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Reactivity towards Nucleophilic Addition
Concepts tested here
- Steric and electronic effects on carbonyl reactivity
All Questions
2006 AIPMT-PRE 1 question
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Nucleophilic addition reaction will be most favoured in :-In nucleophilic addition the nucleophile attacks the electron-deficient carbonyl carbon.
Reactivity $\propto \dfrac{1}{\text{steric hindrance}}$: bulky groups around the carbonyl carbon hinder the approach of the nucleophile.
Alkyl groups are also electron releasing ($+I$ effect); they reduce the positive charge on the carbonyl carbon and lower its reactivity.
So aldehydes (one alkyl group) are more reactive than ketones (two alkyl groups), and among aldehydes the smaller alkyl group gives the higher reactivity.
$CH_3CHO$ has the smallest alkyl group among the given compounds; $CH_3CH_2CHO$ has a larger ethyl group, and the two ketones are still less reactive.
Order: $CH_3CHO \gt CH_3CH_2CHO \gt (CH_3)_2CO \gt CH_3CH_2CH_2COCH_3$
So nucleophilic addition is most favoured in $CH_3CHO$.