Which of the following would be the best synthesis of benzoic acid from bromobenzene?

Options

A.
B.
C.
D.

Answer

To synthesize benzoic acid from bromobenzene, we can analyze the provided reactions step by step. Here’s a detailed breakdown of the best synthesis method:

### Step 1: Understanding the Reactants
- **Bromobenzene** is our starting material. It is an aryl halide, meaning it has a bromine atom attached to a benzene ring.
- We want to convert this compound into **benzoic acid**, which is a carboxylic acid with the formula C6H5COOH.

### Step 2: Evaluating the Reactions
1. **First Reaction: Bromobenzene + KCN**
- This reaction involves a nucleophilic substitution where cyanide (CN⁻) acts as the nucleophile.
- The bromine atom is replaced by the cyanide group, forming **benzonitrile** (C6H5CN).
- Hydrolysis of benzonitrile leads to the formation of benzoic acid.
- **Limitations**: Aryl halides like bromobenzene do not favor nucleophilic substitution reactions, making this method less efficient.

2. **Second Reaction: Bromobenzene + AgCN**
- Similar to the first reaction, AgCN also leads to the formation of benzonitrile.
- Hydrolysis again yields benzoic acid.
- **Limitations**: The reaction with silver cyanide is also slow due to the nature of the metal, making it less favorable.

3. **Third Reaction: Bromobenzene + Mg (in THF)**
- Here, bromobenzene reacts with magnesium to form a **Grignard reagent** (C6H5MgBr).
- The Grignard reagent then reacts with carbon dioxide (CO2), which acts as an electrophile.
- The carbon from CO2 is attacked by the negatively charged carbon in the Grignard reagent, leading to the formation of benzoic acid upon hydrolysis.
- **Advantages**: This method is efficient, as Grignard reagents readily react with CO2, and the process is fast.

4. **Fourth Reaction: Bromobenzene + Grignard Reagent + KCN**
- This reaction also starts with the formation of a Grignard reagent.
- However, the subsequent reaction with KCN and hydrolysis is slower compared to the reaction with CO2.
- **Limitations**: The presence of KCN slows down the process, making this method less preferable.

### Step 3: Conclusion
After evaluating all four reactions, the **third reaction** (bromobenzene with magnesium followed by CO2) is the best synthesis method for producing benzoic acid due to its efficiency and speed.

### Final Answer
The best synthesis of benzoic acid from bromobenzene is through the third reaction: **Bromobenzene + Mg (in THF) → Grignard Reagent → CO2 → Benzoic Acid**.

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