Derivatives of Haloalkanes and Haloarenes - Dichloroethanes

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Polyhalogen derivatives of alkanes are the halogen derivatives of alkanes containing more than one halogen atom per molecule. For example, ethylene dichloride, ethylidene chloride, chloroform, iodoform, carbon tetrachloride, etc.

Dichloroethanes

Ethane has two dichloro derivatives:

• If two halogen atoms are attached to adjacent carbons, the compound is called vicinal (vic-) dihalide. For example, 1,2-dichloroethane or ethylene chloride.

• If two halogen atoms are attached to the same carbon atom, the compound is known as geminal (gem-) dihalide. For example, 1,1-dichloroethane or ethylidene chloride.

Preparation

1,1-Dichloroethane (Ethylidene dichloride) CH³CHCl²

It is prepared by

• Treating acetaldehyde with phosphorus pentachloride

• Adding hydrogen chloride to acetylene

1,2-Dichloroethane (Ethylene chloride), CH

₂ClCH₂Cl

It is prepared by,

• The addition of chlorine to ethene

• The action of HCl, PCl₅ or SOCl₂ on 1,2-ethane diol.

Properties

• Dihalides are sweet smelling, colorless liquids with relatively high boiling points. The boiling point of ethylidene chloride is less than that of ethylene dichloride.

Hydrolysis

1,1-Dichloroethane gets hydrolyzed with aqueous NaOH or KOH to give ethanal (acetaldehyde).

The hydrolysis of 1,2-dichloroethane with aqueous KOH gives ethylene glycol.

This reaction is being used to distinguish the two compounds.

• Both react with alcoholic KOH to give acetylene.

• 1,2-Dichloroethane reacts with zinc dust in the presence of methanol and gives ethene.