Distinction Between Alcohols

Lucas test

Lucas reagents (anhydrous ZnCl2+HCl) are used to distinguish primary, secondary and tertiary alcohols. This test is known as Lucas test. Their effect on primary, secondary and tertiary alcohols is given below :

  1. Primary alcohols do not react with Lucas reagents.
  2. White turbudity appears after 5 to 10 minutes when Lucas reagent reacts with secondary alcohols.
  3. Turbudity appears immediately when Lucas reagent reacts with tertiary alcohols.

Lucas test to distinguish 1°, 2° and 3° alcohols

Primary alcohol

Lucas test to distinguish primary alcohols

Secondary alcohol

Lucas test to distinguish secondary alcohols

Tertiary alcohol

Lucas test to distinguish tertiary alcohols

Cu at 573K

The vapours of alcohols react with Cu at 573 K, to form different products depending on whether the alcohol is primary, secondary or tertiary. Primary, secondary and tertiary alcohols form aldehydes, ketones and alkenes respectively. We have already discussed it in dehydration and oxidation reactions.

distinction of primary, secondary, and tertiary alcohols using Cu

Iodoform test

Take a look at the structure below :

alccohols that give iodoform must satisfy this structure

Alcohols which satisfy the above structure form a yellow ppt of iodoform when heated with iodine and aqueous NaOH (or Na2CO3).

Example : CH3CH2OH gives yellow ppt of iodoform (CHI3)

CH3CH2OH satisfies iodoform test

because it satisfies the above condition.

Reason for why CH3CH2OH satisfies iodoform test

You should note that ethanol is the only primary alcohol that gives iodoform test.

Example : CH3CH2CH2OH does not give yellow ppt of iodoform

CH3CH2CH2OH does not satisfy iodoform test

because it does not satisfy the required condition.

Reason for why CH3CH2CH2OH does not satisfy iodoform test

Example : Tertiary alcohols do not show yellow ppt of iodoform.

Tertiary alcohols do not satisfy iodoform test

Distinguish between 1-phenylethanol and 2-phenylethanol.