Monday, January 21, 2008


General methods of preparation, properties and reactions; Nature of C-X bond; Mechanisms of substitution reactions.
Uses; Environmental effects of chloroform, iodoform, freons and DDT.

When hydrogen atom or atoms of alkanes are replaced by the corresponding number of halogen atoms, the compounds are called halogen derivatives of alkanes.

They are classified according to the number of halogen atoms that replace hydrogen atoms in the alkane.

Monohalogen derivatives: They contain only one halogen atom.

e.g. CH-3Cl Methyl chloride
CH-3-CH-CH-3 2-Bromopropane
Monohalogen derivatives of alkane are called alkyl halides

Dihalogen alkanes contain two halogen atoms.
Trihalog alkanes contain three halogen atoms.

Monohalo alkanes

The general formula is RX where is a alkyl group and X is a halogen.

Classification of Haloalkanes

A. Type of halogen atoms; Fluorides, chlorides, bromides, iodides
B. Number of halogena atoms, monohalo, dihalo, trihalo, tetra halo.

Classification of Haloalkanes
1. Compounds containing sp3 hybridization
2. Compounds containing sp2 hybridization

Compounds containing sp3 hybridization
a) halo alkances or alkyl halides
(i)Primary halogenoalkanes

In a primary (1°) halogenoalkane, the carbon which carries the halogen atom is only attached to one other alkyl group.

Secondary halogenoalkanes

In a secondary (2°) halogenoalkane, the carbon with the halogen attached is joined directly to two other alkyl groups, which may be the same or different.

Tertiary halogenoalkanes

In a tertiary (3°) halogenoalkane, the carbon atom holding the halogen is attached directly to three alkyl groups, which may be any combination of same or different.

b) Allylic halides: Halogens attached to alkenes to a carbon atom next to carbon-carbon double bond.

c) Benzylic halide (aralalkyl halide): Halogens attached to alkenes to a carbon atom next to to an aromatic ring (not to a carbon atom in the aromatic ring). It is attached to a carbon atom which is inturn attached to a carbon atom in the aromatic ring.

2. Compounds containing sp2 hybridization

a) Vinylic halides: Halogens attached to alkenes to a carbon atom of one of the carbon atoms of a double bond.

B) Aryl halides: Halogens attached to alkenes to a carbon atom of an aromatic ring

Methods of preparation

1. From hydrocarbons
a) from alkanes: halogens react with alkanes in the presence of uv light to form haloalkanes.
b) from alkenes: by the electrophylic addition of halogen acids (HBr, HCl, or HI)

Markownikov rule and anti Markownikov rule are applicable in the reaction between alkene and halogen acids.

2. From alcohols: These reactions are covered in alcohol chapter also.

Reaction with phosphorus halides

Phosphorus halides such as PCl5, Pcl3, PBr3 and PI3 react with alcohols to form corresponding haloalkanes.

Haloalkanes : Chloroethane, Bromoethane, Iodoethane

Reaction with ZnCl2/conc.-HCl

This is a reaction or test to distinguish various categories of alcohols and is termed Lucas test.

In this test, an alcohol is treated with an equimolar mixture of concentrated hydrochloric acid and anhydrous ZnCl2 (called Lucas reagent).

Alcohols get converted into alkylhalides. As alkyl halides are insoluble in water, their presence is indicated by the appearance of turbidity in the reaction mixture.

Physical properties of halogenoalkanes

Boiling Points

the only methyl halide which is a liquid is iodomethane;

chloroethane is a gas.

The examples show that the boiling points fall as the isomers go from a primary to a secondary to a tertiary halogenoalkane. This is a simple result of the fall in the effectiveness of the dispersion forces.

Solubility in water

The halogenoalkanes are at best only very slightly soluble in water.

In order for a halogenoalkane to dissolve in water you have to break attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and break the hydrogen bonds between water molecules. Both of these cost energy.

Solubility in organic solvents

Halogenoalkanes tend to dissolve in organic solvents because the new intermolecular attractions have much the same strength as the ones being broken in the separate halogenoalkane and solvent.

bond strength falls as you go from C-F to C-I, and notice how much stronger the carbon-fluorine bond is than the rest.

In order for anything to react with the halogenoalkanes, the carbon-halogen bond has got to be broken. Because that gets easier as you go from fluoride to chloride to bromide to iodide, the compounds get more reactive in that order.

Iodoalkanes are the most reactive and fluoroalkanes are the least. In fact, fluoroalkanes are so unreactive that we shall pretty well ignore them completely in discussion on reactions.

Nucleophilic substitution in primary halogenoalkanes


A nucleophile is a species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else.

Nucleophiles are either fully negative ions, or else have a strongly - charge somewhere on a molecule. Common nucleophiles are hydroxide ions, cyanide ions, water and ammonia.

The nucleophilic substitution reaction - an SN2 reaction

Take bromoethane as a typical primary halogenoalkane. The bromoethane has a polar bond between the carbon and the bromine.The general purpose nucleophilic ion is referred to as Nu-. This will have at least one lone pair of electrons. Nu- could, for example, be OH- or CN-.

The lone pair on the Nu- ion will be strongly attracted to the + carbon, and will move towards it, beginning to make a co-ordinate (dative covalent) bond. In the process the electrons in the C-Br bond will be pushed even closer towards the bromine, making it increasingly negative.
The movement goes on until the -Nu is firmly attached to the carbon, and the bromine has been expelled as a Br- ion.

There is obviously a point in the reaction in which the Nu- is half attached to the carbon, and the C-Br bond is half way to being broken. This is called a transition state. It isn't an intermediate. You can't isolate it - even for a very short time. It's just the mid-point of a smooth attack by one group and the departure of another.

Technically, this is known as an SN2 reaction. S stands for substitution, N for nucleophilic, and the 2 is because the initial stage of the reaction involves two species - the bromoethane and the Nu- ion.

Nucleophilic substitution in tertiary halogenoalkanes

A tertiary halogenoalkane has three alkyl groups attached to the carbon with the halogen on it. These alkyl groups can be the same or different. The example uses a simple compound, (CH3)3CBr - 2-bromo-2-methylpropane.

The nucleophilic substitution reaction - an SN1 reaction (1 denotes 1st order)

The general purpose nucleophilic ion is termed as Nu-. This will have at least one lone pair of electrons.

The reaction happens in two stages. In the first, a small proportion of the halogenoalkane ionises to give a carbocation and a bromide ion.

Once the carbocation is formed, however, it would react immediately it came into contact with a nucleophile like Nu-. The lone pair on the nucleophile is strongly attracted towards the positive carbon, and moves towards it to create a new bond.

How fast the reaction happens is going to be governed by how fast the halogenoalkane ionises. Because this initial slow step only involves one species, the mechanism is described as SN1 - substitution, nucleophilic, one species taking part in the initial slow step.

Nucleophilic substitution reactions

1. substitution by hydroxylgroup (OH) leads to the formation of alcohols
2. substitution by alkoxy group leads to the formation of ether.
3.substitution by cyano group leads to the formation of cyanides or nitriles.

4. Substitution by isocyanide group leads to the formation of isocyanides.
When haloalkane is treated with alcoholic silver cyanide (AgCN), isocynaides (R-N≡C) are obtained. They are also called carbyl amines.

RX + AgCN ---> RNC + AGX

C2H5Br + AgCN ---> C2H5NC + Ag Br

5. substitution by amino group leads to the formation of amines.
6. substitution by nitrite group leads to the formation of nitrite.
7. substitution by nitro group leads to the formation of nitro alkanes.
8. substitution by carboxyl group leads to the formation of esters.
9. substitution by hydrosulphide group leads to the formation of thioalcohols.
10. substitution by mercaptide group leads to the formation of thioethers.
11. substitution by alkyl group leads to the formation of alkynes.

Dibromoethane can refer to either of two isomeric organobromides with the molecuar formula C2H4Br2:

1,1-Dibromoethane (ethylidene dibromide)
1,2-Dibromoethane (ethylene dibromide)

For IIT JEE material on the same topic

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