Lecture 8

Halogeno-compounds

1. Introduction

2. Nomenclature of Halogeno-compounds

3. Physical Properties of Halogeno-compounds

4. Preparation of Halogeno-compounds

5. Reactions of Halogeno-compounds

6. Nucleophilic Substitution Reactions

7. Elimination Reactions

8. Uses of Halogeno-compounds

1 Introduction

• Haloalkanes are organic compounds having one or more halogen atoms replacing hydrogen atoms in alkanes

• Haloalkanes are classified into primary, secondary and tertiary, based on the number of alkyl groups attached to the carbon atom which is bonded to the halogen atom

Halobenzenes are organic compounds in which the halogen atom is directly attached to a benzene ring e.g.

(Cloromethyl)benzene is not a halobenzene, because the chlorine atom is not directly attached to the benzene ring

2 Nomenclature of Halogeno-compounds

• Naming haloalkanes are similar to those for naming alkanes

• The halogens are written as prefixes: fluoro- (F), chloro- (Cl), bromo- (Br) and iodo- (I)

e.g.

When the parent chain has both a halogen and an alkyl substituent, the chain is numbered from the end nearer the first substituent regardless of what substituents are

e.g.

In case of halobenzenes, the benzene ring is numbered so as to give the lowest possible numbers to the substituents

e.g.

3 Physical Properties of Halogeno-compounds

Boiling Point and Melting Point

Haloalkanes have higher b.p. and m.p. than alkanes. Dipole-dipole interactions are present between haloalkane molecules

m.p. and b.p. increase in the order: RCH₂F < RCH₂Cl < RCH₂Br < RCH₂I Larger, more polarizable halogen atoms increase the dipole-dipole interactions between the molecules

No. of carbon ­ Þ m.p. and b.p. ­

Density

• Relative molecular mass ­ Þ density ¯. Closer packing of the smaller molecules in the liquid phase

• Bromo and iodoalkanes are all denser than water at 20°C

Solubility

Although C — X bond is polar, it is not polar enough to have a significant effect on the solubility of haloalkanes and halobenzenes Þ Immiscible with water Þ Soluble in organic solvents

4 Preparation of Halogeno-compounds

Preparation of Haloalkanes

Substitution of Alcohols

• Prepared by substituting –OH group of alcohols with halogen atoms

• Common reagents used: HCl, HBr, HI, PCl₃ or PBr₃

• The ease of substitution of alcohols: 3° alcohol > 2° alcohol > 1° alcohol > CH₃OH

• This is related to the stability of the reaction intermediate (i.e. stability of carbocations)

Reaction with Hydrogen Halides

• Dry HCl is bubbled through alcohols in the presence of ZnCl₂ catalyst

For the preparation of bromo- and iodoalkanes, no catalyst is required

The reactivity of hydrogen halides: HI > HBr > HCl

e.g.

Reaction with Phosphorus Halides

Haloalkanes can be prepared from the vigorous reaction between cold alcohols and phosphorus(III) halides

Addition of Alkenes and Alkynes

Addition of halogens or hydrogen halides to an alkene or alkyne can form a haloalkane

e.g.

Preparation of Halobenzenes

Halogenation of Benzene

Benzene reacts readily with chlorine and bromine in the presence of catalysts (e.g. FeCl₃, FeBr₃, AlCl₃)

From Benzenediazonium Salts

5 Reactions of Halogeno-compounds

• Carbon-halogen bond is polar

• Carbon atom bears a partial positive charge

• Halogen atom bears a partial negative charge

Characteristic reaction: Nucleophilic substitution reaction

Alcohols, ethers, esters, nitriles and amines can be formed by substituting – OH, – OR, RCOO –, – CN and – NH₂ groups respectively

Another characteristic reaction: Elimination reaction

• Bases and nucleophiles are the same kind of reagents

• Nucleophilic substitution and elimination reactions always occur together and compete each other