Which one of the following statement is incorrect?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Ketones are good reducing agents
A reducing agent is itself oxidised while it reduces something else, so it must be easily oxidised.
An aldehyde has a hydrogen atom on its carbonyl carbon and is oxidised even by mild oxidising agents such as Tollens’ reagent.
A ketone has no such hydrogen; to oxidise it, a carbon–carbon bond must be broken, which needs strong oxidising agents and high temperature. So ketones do not reduce Tollens’ or Fehling’s reagent, and they are not good reducing agents.
Option (1) Ketones are good reducing agents — An incorrect statement. Ketones resist oxidation by mild oxidising agents; they are oxidised only under vigorous conditions, with C–C bond cleavage, so they are poor reducing agents.
Option (2) Aldehydes and ketones are polar compounds — A correct statement. Oxygen is more electronegative than carbon, so the \( \ce{C=O} \) bond is polarised: the carbon is slightly positive and the oxygen slightly negative. Carbonyl compounds have substantial dipole moments.
Option (3) Aldehydes and ketones have higher boiling points than hydrocarbons of comparable molecular masses — A correct statement.
Their polar molecules attract one another by dipole–dipole interactions, which hydrocarbon molecules lack.
Their boiling points are still lower than those of alcohols, because they cannot form hydrogen bonds with one another.
Option (4) Aldehydes and ketones are reduced to alcohols — A correct statement. With \( \ce{NaBH4} \), \( \ce{LiAlH4} \) or by catalytic hydrogenation, aldehydes give primary alcohols and ketones give secondary alcohols.
NCERT (2026-27 edition):
Oxidation – “… Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes. Ketones are generally oxidised under vigorous conditions, i.e., strong oxidising agents and at elevated temperatures.”
8.1.2 Structure of the Carbonyl Group – “The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon. … Carbonyl compounds have substantial dipole moments and are polar than ethers.”
Physical Properties – “… The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions. Also, their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding.”
Reduction – “(i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation (Unit 7, Class XII).”