When neopentyl alcohol is heated with an acid , it slowly converted into an 85 : 15 mixture of alkenes A and B , respectively. What are these alkenes ?

Options

A.
B.
C.
D.

Answer

To solve the problem of identifying the alkenes A and B formed from the dehydration of neopentyl alcohol, we can follow these steps:

### Step 1: Understand the Structure of Neopentyl Alcohol
Neopentyl alcohol (2-methyl-2-butanol) has the following structure:
```
CH3
|
CH3 - C - CH2 - OH
|
CH3
```
This molecule consists of a central carbon (C) bonded to three methyl groups (CH3) and one hydroxyl group (OH).

### Step 2: Identify the Reaction Conditions
When neopentyl alcohol is heated with an acid, it undergoes dehydration. This process involves the removal of a water molecule (H2O) from the alcohol, leading to the formation of a carbocation intermediate.

### Step 3: Formation of Carbocation
Upon heating, the hydroxyl group (OH) is protonated by the acid, making it a better leaving group. The water molecule leaves, forming a carbocation. In the case of neopentyl alcohol, the most stable carbocation formed is a tertiary carbocation due to the presence of three alkyl groups that stabilize the positive charge.

### Step 4: Rearrangement of the Carbocation
The carbocation can undergo rearrangement. In this case, a methyl group can migrate to the positively charged carbon, leading to a more stable carbocation:
```
CH3
|
CH3 - C+ - CH2 - CH3
|
CH3
```
This rearrangement leads to a stable tertiary carbocation.

### Step 5: Elimination to Form Alkenes
From the stable carbocation, elimination occurs to form alkenes. According to Zaitsev's rule, the more substituted alkene will be the major product.

### Step 6: Identify the Major and Minor Products
1. **Major Product (Alkene A)**: The major product will be formed by removing a hydrogen from the beta carbon that has the least number of hydrogens, leading to the most substituted alkene:
- Alkene A: 2-methylpropene (CH2=C(CH3)2)

2. **Minor Product (Alkene B)**: The minor product will be formed by removing a hydrogen from one of the other beta carbons:
- Alkene B: 2,3-dimethylpropene (CH3-CH=C(CH3)-CH3)

### Step 7: Conclusion
Thus, the two alkenes formed from the dehydration of neopentyl alcohol are:
- Alkene A: 2-methylpropene (85% of the mixture)
- Alkene B: 2,3-dimethylpropene (15% of the mixture)