Preparation of alcohols

Preparation of alcohols:

We have already learnt that the nucleophilic substitution reactions of alkyl halides with dilute alkali, conversion of alkenes to alcohols by hydration and the preparation of alcohols using Grignard reagent in XI standard. These reactions are summarised below.

1. From Alkyl halides:

Alkyl halides on heating with dilute aqueous NaOH gives alcohols. Primary alkyl halides undergo substitution by SN² reaction. Secondary and tertiary alkyl halides usually undergo nucleophilic substitution by SN¹ mechanism.

R-X + NaOH(aq) -----^∆→ R-OH + NaX

If R =t-butyl, the reaction proceeds through the formation of t-butyl carbocation

2. From alkenes:

Addition of water across the double bond of an alkene in presence of concentrated sulphuric acid gives alcohols. This addition reaction follows Markownikoff’s rule.

Example: CH₃ -CH=CH₂ + H₂O ---------^Conc.H2SO4→ CH₃ -CH(OH)-CH₃

(propylene ---------^Conc.H2SO4→  propan-2-ol )

3. From Grignard reagent:

Nucleophilic addition of Grignard reagent to aldehydes/ketones in presence of dry ether followed by the acid hydrolysis gives alcohols. Formaldehyde gives primary alcohol and other aldehydes give secondary alcohols. Ketones give tertiary alcohols.

Examples

4. Hydroboration:

Diborane reacts with an alkene to form trialkyl borane which on treatment with H₂O₂ in presence of NaOH gives an alcohol. (Refer reactions of diborane) The overall reaction is hydration of an alkene. This reaction yields an anti-Markownikoff's product.

6CH₃ -- CH = CH₂ + B₂H₆  ------→ 2(CH₃ --CH₂--CH₂)₃B (Tripropylborane)

(CH₃ -- CH₂ -- CH₂)₃B + 3H₂O₂ ---^OH-→ 3CH₃ --CH₂ --CH₂ OH (propan-1-ol) + B (OH)₃

5. Reduction of carbonyl compounds:

Reduction of aldehydes/ketones with LiAlH₄ in the presence of solvents like THF (Tetrahydrofuran) followed by hydrolysis gives alcohols. Unlike other reducing agents such as Raney Ni, Na-Hg/H₂O, the lithium aluminium hydride does not reduce the carbon–carbon double bond present in unsaturated carbonyl compound and hence it is a best reagent to prepare unsaturated alcohols.

Examples

Preparation of glycol

We have already learnt that the hydroxylation of ethylene using cold alkaline solution of potassium permanganate (Baeyer’s reagent) gives ethylene glycol.

Preparation of glycerol

Glycerol occurs in many natural fats and it is also found in long chain fatty acids in the form of glyceryl esters (Triglycerides). The alkaline hydrolysis of these fats gives glycerol and the reaction is known as saponification.

Evaluate Yourself?

1. Suggest a suitable carbonyl compound for the preparation of pent-2-en-1-ol using LiAlH₄ .

2. 2-methylpropene  -----^{H2SO4 /H2O} →?

3. How will you prepare the following using Grignard reagent. i) t-butyl alcohol ii) allyl alcohol

Methods to differentiate primary, secondary and tertiary alcohols.

The following tests are used to distinguish between 1°, 2° and 3° alcohols.

a) Lucas test:

When alcohols are treated with Lucas agent (a mixture of concentrated HCl and anhydrous ZnCl₂ ) at room temperature, tertiary alcohols react immediately to form a turbidity due to the formation of alkyl chloride which is insoluble in the medium. Secondary alcohols react within 10 minutes to form a turbidity of alkyl chloride where primary alcohols do not react at room temperature.

b) Victor Meyer’s test:

This test is based on the behaviour of the different nitro alkanes formed by the three types of alcohols with nitrous acid and it consists of the following steps.

i) Alcohols are converted into alkyl iodide by treating it with I₂ /P .

ii) Alkyl iodide so formed is then treated with AgNO₂ to form nitro alkanes.

iii) Nitro alkanes are finally treated with HNO₂ (mixture of NaNO₂ / HCl ) and the resultant solution is made alkaline with KOH.

Result:

• Primary alcohol gives red colour

• Secondary alcohol gives blue colour.

• No colouration will be observed in case of tertiary alcohol.