IIT JEE , AIEEE Forum - Articles Category , Community shelf

coolriya

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

Example:
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.

* ii. A dihydric alcohol contains two -OH groups in the molecule

ex. ethylene glycol

CH2OH
|
CH2OH

iii. A trihydric alcohol contains three -OH groups in its molecules

ex.glycerol

CH2OH

|
CH2OH
|
CH2OH

Monohydric alcohols are represented by the general formula R-OH or CnH2n+1OH.

Monohydric alcohols are further classified according to the carbon atom to which the hydroxyl group is attached.

1. Primary alcohols: -OH group is attached to primary carbon atom. They contain the group -CH2OH

eg.: CH3CH2OH, CH3CH2CH2OH

2. Secondry alcohol: -OH group is attached to a secondary carbon atom. It contains a divalent >CHOH group.

Ex. iso-propyl alcohol

3. Tertiary alcohol: -Oh group is attached to tertiary carbon atom. C-OH group is present

* alcohols physical props :

A. Physical state: the lower members are colourless liquids and have a characteristic smell and burning taste.

The higher members(with more than 12 carbons) are colourless wax like solids.

B. Solubility: The lower members are highly soluble in water.

Amongst isomeric alcohols, the solubility increases with branching.

C. Alcohols exists associated molecules due to intermolecular hydrogen bonds.

D. Boiling points: The lowers members have low boiling points.
With the increase in molecular weight, the boiling points keep on increasing gradually.

e. Density: Generally alcohols are lighter than water.

Density of alcohols increases with molecular mass.

F. Alcohols have intoxicating effects.

Methanol is poisonous.
Ethanol is used for drinking purposes.

ALCOHOLS ACIDIC NATURE :
Alcohols are weakly acidic in nature.

They react with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide.

Liberation of hydrogen shows that alcohols are acidic in nature.

The acidic nature of alcohols is due to the presence of polar O-H bond.

As oxygen withdraws shared electron pair between O and H atoms towards itself, it can lose the proton (H+).

However, alcohols are weak acids

A COMMENT PLS........................

SOURCE: chemistry blog spots & net.

Ask & Discuss Questions with Community & Experts