of dyes, pharmaceuticals, and agrochemicals.
• Chloroform and carbon tetrachloride are well-known haloalkanes used as solvents.

8. Important Tips for JEE

• Know the difference between haloalkanes and haloarenes in terms of reactivity.
• Memorize major preparation methods and conditions.
• Understand S_N1 and S_N2 mechanisms thoroughly, including stereochemistry.
• Practice mechanism-based questions and reaction prediction.
• Be aware of exceptions and special cases like benzylic and allylic halides.