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 :
Lucas test to distinguish 1°, 2° and 3° alcohols
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.
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)
because it satisfies the above condition.
You should note that ethanol is the only primary alcohol that gives iodoform test.
Example : CH3CH2CH2OH does not give yellow ppt of iodoform
because it does not satisfy the required condition.
Example : Tertiary alcohols do not show yellow ppt of iodoform.