Assertion (A): Alkenes undergo electrophilic addition reactions, whereas aldehydes and ketones undergo nucleophilic addition reactions.
Reason (R): In alkenes, the double bond joins two carbon atoms and has no resultant polarity, whereas in aldehydes and ketones the carbonyl group is highly polar and the partial positive charge on its carbon atom makes it open to nucleophilic attack.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
The kind of reagent that attacks a double bond depends on how its electrons are shared: an electrophile seeks electrons, a nucleophile seeks a positive centre.
Assertion (A): Alkenes undergo electrophilic addition reactions, whereas aldehydes and ketones undergo nucleophilic addition reactions. — True. Alkenes add reagents such as HBr by electrophilic addition; aldehydes and ketones add reagents such as HCN by nucleophilic addition.
Reason (R): In alkenes, the double bond joins two carbon atoms and has no resultant polarity, whereas in aldehydes and ketones the carbonyl group is highly polar and the partial positive charge on its carbon atom makes it open to nucleophilic attack. — True. The two carbon atoms of C=C have the same electronegativity, so the bond is not polarised. In C=O, oxygen is more electronegative, so the carbonyl carbon carries a partial positive charge.
Why Reason (R) explains Assertion (A)?
The non-polar C=C bond offers only its exposed π electrons, so it is attacked by an electron-seeking electrophile.
The polar C=O bond has an electrophilic carbon, so it is attacked first by a nucleophile. The difference in polarity is what decides the type of addition.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “1. Nucleophilic addition reactions
Contrary to electrophilic addition reactions observed in alkenes, the aldehydes and ketones undergo nucleophilic addition reactions.”
8.1.2 Structure of the Carbonyl Group – “The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen relative to carbon. Hence, the carbonyl carbon is an electrophilic (Lewis acid), and carbonyl oxygen, a nucleophilic (Lewis base) centre.”