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Compound A is pentan-3-one (CH3CH2COCH2CH3) and compound B is pentan-2-one (CH3COCH2CH2CH3). Which of the following statements about their reaction with NaHSO3 (sodium bisulfite) is correct?

1. Both A and B give a white precipitate with NaHSO3
2. Only B gives a white crystalline precipitate with NaHSO3; A does not react
3. Only A gives a white precipitate with NaHSO3; B does not react
4. Neither A nor B reacts with NaHSO3

Correct answer: Only B gives a white crystalline precipitate with NaHSO3; A does not react

Solution

NaHSO3 forms a bisulfite addition product with carbonyl compounds only when steric hindrance is low: aldehydes and methyl ketones (RCOCH3) react readily. Compound B (pentan-2-one, CH3-CO-CH2CH2CH3) is a methyl ketone and reacts with NaHSO3 to give a white crystalline precipitate. Compound A (pentan-3-one, CH3CH2-CO-CH2CH3) is a diethyl ketone; the two ethyl groups create too much steric bulk around the carbonyl, preventing NaHSO3 addition. So B gives the test, A does not.

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