Question

How would you distinguish among primary, secondary and tertiary alcohols ?

Answer 1

Primary alcohol:-

• These are the alcohols where the carbon to which the hydroxide group is linked is 1°C carbon, as it has an alkyl group attached to it.
• Example: Ethanol (CH₃-CH₂-OH)

 Secondary alcohol:-

• These are the alcohols where the carbon to which the hydroxide group is linked is 2°C carbon, as it 2 alkyl groups attached to it.
• Example: Propna-2-ol

 \(CH_3 - \overset {CH_3}{\overset{|}{CH}}-OH\)

Tertiary alcohol:-

• These are the alcohols where the carbon to which the hydroxide group is linked is 3°C carbon, as it has 3 alkyl groups attached to it.
• Example: 2-methylpropna-2-ol

\(CH_3 - \underset{CH_3}{\underset{|}{\overset{CH_3}{\overset{|}C}}}-OH\)

Distinguishing between alcohols:-

Lucas test is used to distinguish between primary 1°C, secondary 2°C, and tertiary 3°C alcohols.

Reagent used here is anhydrous Zinc chloride(ZnCl₂) and Hydrochloric acid and is called as Lucas reagent.

When this Lucas reagent is used to treat tertiary 3°C alcohol, they form a turbidity almost immediately:

 

When this Lucas reagent is used to treat secondary 2°C alcohol, they form a turbidity slowly:

When this Lucas reagent is used to treat primary 1°C alcohol, they do not form a turbidity.

The turbidity only appears upon heating the alcohol.

Therefore, tertiary alcohol responds to Lucas test by forming turbidity immediately, secondary alcohols form turbidity slowly and primary alcohols do not form turbidity.