Predict the major product of the acid catalysed dehydration of the following alcohols :
(a)`(CH_3)_2C(OH)CH_2CH_3` , (b)`CH_3CH_2CH_2CH(OH)CH_3`
(c )`(CH_3)_2C(OH)CH(CH_3)_2` , (iv)`(CH_3)_3C CH_2OH`

Answer

To predict the major product of the acid-catalyzed dehydration of the given alcohols, we will follow a systematic approach. The dehydration of alcohols involves the formation of alkenes through the elimination of a water molecule, typically under acidic conditions. The major product is usually the more substituted alkene due to stability reasons (Zaitsev's rule).

### Step-by-Step Solution:

#### (a) For `(CH_3)_2C(OH)CH_2CH_3`:

1. **Identify the structure**: The alcohol can be represented as:
```
CH3
|
C(OH)
|
CH2
|
CH3
```

2. **Protonation of the hydroxyl group**: In the presence of acid, the hydroxyl group (OH) is protonated to form a better leaving group (water).

3. **Formation of carbocation**: The protonation leads to the formation of a carbocation. The carbocation can be formed at the carbon where the OH group was attached.

4. **Elimination of water**: Two elimination pathways are possible:
- Elimination of a hydrogen from the adjacent CH2 group leads to:
```
CH3
|
C+
|
CH2
|
CH3
```
- Elimination of a hydrogen from the other CH3 group leads to:
```
CH2
|
C+
|
CH3
|
CH3
```

5. **Determine the major product**: The more stable product is the one formed by the elimination from the CH3 group, leading to:
```
CH3
|
C=CH2
|
CH3
```
This is the major product (more substituted alkene).

#### (b) For `CH_3CH_2CH_2CH(OH)CH_3`:

1. **Identify the structure**: The alcohol can be represented as:
```
CH3
|
CH2
|
CH2
|
C(OH)
```

2. **Protonation of the hydroxyl group**: The hydroxyl group is protonated.

3. **Formation of carbocation**: The carbocation is formed at the carbon where the OH group was attached.

4. **Elimination of water**: The elimination can occur from either side:
- Elimination from the CH3 side leads to:
```
CH3
|
CH2
|
C+
|
CH2
```
- Elimination from the CH2 side leads to:
```
CH3
|
CH2
|
CH2
|
C+
```

5. **Determine the major product**: The more substituted product is formed by elimination from the CH2 group, leading to:
```
CH3
|
CH2
|
C=CH2
```
This is the major product.

#### (c) For `(CH_3)_2C(OH)CH(CH_3)_2`:

1. **Identify the structure**: The alcohol can be represented as:
```
CH3
|
C(OH)
|
CH
|
CH3
```

2. **Protonation of the hydroxyl group**: The hydroxyl group is protonated.

3. **Formation of carbocation**: The carbocation is formed at the carbon where the OH group was attached.

4. **Elimination of water**: The elimination can occur from either side:
- Elimination from the CH3 group leads to:
```
CH3
|
C+
|
CH(CH3)
```
- Elimination from the adjacent CH group leads to:
```
CH3
|
C+
|
CH3
```

5. **Determine the major product**: The more substituted product is formed by elimination from the CH group, leading to:
```
CH3
|
C=CH2
|
CH3
```
This is the major product.

#### (iv) For `(CH_3)_3CCH_2OH`:

1. **Identify the structure**: The alcohol can be represented as:
```
CH3
|
C(OH)
|
CH2
```

2. **Protonation of the hydroxyl group**: The hydroxyl group is protonated.

3. **Formation of carbocation**: The carbocation is formed at the carbon where the OH group was attached.

4. **No beta-hydrogens**: In this case, the carbon bearing the OH does not have any beta-hydrogens available for elimination.

5. **Rearrangement**: A rearrangement occurs where a methyl group shifts to form a more stable carbocation:
```
CH3
|
C+
|
CH3
```

6. **Elimination of water**: The elimination leads to:
```
CH3
|
C=CH2
|
CH3
```
This is the major product.

### Summary of Major Products:
- (a) Major product: More substituted alkene from `(CH_3)_2C(OH)CH_2CH_3` is `C=CH(CH_3)`.
- (b) Major product: More substituted alkene from `CH_3CH_2CH_2CH(OH)CH_3` is `C=CH(CH_3)`.
- (c) Major product: More substituted alkene from `(CH_3)_2C(OH)CH(CH_3)_2` is `C=CH(CH_3)`.
- (iv) Major product: Rearranged alkene from `(CH_3)_3CCH_2OH` is `C=CH(CH_3)`.