What product (s) is (are) obtained when 2 - bromobutane undergoes an elimination reaction with a strong base ?

Options

A. Only 1 - butene
B. Only 2 - butene
C. 1 - butene and 2 - butene , with more 1 - butene
D. 1 - butene and 2 - butene , with more 2 - butene

Answer

To determine the products obtained when 2-bromobutane undergoes an elimination reaction with a strong base, we can follow these steps:

### Step 1: Identify the Structure of 2-Bromobutane
2-bromobutane has the following structure:
```
CH3
|
CH3-CH-CH2-Br
|
CH2
```
In this structure, the bromine atom (Br) is attached to the second carbon (C-2) of the butane chain.

### Step 2: Understand the Elimination Reaction
An elimination reaction involves the removal of a leaving group (in this case, Br) and a hydrogen atom (H) from adjacent carbon atoms. This type of reaction is often referred to as a beta elimination reaction.

### Step 3: Identify the Beta Hydrogens
In 2-bromobutane, the carbon to which Br is attached (C-2) has two neighboring carbons (C-1 and C-3). The hydrogen atoms attached to these carbons are the beta hydrogens that can be removed during the elimination reaction.

### Step 4: Perform the Elimination Reaction
When a strong base abstracts a beta hydrogen, two possible scenarios arise:

1. **Removing a hydrogen from C-1 (beta position)**:
- The base removes a hydrogen from C-1, and Br leaves, forming but-1-ene (1-butene).
- Structure of but-1-ene:
```
CH2=CH-CH2-CH3
```

2. **Removing a hydrogen from C-3 (beta position)**:
- The base removes a hydrogen from C-3, and Br leaves, forming but-2-ene (2-butene).
- Structure of but-2-ene:
```
CH3-CH=CH-CH3
```

### Step 5: Determine the Major Product
According to Zaitsev's rule, the more substituted alkene is favored. In this case, but-2-ene is more substituted than but-1-ene because it has a double bond between two carbons that each have two alkyl groups attached.

### Conclusion
The products obtained when 2-bromobutane undergoes an elimination reaction with a strong base are:
- **1-butene (but-1-ene)**
- **2-butene (but-2-ene)**

However, the major product is **2-butene** due to greater substitution.

### Final Answer
The products are 1-butene and 2-butene, with 2-butene being the major product.

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