For preparing monoalkyl benzene, acylation process is preferred than direct alkylation because

Options

A. In alkylation, a poisonous gas is evolved
B. In alkylation, large amount of heat is evolved
C. In alkylation, polyalkylated product is formed
D. Alkylation is very costly

Answer

To understand why the acylation process is preferred over direct alkylation for preparing monoalkyl benzene, we can break down the explanation into several steps:

### Step 1: Understanding Direct Alkylation
In direct alkylation, benzene reacts with an alkyl halide (RX) in the presence of a Lewis acid catalyst like FeCl3. This process leads to the formation of an alkylated benzene.

**Hint:** Direct alkylation can lead to multiple products due to the nature of the alkyl group.

### Step 2: Electron Donating Effects
The alkyl group (R) is an electron-donating group. When it is introduced onto the benzene ring, it increases the electron density on the ring. This activation of the ring makes it more reactive towards further electrophilic substitution.

**Hint:** Increased electron density on the ring can lead to unwanted side reactions.

### Step 3: Formation of Polyalkylated Products
Due to the increased electron density, the benzene ring can undergo further alkylation, resulting in polyalkylated products. This is not desirable when the goal is to obtain a monoalkylated product.

**Hint:** Polyalkylation occurs because the activated benzene can react multiple times.

### Step 4: Understanding Acylation
In the acylation process, benzene reacts with an acyl chloride (like CH3COCl) in the presence of a Lewis acid (like AlCl3). This reaction introduces an acyl group (COCH3) onto the benzene ring.

**Hint:** Acylation introduces an electron-withdrawing group instead of an electron-donating group.

### Step 5: Electron Withdrawing Effects
The acyl group is an electron-withdrawing group. This means that it decreases the electron density on the benzene ring, making it less reactive towards further electrophilic substitution.

**Hint:** Lower electron density on the ring helps prevent multiple substitutions.

### Step 6: Reduction of Acylated Product
After acylation, the product (like acetophenone) can be reduced using methods such as Wolff-Kishner reduction or Clemmensen reduction to convert the carbonyl group into a methylene group (–CH2–), resulting in the desired monoalkylated benzene.

**Hint:** Reduction is a crucial step to achieve the final monoalkylated product from the acylated intermediate.

### Conclusion
The acylation process is preferred over direct alkylation because it allows for the formation of a monoalkylated product without the risk of polyalkylation. The introduction of an electron-withdrawing acyl group stabilizes the benzene ring and prevents further reactions, making it easier to obtain the desired product through subsequent reduction.