Tuesday, February 5, 2008

IIT JEE Revision - Ch. 25. Alcohols - Core Points


dehydration and oxidation,
reaction with sodium,
reaction with phosphorus halides,
reaction with ZnCl2/conc.-HCl,
conversion of alcohols into aldehydes and ketones;

1. The hydroxy derivatives of aliphatic hydrocarbons are termed alcohols. They contain one or more hydroxyl (OH) groups.

Methyl Alcohol CH-3OH
Ehtyl alcohol C-2H-5OH also written as CH-3CH-2OH
Propyl alcohol C-3H-7OH also written as CH-3CH-2CH-2OH

2. Methods of Preparation of Alcohols

1. preparation from haloalkanes
2. By reduction of aldehydes, ketones and esters

3. Physical Properties:

4. Reaction with active metals - acidic character

5. Esterification

Alcohols react with monocarboxylic acids, in the presence of concentrated sulphuric acid or dry HCL gas as catalyst, to from esters. This reaction is known as esterification.

6. Dehydration

When alcohols are heated with conc. or H3PO4, at 443 K, they get dehydrated to form alkenes.

The ease of dehydration of alcohol follows the order 3>2>1 which is also the order of stability of carbocation.

7. Oxidation

The oxidation of alcohols can be carried out by a number of reagents such as acqueous, alkaline or acidified KMnO4, acidified Na2Cr2O7, nitric acid, chromic acid, etc.

8. Reaction with sodium

The cleavage in this reaction will be in the OH bond. Alcohols react with active metals to liberate hydrogen gas and form metal alkoxide.

Ethanol or Ethyl alcohol reacts with sodium to give Sodium ethoxide and hydrogen

9. Reaction with phosphorus halides

Phosphorus halides such as PCl5, PCl3, PBr3 and PI3 react with alcohols to form corresponding haloalkanes.

10. 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).

11. Conversion of alcohols into aldehydes and ketones

Oxidation of primary alcohol gives aldehydes.
Oxidation of secondary alcohols gives ketones.
It is difficult to oxidize tertiary alcohols.

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