An organic compound ‘A’ with molecular formula CH4O reacts with HI to give an alkyl iodide, which then reacts with potassium cyanide to produce ‘B’. Compound ‘B’ on reduction followed by reaction with HNO2 gives an alcohol, which on oxidation gives ‘C’. Compound ‘C’ reacts with CH3MgBr to give ‘D’, which on hydrolysis gives compound ‘E’, which is also an alcohol.
In the reaction sequence given in the passage, an alcohol is oxidised to give compound ‘C’. Consider the following statements about the oxidation of alcohols.
(A) Oxidation of an alcohol involves the formation of a carbon–oxygen double bond with cleavage of an O–H bond and a C–H bond.
(B) Acidified potassium permanganate is used for getting carboxylic acids directly from primary alcohols.
(C) Secondary alcohols are oxidised to aldehydes by chromic anhydride (CrO3).
(D) CrO3 in an anhydrous medium is used as the oxidising agent for the isolation of aldehydes from primary alcohols.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (B) and (D) only
In the oxidation of an alcohol a C=O bond is formed. What a primary alcohol gives, an aldehyde or a carboxylic acid, depends on the oxidising agent used.
(A) Oxidation of an alcohol involves the formation of a carbon–oxygen double bond with cleavage of an O–H bond and a C–H bond. — True. The alcohol loses one hydrogen from O–H and one from C–H, so these reactions are also called dehydrogenation reactions.
(B) Acidified potassium permanganate is used for getting carboxylic acids directly from primary alcohols. — True. It is a strong oxidising agent, so the aldehyde formed first is oxidised further to the acid:
\( \displaystyle \ce{RCH2OH + 2[O]} \xrightarrow{\text{acidified } \ce{KMnO4}} \ce{RCOOH + H2O} \)
(C) Secondary alcohols are oxidised to aldehydes by chromic anhydride (CrO3). — False. The carbon carrying the –OH group of a secondary alcohol is joined to two carbon atoms, so oxidation gives a ketone, not an aldehyde:
\( \displaystyle \ce{R2CH-OH + [O]} \xrightarrow{\ce{CrO3}} \ce{R2C=O + H2O} \)
(D) CrO3 in an anhydrous medium is used as the oxidising agent for the isolation of aldehydes from primary alcohols. — True. Under these conditions the oxidation stops at the aldehyde stage.
\( \displaystyle \ce{RCH2OH + [O]} \xrightarrow{\text{anhydrous } \ce{CrO3}} \ce{RCHO + H2O} \)
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “4. Oxidation: Oxidation of alcohols involves the formation of a carbon- oxygen double bond with cleavage of an O-H and C-H bonds.”
“…These are also known as dehydrogenation reactions as these involve loss of dihydrogen from an alcohol molecule. Depending on the oxidising agent used, a primary alcohol is oxidised to an aldehyde which in turn is oxidised to a carboxylic acid.”
“Strong oxidising agents such as acidified potassium permanganate are used for getting carboxylic acids from alcohols directly. CrO3 in anhydrous medium is used as the oxidising agent for the isolation of aldehydes.”
“Secondary alcohols are oxidised to ketones by chromic anhyride (CrO3).”