The order of increasing reactivity of the following compounds towards HCl will be
(1) `CH_2=CH_2`
(2) `(CH_3)_2C=CH_2`
(3) `CH_3CH=CHCH_3`

Options

A. `1 lt 2 lt 3`
B. `1 lt 3 lt 2`
C. `3 lt 2lt 1`
D. ` 2 lt 1 lt 3`

Answer

To determine the order of increasing reactivity of the given compounds towards HCl, we need to analyze the stability of the carbocation intermediates formed during the reaction. The more stable the carbocation, the more reactive the alkene will be.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (1) Ethylene: \(CH_2=CH_2\)
- (2) Isobutylene: \((CH_3)_2C=CH_2\)
- (3) 2-Butene: \(CH_3CH=CHCH_3\)

2. **Understand the Reaction Mechanism**:
- When alkenes react with HCl, the double bond attacks the hydrogen (H+) from HCl, leading to the formation of a carbocation. The stability of this carbocation is crucial for the reactivity of the alkene.

3. **Analyze Each Compound**:
- **Ethylene (\(CH_2=CH_2\))**:
- When it reacts with HCl, it forms a primary carbocation (\(CH_3-CH_2^+\)), which is the least stable type of carbocation.
- **Isobutylene (\((CH_3)_2C=CH_2\))**:
- This compound can form a tertiary carbocation upon reaction with HCl, which is highly stable due to the presence of three alkyl groups stabilizing the positive charge.
- **2-Butene (\(CH_3CH=CHCH_3\))**:
- This compound can form a secondary carbocation. While it is more stable than a primary carbocation, it is less stable than a tertiary carbocation.

4. **Rank the Stability of Carbocations**:
- Primary carbocation (Ethylene) < Secondary carbocation (2-Butene) < Tertiary carbocation (Isobutylene)

5. **Determine the Order of Reactivity**:
- Since reactivity is inversely related to the stability of the carbocation formed:
- Ethylene (least reactive) < 2-Butene < Isobutylene (most reactive)

### Final Order of Increasing Reactivity:
1. \(CH_2=CH_2\) (Ethylene)
2. \(CH_3CH=CHCH_3\) (2-Butene)
3. \((CH_3)_2C=CH_2\) (Isobutylene)

Thus, the order of increasing reactivity towards HCl is:
\[ CH_2=CH_2 < CH_3CH=CHCH_3 < (CH_3)_2C=CH_2 \]