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 will analyze each compound based on the stability of the carbocations formed during the reaction.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Compound 1: Ethylene (CH₂=CH₂)
- Compound 2: Isobutylene ((CH₃)₂C=CH₂)
- Compound 3: 2-Butene (CH₃CH=CHCH₃)

2. **Understand the Reaction with HCl**:
- When these alkenes react with HCl, the H⁺ ion from HCl will attack the double bond, leading to the formation of a carbocation intermediate. The stability of this carbocation is crucial for determining the reactivity of the alkene.

3. **Form the Carbocations**:
- **For Compound 1 (CH₂=CH₂)**:
- The reaction leads to the formation of a primary carbocation (C⁺), which is less stable.
- **For Compound 2 ((CH₃)₂C=CH₂)**:
- The reaction leads to the formation of a tertiary carbocation (C⁺), which is more stable than a primary carbocation.
- **For Compound 3 (CH₃CH=CHCH₃)**:
- The reaction leads to the formation of a secondary carbocation (C⁺), which is more stable than a primary but less stable than a tertiary.

4. **Evaluate Carbocation Stability**:
- Stability order of carbocations: Tertiary (3°) > Secondary (2°) > Primary (1°).
- Therefore:
- Compound 1 forms a 1° carbocation (least stable).
- Compound 2 forms a 3° carbocation (most stable).
- Compound 3 forms a 2° carbocation (moderately stable).

5. **Determine Reactivity Order**:
- Since the reactivity of the compounds towards HCl is directly related to the stability of the carbocations formed:
- Compound 2 ((CH₃)₂C=CH₂) will be the most reactive.
- Compound 3 (CH₃CH=CHCH₃) will be less reactive than Compound 2 but more reactive than Compound 1.
- Compound 1 (CH₂=CH₂) will be the least reactive.

6. **Final Order of Increasing Reactivity**:
- Therefore, the order of increasing reactivity towards HCl is:
- CH₂=CH₂ < CH₃CH=CHCH₃ < (CH₃)₂C=CH₂.

### Conclusion:
The order of increasing reactivity of the compounds towards HCl is:
**CH₂=CH₂ < CH₃CH=CHCH₃ < (CH₃)₂C=CH₂**.