Aldehydes and ketones can be prepared by:
(A) Oxidation of Alcohol
(B) Dehydrogenation of Alcohol
(C) Ozonolysis of alkenes
(D) Hydration of Alkyne
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B), (C) and (D)
An aldehyde or ketone has a C=O group. It can be formed by removing hydrogen from an alcohol (oxidation or dehydrogenation), by breaking a C=C bond (ozonolysis), or by adding water to a C≡C bond (hydration). Each of the four methods does one of these.
(A) Oxidation of Alcohol — Correct. Primary alcohols give aldehydes and secondary alcohols give ketones. To stop a primary alcohol at the aldehyde stage, a mild oxidant such as PCC is used:
\( \displaystyle \ce{CH3CH2OH} \xrightarrow{\text{PCC}} \ce{CH3CHO} \qquad \ce{(CH3)2CHOH} \xrightarrow{\ce{K2Cr2O7}/\ce{H+}} \ce{CH3COCH3} \)
(B) Dehydrogenation of Alcohol — Correct. Vapours of a primary or secondary alcohol passed over heated copper lose hydrogen; this method suits volatile alcohols and is used in industry:
\( \displaystyle \ce{CH3CH2OH} \xrightarrow{\ce{Cu}, 573\ \mathrm{K}} \ce{CH3CHO + H2} \)
(C) Ozonolysis of alkenes — Correct. Ozone adds to the double bond to give an ozonide, which zinc dust and water split into two carbonyl compounds. 2-Methylbut-2-ene, for example, gives an aldehyde and a ketone:
\( \displaystyle \ce{CH3-CH=C(CH3)2} \xrightarrow[\text{(ii) }\ce{Zn}/\ce{H2O}]{\text{(i) }\ce{O3}} \ce{CH3CHO + CH3COCH3} \)
(D) Hydration of Alkyne — Correct. With dilute sulphuric acid and mercury(II) sulphate, water adds to the triple bond; the enol formed changes into the carbonyl compound. Ethyne gives ethanal, and other alkynes give ketones:
\( \displaystyle \ce{HC#CH + H2O} \xrightarrow{\ce{HgSO4}/\text{dil. }\ce{H2SO4}} \ce{CH3CHO} \qquad \ce{CH3-C#CH + H2O} \xrightarrow{\ce{HgSO4}/\text{dil. }\ce{H2SO4}} \ce{CH3COCH3} \)
NCERT (2026-27 edition):
Preparation of Aldehydes and Ketones – “1. By oxidation of alcohols
Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols, respectively (Unit 7, Class XII).
2. By dehydrogenation of alcohols
This method is suitable for volatile alcohols and is of industrial application. In this method alcohol vapours are passed over heavy metal catalysts (Ag or Cu). Primary and secondary alcohols give aldehydes and ketones, respectively (Unit 7, Class XII).
… (i) By ozonolysis of alkenes: As we know, ozonolysis of alkenes followed by reaction with zinc dust and water gives aldehydes, ketones or a mixture of both depending on the substitution pattern of the alkene (Unit 9, Class XI).
(ii) By hydration of alkynes: Addition of water to ethyne in the presence of H2SO4 and HgSO4 gives acetaldehyde. All other alkynes give ketones in this reaction (Unit 9, Class XI).”