Assertion : Most carboxylic acids exist as dimers in the vapour phase or in aprotic solvents.
Reason : Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided:

### Step 1: Understand the Assertion
The assertion states that "Most carboxylic acids exist as dimers in the vapor phase or in aprotic solvents."

**Explanation**:
Carboxylic acids can form dimers due to the presence of strong hydrogen bonding between the hydroxyl (–OH) group of one molecule and the carbonyl (C=O) group of another. This dimerization occurs because the hydrogen bond is strong enough to hold the two molecules together, even in the vapor phase or in solvents that do not donate protons (aprotic solvents).

### Step 2: Understand the Reason
The reason provided is that "Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of the hydrocarbon part."

**Explanation**:
Higher carboxylic acids have long hydrocarbon chains, which are hydrophobic (water-repelling). As the length of the hydrocarbon chain increases, the solubility of the carboxylic acid in water decreases because the hydrophobic interactions dominate over the polar interactions that the carboxylic acid can have with water. Thus, higher carboxylic acids are less soluble in water.

### Step 3: Evaluate the Relationship
Now we need to determine if the reason correctly explains the assertion.

- The assertion is true: Carboxylic acids do exist as dimers due to hydrogen bonding.
- The reason is also true: Higher carboxylic acids are less soluble in water due to their hydrophobic nature.

However, the reason does not explain why carboxylic acids exist as dimers. The dimerization is due to hydrogen bonding, not hydrophobic interactions.

### Conclusion
Both the assertion and the reason are true, but the reason is not the correct explanation for the assertion.

### Final Answer
The correct option is: **B. Both assertion and reason are true, but the reason is not the correct explanation of assertion.**

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