Preparation of Alcohol

INTRODUCTION

Hydroxy compound can be classified in the following three categories.

1. Aliphatic hydroxy compound

Contain three and more than three hydroxyl group.

Monohydric alcohols are of three types.

Illustration 1. Classify the following into primary, secondary and tertiary alcohols:

Solution: (a) Tertiary

(b) Secondary

(c) Tertiary

PREPARATION OF ALCOHOLS

1. From Alkanes

Alkanes having tertiary carbon on oxidation with cold alkaline KMnO₄ give tertiary alcohol.

2. From Alkenes

Alkenes can be converted into alcohol by the following reactions:

3. From alkyl halides

Alkyl halides give alcohol with KOH/NaOH or with moist Ag₂O.

4. Reduction of aldehydes and ketones

(a) Reduction by reducing agents

(i) Aldehyde gives primary alcohol

Reducing agents

(i) LiAlH₄ (ii) NaBH₄

(iii) Na/C₂H₅OH (iv) Metal (Zn, Fe or Sn)/Acid (HCl, dil H₂SO₄ or CH₃COOH)

(v) (a) Aluminium isopropoxide/isopropylalcohol (b) H₂O

(vi) H_­2/Ni

$\bullet$ NaBH₄ and aluminium isopropoxide reduces only carbonyl group and has no effect on any other group.

$C{H}_3-CH=CH-CHO\stackrel{NaB{H}_4}{\to }C{H}_3-CH=CH-C{H}_{2}OH$

$\bullet$ Reduction with aluminium isopropoxide is known as Meerwein – Ponndorf Verley (MPV) reduction.

$\bullet$ LiAlH₄ has no effect on double and triple bonds but if compound is b - aryl, $\alpha$, b - unsaturated carbonyl compound then double bond also undergoes reduction.

(b) Reduction by Grignard reagents

Addition followed by hydrolysis

$\bullet$ Methanol can not be prepared by this method.

5. Reduction of carboxylic acid, Acid chlorides and esters:

(a) Reduction by LiAlH₄

G = OH (acid)

G = Cl (acid chloride)

G = OR¢ (ester)

(b) Reduction by BH₃

Carboxylic acids and esters are reduced in to primary alcohol by BH₃.

(c) Bouveault – Blanc reaction

Illustration 2.

If compound has ¾COOH as well as ester group then reactivity of ester is more than acid towards LiAlH₄.

Acid is more reactive than ester towards BH₃.

6. From aliphatic primary amines

It react with nitrous acid to give alcohol.

$\bullet$ Nature of alcohol depends on the nature of carbon having $-$NH₂ group.

$\bullet$ Reaction proceeds through carbocation hence rearranged alcohol is obtained.

7. From Oxiranes

Oxiranes react with Grignard reagent to give mono hydric alcohol. Nature of G.R is basic hence it attack on less hindered carbon of oxirane ring.

Illustration 3. (a) Find A, B, C, D, E.

Solution:

8. Fermentation of carbohydrates

Illustration 4.

Solution: