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.

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.

Iodoform test

Take a look at the structure below :

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)

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

Example : CH₃CH₂CH₂OH does not give yellow ppt of iodoform

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