Reactivity order of the following alkenes for hydrochlorination (HCl) is
(I) `CH_(2) = CH - Cl` (II) `(CH_(3))_(2)C=CH_(2)`
(III) `CH_(2) = CH - CHO`

Options

A. `I gt II gt III`
B. `II gt III gt I`
C. `II gt I gt III`
D. `III gt I gt II`

Answer

To determine the reactivity order of the given alkenes for hydrochlorination (HCl), we need to analyze the stability of the carbocations formed during the reaction. The more stable the carbocation, the more reactive the alkene will be.

### Step-by-Step Solution:

1. **Identify the Alkenes**:
- (I) `CH2 = CH - Cl`
- (II) `(CH3)2C=CH2`
- (III) `CH2 = CH - CHO`

2. **Understand Hydrochlorination**:
Hydrochlorination involves the addition of HCl across the double bond of the alkene. According to Markovnikov's rule, the hydrogen atom will attach to the carbon with the most hydrogen atoms, leading to the formation of a more stable carbocation.

3. **Analyze Alkene (I) `CH2 = CH - Cl`**:
- The double bond can react with HCl to form a carbocation.
- The positive charge will be on the carbon that is less substituted, leading to a primary carbocation, which is less stable.

4. **Analyze Alkene (II) `(CH3)2C=CH2`**:
- The double bond can react with HCl, and the positive charge will form on the tertiary carbon after the addition of H.
- This results in a tertiary carbocation, which is highly stable due to hyperconjugation and inductive effects from the surrounding alkyl groups.

5. **Analyze Alkene (III) `CH2 = CH - CHO`**:
- The reaction with HCl will lead to a carbocation where the positive charge is on the carbon adjacent to the carbonyl group (CHO).
- This results in a less stable carbocation compared to the tertiary carbocation in (II) due to the electron-withdrawing nature of the carbonyl group, which destabilizes the carbocation.

6. **Stability of Carbocations**:
- Carbocation from (II) is tertiary (most stable).
- Carbocation from (I) is primary (least stable).
- Carbocation from (III) is secondary but destabilized by the carbonyl group.

7. **Reactivity Order**:
Based on the stability of the carbocations formed:
- (II) `(CH3)2C=CH2` (most stable)
- (I) `CH2 = CH - Cl` (less stable)
- (III) `CH2 = CH - CHO` (least stable)

Therefore, the reactivity order for hydrochlorination is:
**(II) > (I) > (III)**

### Final Answer:
The reactivity order of the given alkenes for hydrochlorination (HCl) is:
**(II) > (I) > (III)**