Question

What is obtained on oxidation of ${2^{\rm 0}}$ - alcohol by chromic acid?
A. Ester
B. Aldehyde
C. Carboxylic acid
D. Ketone

Answer 1

Hint – The primary alcohols when oxidised yield either aldehydes or carboxylic acids depending on the reaction conditions. And secondary alcohols are oxidised to ketones.
Secondary alcohols get easily oxidized without involving any breakage of the carbon-carbon bonds, but only as far as the ketone stage. Then there is no further oxidation seen except under very stringent conditions. Whereas in the case of tertiary alcohols, they cannot be oxidized at all without breaking of the carbon-carbon bonds, whereas primary alcohols can be oxidized to yield aldehydes or further oxidized to carboxylic acids.

Complete step-by-step answer:
Chromic acid ($H{\,_2}Cr{O_4}$, which is generated by mixing sodium dichromate, $N{a_2}C{r_2}{O_7}$, with sulfuric acid, ${H_2}S{O_4}$) is considered to be an effective oxidizing agent for most alcohols. It is a very strong oxidant, and it oxidizes the alcohol as far as possible without involving the breakage of the carbon-carbon bonds.
Chromic acid is considered to oxidize primary alcohols to carboxylic acids, and it oxidizes secondary alcohols to ketones. Tertiary alcohols do not react with chromic acid under mild conditions but can oxidize under several special chemical conditions.

The chromic acid oxidizes secondary alcohol to a ketone.
As shown in the below example it oxidizes isopropyl alcohol to acetone.
${\left( {C{H_3}} \right)_2}CHOH + \left[ O \right]\xrightarrow{{{H_2}Cr{O_4}}}{\left( {C{H_3}} \right)_2}C \doteq O$
Therefore, the correct option for the above answer is – option (D) ie Ketone.

Note – There are a wide variety of reagents that can be used for the process of oxidation of alcohols. But the two most commonly used are chromic acid, $ {H_2}{Cr_2}{O_7}$, and pyridinium chlorochromate, PCC. Chromic acid is said to be prepared by treatment of sodium or potassium dichromate with aqueous sulfuric acid. The most common use of chromic acid is to oxidize $2^0$ alcohols to ketones. Oxidation of alcohol is usually a two step process. The first step involves the generation of chromate esters. And once the chromate ester is formed, it then undergoes an elimination reaction to produce the carbonyl group of the aldehyde or ketone.