Comprehension:
Nucleophilic Addition Reactions
Organic compounds containing carbon-oxygen double bond (>C=O ) are called carbonyl group, which is one of the most important functional group in organic chemistry. In aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in ketones, it is bonded to two carbon atoms.The carbonyl compounds in which carbon of carbonyl group is bonded to carbon or hydrogen and oxygen of hydroxyl moiety (-OH) are known as carboxylic acids , while in compounds where carbon is attached to carbon or hydrogen and nitrogen of -NH2 moiety or to halogens are called amides and acyl halides respectively. Aldehydes, ketones, and carboxylic acids are widespread in plants and animal kingdom. They add fragrance and flavour to nature, for example, vanillin ( from vanilla bean), salicylaldehyde ( from meadow sweet ) and cinnamaldehyde ( cinnamon) have pleasant fragrances.
The carbonyl carbon atom is sp2-hybridised and forms three sigma ( σ ) bonds. The fourth valence electron of carbon remains in its p-orbital and forms a π-bond with oxygen by overlap with p-orbital of an oxygen. In addition , the oxygen atom also has two non bonding electron pairs. Thus, the carbonyl carbon and the three atoms attached to it lie in the same plane and the π-electron cloud is above and below this plane. The bond angles are approximately 120° and trigonal coplanar structure.
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. Carbonyl compounds have substantial dipole moments and are more polar than ethers. Aldehyde and ketone can be prepared by oxidation of alcohols, dehydrogenation of alcohols, and ozonolysis of alkenes etc.
Which of the following compounds will exhibit slowest rate of reaction with HCN in the presence of NaOH?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Acetophenone
Addition of HCN is a nucleophilic addition: \( \ce{CN^{-}} \) (formed from HCN by the base) attacks the electrophilic carbonyl carbon. The reaction is faster when the carbonyl carbon is more positive and less crowded:
Substituents on the ring: an electron withdrawing group (–\( \ce{NO2} \)) makes the carbonyl carbon more positive and speeds up the addition; an electron releasing group (–\( \ce{CH3} \)) slows it down.
\( \displaystyle \ce{R2C=O + HCN} \xrightarrow{\ce{OH^-}} \ce{R2C(OH)CN} \)
The rate order for the given compounds is:
4-nitrobenzaldehyde > benzaldehyde > p-tolualdehyde > acetophenone
So acetophenone, the only ketone, reacts most slowly.
Why the other options are incorrect
Option (1) Benzaldehyde — Incorrect. An aldehyde, with only one group on the carbonyl carbon, so it is faster than any ketone.
Option (2) p-Tolualdehyde — Incorrect. The para –\( \ce{CH3} \) group slows it a little compared with benzaldehyde, but it is still an aldehyde and faster than acetophenone.
Option (3) 4-Nitrobenzaldehyde — Incorrect. The strongly electron withdrawing –\( \ce{NO2} \) group makes it the fastest of the four.
NCERT (2026-27 edition):
Reactivity
