Question

How will you distinguish primary and secondary alcohols using Luca’s test?

Answer 1

There are different types of tests to distinguish primary, secondary and tertiary alcohols. They are Lucas tests and oxidation of alcohols. Lucas test is used to distinguish between primary and secondary alcohols. Both alcohols behave differently on the Lucas test.

Complete step by step answer:
Lucas reagent is a solution of zinc chloride in concentrated ${\text{HCl}}$. This is based on the reactivity difference of alcohols towards ${\text{HCl}}$. This reaction produces alkyl halides by ${{\text{S}}_{\text{N}}}^1$ reaction. It is the nucleophilic substitution. Lucas reagent has equal moles of ${\text{ZnC}}{{\text{l}}_2}$ and ${\text{HCl}}$.
The experiment part is explained below:
We have to take a test tube with alcohol and add the reagent. Shake it well.
A tertiary alcohol reacts rapidly with the reagent and forms alkyl halides. The alkyl halide formed is insoluble and forms an oily layer. The product formed will show a rapid turbidity.
E.g. ${\left( {{\text{C}}{{\text{H}}_3}} \right)_3}{\text{COH + HCl}}\xrightarrow{{{\text{ZnC}}{{\text{l}}_2}}}{\left( {{\text{C}}{{\text{H}}_3}} \right)_3}{\text{CCl + }}{{\text{H}}_2}{\text{O}}$
${3^ \circ }$ alcohol ${3^ \circ }$ alkyl halide
A secondary alcohol react comparatively slower than ${3^ \circ }$ alcohols. It takes $3 - 5{\text{min}}$ to show the turbidity.
E.g. ${\text{C}}{{\text{H}}_3}{\text{CH}}\left( {{\text{OH}}} \right){\text{C}}{{\text{H}}_3} + {\text{HCl}}\xrightarrow{{{\text{ZnC}}{{\text{l}}_2}}}{\text{C}}{{\text{H}}_3}{\text{CH}}\left( {{\text{Cl}}} \right){\text{C}}{{\text{H}}_3} + {{\text{H}}_2}{\text{O}}$
${2^ \circ }$ alcohol ${2^ \circ }$ alkyl halide
A primary alcohol does not give any turbidity when reacted with the reagent.
E.g. ${\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}} + {\text{HCl}}\xrightarrow{{{\text{ZnCl}}}}$ No reaction
There are other tests also to distinguish them.

Additional Information:
A positive test occurs when the alkyl halide formed in the reaction begins to separate from the aqueous solution to form a second layer.

Note:
Another test for distinguishing primary and secondary alcohols is oxidation using ${\text{KMn}}{{\text{O}}_4}$ or ${{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}$. When the alcohol is treated with ${\text{KMn}}{{\text{O}}_4}$ or ${{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}$ in the presence of concentrated sulfuric acid, primary or secondary alcohols disappear the color of ${\text{KMn}}{{\text{O}}_4}$, i.e. purple or the color of ${{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}$ changes from orange to green. But tertiary alcohols do not react with this reagent.