Which one is a nucleophilic substitution reaction among the following?

Options

A. `CH_(3)CHO+HCNtoCH_(3)CH(OH)CN`
B. `CH_(3)+CH=CH_(2)+H_(2)Ooverset(H^(+))toCH_(3)-underset(OH)underset(|)(CH)-CH_(3)`
C. `RCHO+R'MgXtoR-underset(OH)underset(|)(CH)-R'`
D. `CH_(3)-CH_(2)-overset(CH_(3))overset(|)(CH)-CH_(2)Br+NH_(3)`
`toCH_(3)-CH_(2)overset(CH_(3))overset(|)(CH)-CH_(2)NH_(2)`

Explanation

To determine which of the given reactions is a nucleophilic substitution reaction, we will analyze each option step by step.

### Step 1: Analyze the First Reaction
**Reaction:** CH₃C(=O)H + SCN⁻

- In this reaction, the cyanide ion (CN⁻) acts as a nucleophile.
- The CN⁻ attacks the carbonyl carbon, leading to the formation of a new bond with CN⁻.
- This results in the formation of CH₃C(OH)(CN) and does not involve the replacement of any atom or group.
- **Conclusion:** This is a nucleophilic addition reaction, not a substitution.

### Step 2: Analyze the Second Reaction
**Reaction:** CH₃C(=CH₂) + H₂O (in acidic conditions)

- Here, the double bond is attacked by H⁺ from the acid, leading to the formation of a carbocation.
- Water (H₂O) then attacks the carbocation, leading to the formation of an alcohol.
- There is no group being replaced in this reaction; instead, we see the addition of water across the double bond.
- **Conclusion:** This is also a nucleophilic addition reaction, not a substitution.

### Step 3: Analyze the Third Reaction
**Reaction:** RCHO + R'MgX

- In this reaction, the Grignard reagent (R'MgX) breaks down to R'⁻ and MgX⁺.
- The R'⁻ acts as a nucleophile and attacks the carbonyl carbon of the aldehyde (RCHO).
- This results in the formation of an alcohol (RCH(OH)R').
- Again, there is no substitution occurring; it is an addition of the nucleophile to the carbonyl.
- **Conclusion:** This is a nucleophilic addition reaction, not a substitution.

### Step 4: Analyze the Fourth Reaction
**Reaction:** CH₃CH₂CH₂Br + NH₃

- In this reaction, ammonia (NH₃) acts as a nucleophile due to its lone pair of electrons.
- The lone pair from NH₃ attacks the carbon bonded to Br, leading to the breaking of the C-Br bond.
- The bromine (Br⁻) is expelled, and NH₂ group replaces the Br atom.
- **Conclusion:** This is a nucleophilic substitution reaction because one nucleophile (Br⁻) is replaced by another (NH₂).

### Final Answer
The reaction that is a nucleophilic substitution reaction is the **fourth reaction (D part)**: CH₃CH₂CH₂Br + NH₃.