Which of the following reaction is an example of elimination reaction :

Options

A. `CH_3-CHOunderset(H_2O)overset(CH_3MgBr)to CH_3-undersetunderset(OH)(|)CH-CH_3`
B. `CH_3-undersetunderset(OH)(|)CH-CH_3underset(Delta)overset(Conc. H_2SO_4)to CH_3-CH=CH_2`
C. `CH_3-CH_2-OH overset(PBr_3)to CH_3-CH_2-Br`
D. `CH_3-CH_2-COOH overset(NaNH_2)to CH_3-CH_2-COONa+NH_3`

Answer

To determine which of the given reactions is an example of an elimination reaction, we will analyze each reaction step by step.

### Step-by-Step Solution:

1. **Understanding Elimination Reactions**:
- Elimination reactions involve the removal of elements from a compound to form a double bond or a ring structure. Typically, this results in the loss of small molecules such as water (H2O) or hydrogen halides (HX).

2. **Analyzing the First Reaction**:
- The first reaction involves a carbonyl compound reacting with a Grignard reagent (CH3MgBr).
- The product formed is a tertiary alcohol after addition of the Grignard reagent.
- **Conclusion**: This is an addition reaction, not an elimination reaction.

3. **Analyzing the Second Reaction**:
- The second reaction shows a secondary alcohol (CH3CHOHCH2CH3) being converted to an alkene (CH3CH=CH2) in the presence of concentrated sulfuric acid (H2SO4) and heat.
- In this reaction, water (H2O) is eliminated during the process.
- The mechanism involves the protonation of the hydroxyl group, followed by the loss of water and formation of a double bond.
- **Conclusion**: This is an elimination reaction.

4. **Analyzing the Third Reaction**:
- The third reaction involves the substitution of the hydroxyl group (OH) with a bromine atom (Br).
- This is a nucleophilic substitution reaction where the OH group is replaced by Br.
- **Conclusion**: This is not an elimination reaction.

5. **Analyzing the Fourth Reaction**:
- The fourth reaction involves the abstraction of an acidic hydrogen by the amide ion (NH2-), resulting in the formation of ammonia (NH3) and a carboxylate ion (CH3CH2COO-).
- This reaction does not involve the formation of a double bond or the elimination of a small molecule like H2O.
- **Conclusion**: This is also not an elimination reaction.

### Final Conclusion:
The only reaction that qualifies as an elimination reaction is the **second reaction** where a secondary alcohol is converted to an alkene with the elimination of water.