Which of the following is least reactive towards nucleophilic substitution with aqueous KOH

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is least reactive towards nucleophilic substitution with aqueous KOH, we can follow these steps:

### Step 1: Understand the Types of Nucleophilic Substitution Reactions
Nucleophilic substitution reactions can be classified into two main types:
- **SN1 (Unimolecular Nucleophilic Substitution)**: This mechanism involves the formation of a carbocation intermediate. The rate of reaction depends only on the concentration of the substrate.
- **SN2 (Bimolecular Nucleophilic Substitution)**: This mechanism involves a single concerted step where the nucleophile attacks the substrate, leading to the displacement of the leaving group. The rate depends on both the substrate and the nucleophile.

### Step 2: Identify the Leaving Groups
In this case, we need to consider the halides (Cl and Br) as leaving groups. The ability of a leaving group to depart is influenced by its stability as a lone pair after leaving:
- Bromide (Br⁻) is a better leaving group than chloride (Cl⁻) because it is a weaker base and more stable in solution.

### Step 3: Analyze the Substituents on the Benzene Ring
The presence of electron-withdrawing or electron-donating groups on the benzene ring can significantly affect the reactivity of the halides:
- **Electron-withdrawing groups (EWGs)** (like NO₂) stabilize the positive charge on the carbocation formed in SN1 mechanisms and weaken the bond between carbon and the leaving group, making substitution easier.
- **Electron-donating groups (EDGs)** (like OH) increase the electron density on the carbon, making the bond stronger and thus less reactive towards nucleophilic substitution.

### Step 4: Compare the Given Compounds
Let's analyze the four options based on the presence of substituents and the type of halide:
1. **Option A**: Benzene with Cl and NO₂
2. **Option B**: Benzene with Cl and OH
3. **Option C**: Benzene with Cl, NO₂, and another NO₂
4. **Option D**: Benzene with Br and OH

- **Option A** has one EWG (NO₂), which will enhance reactivity.
- **Option B** has an EDG (OH), which will decrease reactivity.
- **Option C** has two EWGs (NO₂), making it the most reactive.
- **Option D** has Br (better leaving group) but also has an EDG (OH), which will reduce its reactivity.

### Step 5: Determine the Least Reactive Compound
Based on the analysis:
- **Option C** is the most reactive due to two EWGs.
- **Option A** is more reactive than **Option D** because of the presence of Cl and an EWG.
- **Option D** is more reactive than **Option B** due to the better leaving group (Br).
- **Option B** is least reactive because the OH group donates electrons, making the C-Cl bond stronger and less prone to nucleophilic attack.

### Conclusion
The least reactive compound towards nucleophilic substitution with aqueous KOH is **Option B**.

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