Which one is a nucleophilic substitution reaction among the following ?

Options

A. `RCHO + R 'Mg x to R - underset(OH) underset(|) CH -R `
B. `CH_3 - CH_2 - oevrset(CH_3) overset(|) CH -CH_2 Br +NH_3 to CH_3 - CH_2 - overset(CH_3) overset(|) CH-CH_2 NH_2`
C. `CH_3 CHO +HCN to CH_3 CH (OH) CN`
D. `CH_3 - CH = CH_2 + H_2 O overset(H^(+)) to CH_3 - underset(OH) underset(|) CH - CH_3`

Explanation

To determine which reaction is a nucleophilic substitution reaction, we will analyze each option provided in the question step by step.

### Step 1: Analyze Reaction A
**Reaction A:** CH3CHO + HCN

- In this reaction, HCN dissociates into H⁺ and CN⁻.
- The H⁺ ion interacts with the oxygen of the carbonyl group (C=O), forming a positively charged intermediate (C=OH).
- The CN⁻ (nucleophile) then attacks the carbon atom of the carbonyl group, leading to the formation of a new bond.
- This reaction results in the addition of the nucleophile (CN⁻) to the carbon atom, converting the C=O double bond into a C-O single bond.

**Conclusion for Reaction A:** This is a nucleophilic addition reaction, not a substitution.

### Step 2: Analyze Reaction B
**Reaction B:** CH2=CH2 + H2O (in the presence of acid)

- The double bond (C=C) reacts with H⁺ from the acid, forming a carbocation intermediate.
- Water (H2O) acts as a nucleophile and attacks the carbocation, leading to the formation of an alcohol.
- The double bond is converted into a single bond, and a new hydroxyl group is added.

**Conclusion for Reaction B:** This is also a nucleophilic addition reaction.

### Step 3: Analyze Reaction C
**Reaction C:** RCHO + R'MgX (in acidic conditions)

- The R' group from the Grignard reagent (R'MgX) acts as a nucleophile and attacks the carbonyl carbon of RCHO.
- This forms a new carbon-carbon bond and results in the formation of an alcohol after the addition of acid.

**Conclusion for Reaction C:** This is again a nucleophilic addition reaction.

### Step 4: Analyze Reaction D
**Reaction D:** CH3CH2CHBr + NH3

- In this reaction, ammonia (NH3) acts as a strong nucleophile.
- The nitrogen in NH3 attacks the carbon atom bonded to bromine (Br), leading to the displacement of Br⁻.
- The product formed is CH3CH2CH(NH2) + HBr, where bromine is replaced by the amine group.

**Conclusion for Reaction D:** This is a nucleophilic substitution reaction.

### Final Answer
The correct option that represents a nucleophilic substitution reaction is **D**.

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