Q33 / 180 · COMEDK 2023 (Evening Shift) · Chemistry
Identify the final product formed during the course of the given reactions.
Pentan-2-one $\xrightarrow{\mathrm{NaCN} / \mathrm{HCl}}[\mathrm{X}]$
$[\mathrm{X}] \xrightarrow[95 \% \mathrm{H}_3 \mathrm{PO}_4]{\Delta \text { with }}[Y]$
- Correct answerA
- B
- C
- D
Solution
The reaction of pentan-2-one with NaCN/HCl is a cyanohydrin formation reaction. Pentan-2-one is $CH_3-C(=O)-CH_2-CH_2-CH_3$.
The nucleophilic addition of cyanide ion ($CN^{-}$) to the carbonyl carbon of pentan-2-one followed by protonation yields 2-hydroxy-2-methylpentanenitrile, which is the intermediate [X]. The structure of [X] is $CH_3-C(OH)(CN)-CH_2-CH_2-CH_3$.
The subsequent reaction of [X] with 95% $H_3PO_4$ at elevated temperature is a dehydration reaction. The hydroxyl group (-OH) is protonated to form a good leaving group ($-OH_2^{+}$), which then leaves to form a carbocation at the C2 position. This is followed by the elimination of a proton from an adjacent carbon to form a double bond, resulting in an unsaturated nitrile.
The most stable alkene formed is the one where the double bond is conjugated with the nitrile group, or the more substituted alkene. Dehydration of 2-hydroxy-2-methylpentanenitrile leads to the formation of 2-methylpent-2-enenitrile, which is $CH_3-C(CN)=CH-CH_2-CH_3$.
Comparing this structure with the given options, the structure $CH_3-C(CN)=CH-CH_2-CH_3$ corresponds to the first option.
Answer:
The nucleophilic addition of cyanide ion ($CN^{-}$) to the carbonyl carbon of pentan-2-one followed by protonation yields 2-hydroxy-2-methylpentanenitrile, which is the intermediate [X]. The structure of [X] is $CH_3-C(OH)(CN)-CH_2-CH_2-CH_3$.
The subsequent reaction of [X] with 95% $H_3PO_4$ at elevated temperature is a dehydration reaction. The hydroxyl group (-OH) is protonated to form a good leaving group ($-OH_2^{+}$), which then leaves to form a carbocation at the C2 position. This is followed by the elimination of a proton from an adjacent carbon to form a double bond, resulting in an unsaturated nitrile.
The most stable alkene formed is the one where the double bond is conjugated with the nitrile group, or the more substituted alkene. Dehydration of 2-hydroxy-2-methylpentanenitrile leads to the formation of 2-methylpent-2-enenitrile, which is $CH_3-C(CN)=CH-CH_2-CH_3$.
Comparing this structure with the given options, the structure $CH_3-C(CN)=CH-CH_2-CH_3$ corresponds to the first option.
Answer:
Hence, the correct answer is option: 'A'.