Comprehensive NEET Chemistry Notes for Organic Compounds Containing Halogens

Organic Compounds Containing Halogens (Haloalkanes and Haloarenes)

1. Introduction

The replacement of hydrogen atom(s) in an aliphatic or aromatic hydrocarbon by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively. Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group whereas haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group.

Did You Know?

Chlorine containing antibiotic, chloramphenicol, is produced by microorganisms and is effective for treating typhoid fever.

2. Classification of Haloalkanes and Haloarenes

2.1 On the Basis of Number of Halogen Atoms

• Monohalogen Compounds: Contain one halogen atom.
• Dihalogen Compounds: Contain two halogen atoms.
• Polyhalogen Compounds: Contain three or more halogen atoms.

Example:

Methyl chloride (CH3Cl), Dichloromethane (CH2Cl2), Chloroform (CHCl3), and Carbon tetrachloride (CCl4).

2.2 Compounds Containing sp3 C—X Bond

• Alkyl Halides (R—X): Classified as primary, secondary, or tertiary based on the nature of the carbon atom to which the halogen is attached.
• Allylic Halides: Halogen attached to an sp3-hybridised carbon atom adjacent to a carbon-carbon double bond (C=C).
• Benzylic Halides: Halogen attached to an sp3-hybridised carbon atom attached to an aromatic ring.

Example:

1-Bromopentane (primary alkyl halide), 2-Bromopentane (secondary alkyl halide), 2-Bromo-2-methylpropane (tertiary alkyl halide).

2.3 Compounds Containing sp2 C—X Bond

• Vinylic Halides: Halogen bonded to an sp2-hybridised carbon atom of a carbon-carbon double bond (C=C).
• Aryl Halides: Halogen bonded to the sp2-hybridised carbon atom of an aromatic ring.

NEET Tip:

For dihalogen derivatives of benzene, common prefixes (o-, m-, p-) are used in the common system, whereas numerals (1,2; 1,3; 1,4) are used in the IUPAC system.

3. Nomenclature

• Common names of alkyl halides are derived by naming the alkyl group followed by the name of halide.
• IUPAC system names alkyl halides as halosubstituted hydrocarbons.
• Dihalogen derivatives are named as alkylidene or alkylene dihalides.

Example:

sec-Butyl chloride (2-Chlorobutane), neo-Pentyl bromide (1-Bromo-2,2-dimethylpropane), Vinyl chloride (Chloroethene).

4. Nature of C—X Bond

• Polarity: Carbon-halogen bond is polar due to the electronegativity difference.
• Bond Length and Strength: Bond length increases and bond strength decreases from C—F to C—I.

Table: Carbon-Halogen (C—X) Bond Lengths, Bond Enthalpies and Dipole Moments

Common Misconception:

A single sharp melting point does not guarantee absolute purity; some impurities may not affect the melting point.

5. Preparation of Haloalkanes and Haloarenes

5.1 From Alcohols

• Reaction with concentrated halogen acids, phosphorus halides, or thionyl chloride.
• Thionyl chloride is preferred due to byproducts being gases (SO2 and HCl).

Example:

$R-OH+HCl\to R-Cl+H_{2}O$

5.2 From Hydrocarbons

• Free Radical Halogenation: Gives a mixture of isomeric mono- and polyhaloalkanes.
• Electrophilic Substitution: For aryl halides, electrophilic substitution with chlorine or bromine in the presence of Lewis acid catalysts.

Example:

$R-H+C{l}_2\to R-Cl+HCl$

5.3 Halogen Exchange

• Finkelstein reaction: Alkyl iodides from alkyl chlorides/bromides with NaI in dry acetone.
• Swarts reaction: Alkyl fluorides from alkyl chlorides/bromides with metallic fluorides.

Example:

$R-Cl+NaI\to R-I+NaCl$

6. Physical Properties

• State: Lower alkyl halides are gases, higher ones are liquids or solids.
• Boiling Points: Higher than corresponding hydrocarbons due to stronger intermolecular forces.
• Solubility: Slightly soluble in water, completely soluble in organic solvents.

NEET Tip:

Boiling points of alkyl halides decrease with increasing branching.

Table: Density of Some Haloalkanes

7. Chemical Reactions

7.1 Nucleophilic Substitution Reactions (SN1 and SN2)

• SN1 Mechanism: Follows first-order kinetics, forms carbocation intermediate.
• SN2 Mechanism: Follows second-order kinetics, involves simultaneous bond making and breaking.

Example:

$R-X+N{u}^{-}\to R-Nu+X^{-}$

Reaction Mechanism:

1. SN1 Reaction: Two steps, formation of carbocation.
2. SN2 Reaction: Single step, inversion of configuration.

Mnemonic:

"SN1 is slow and steps twice, SN2 is swift and steps once."

8. Applications of Haloalkanes and Haloarenes

• Pharmaceuticals: Chloramphenicol for typhoid, Chloroquine for malaria.
• Industrial Use: Solvents, refrigerants (Freon), propellants in aerosols.
• Environmental Impact: Persistent in environment, potential hazards like ozone depletion.

Real-life Application:

Freon used in refrigeration and air conditioning.

Quick Recap

• Purification Techniques: Sublimation, Crystallization, Distillation, Differential Extraction, Chromatography.
• Characterization Techniques: Melting Point, Boiling Point, IR Spectroscopy, NMR Spectroscopy, Mass Spectrometry.
• Chemical Reactions: Nucleophilic Substitution, Elimination, Reactions with Metals.

Practice Questions

1. Explain the mechanism of the SN1 reaction using tert-butyl bromide and hydroxide ion.
2. How does the boiling point of haloalkanes change with the type of halogen and branching?
3. Describe the Finkelstein reaction with an example.
4. Why are aryl halides less reactive towards nucleophilic substitution compared to alkyl halides?
5. Write the structure and IUPAC name of 2-Bromo-2-methylpropane.

Answers to Practice Questions

1. SN1 Mechanism: Formation of carbocation intermediate, followed by nucleophilic attack.
2. Boiling Point: Increases with size and mass of halogen, decreases with branching.
3. Finkelstein Reaction: Halogen exchange, e.g., $R-Cl+NaI\to R-I+NaCl$.
4. Reactivity of Aryl Halides: Due to resonance stabilization and sp2 hybridization of carbon.
5. Structure and IUPAC Name: $(C{H}_3{)}_{3}CBr$, 2-Bromo-2-methylpropane.

Glossary

• Sublimation: Direct transition from solid to vapor phase.
• Crystallization: Purification based on solubility differences.
• Distillation: Separation based on boiling point differences.
• Chromatography: Separation based on differential affinities.
• IR Spectroscopy: Identification of functional groups.
• NMR Spectroscopy: Structural analysis based on magnetic environment.
• Mass Spectrometry: Determination of molecular mass and structure.