CH3CH2CH2Br + Na+ -CN →

CH3CH2CH2Br + Na+ -CN →

The correct answer and explanation is :

The given reaction:

[
\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} + \text{Na}^+ -\text{CN} \rightarrow
]

Correct Answer:
[
\text{CH}_3\text{CH}_2\text{CH}_2\text{CN} + \text{NaBr}
]

Explanation:

1. Type of Reaction:
   This is a nucleophilic substitution reaction (S(_N)2 mechanism). In this reaction, the bromopropane (CH(_3)CH(_2)CH(_2)Br) undergoes substitution with the cyanide ion (CN⁻) from sodium cyanide (NaCN).
2. Mechanism of the Reaction:

• The propyl bromide (CH₃CH₂CH₂Br) has a polar C-Br bond, where the carbon is partially positive due to the electronegativity of bromine.
• The cyanide ion (CN⁻) is a strong nucleophile and attacks the electrophilic carbon from the backside in a one-step concerted mechanism.
• The Br⁻ leaves, and C-CN bond formation occurs simultaneously.
• This follows an S(_N)2 mechanism (bimolecular nucleophilic substitution), which occurs in one step with inversion of configuration.

1. Product Formation:

• The propyl bromide (CH₃CH₂CH₂Br) is converted to propyl cyanide (CH₃CH₂CH₂CN), also known as butanenitrile.
• Sodium bromide (NaBr) is formed as a by-product.

1. Why S(_N)2 Instead of S(_N)1?

• Primary alkyl halides (like CH₃CH₂CH₂Br) favor S(_N)2 mechanisms because the primary carbon is less sterically hindered, making it easier for the nucleophile to attack.
• The S(_N)1 mechanism, which involves a carbocation intermediate, is less favorable here since a primary carbocation is highly unstable.

1. Importance of Reaction:

• This reaction is useful in organic synthesis for converting alkyl halides to nitriles, which can then be hydrolyzed to carboxylic acids or reduced to primary amines.

Thus, the final product is propyl cyanide (butanenitrile), CH₃CH₂CH₂CN.