What is the order of reactivity of these alkenes
`(CH_(3))_(2)C=CH_(2)(I), CH_(3)CH=CH_(2)(II) and `CH_(2)=CH_(2)(III)` when subject to acid - catalysed hydration?

Options

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

Answer

To determine the order of reactivity of the given alkenes (I: (CH₃)₂C=CH₂, II: CH₃CH=CH₂, III: CH₂=CH₂) in acid-catalyzed hydration, we need to analyze the stability of the carbocations formed during the reaction.

### Step-by-Step Solution:

1. **Identify the Alkenes**:
- Alkene I: (CH₃)₂C=CH₂
- Alkene II: CH₃CH=CH₂
- Alkene III: CH₂=CH₂

2. **Mechanism of Acid-Catalyzed Hydration**:
- In acid-catalyzed hydration, the double bond of the alkene attacks a proton (H⁺) from the acid, leading to the formation of a carbocation intermediate.
- The stability of the carbocation determines the reactivity of the alkene. More stable carbocations lead to faster reactions.

3. **Formation of Carbocations**:
- For Alkene I ((CH₃)₂C=CH₂):
- When H⁺ attacks the double bond, a tertiary carbocation is formed (C⁺ is attached to three alkyl groups).
- Stability: Tertiary > Secondary > Primary
- Carbocation: (CH₃)₃C⁺ (tertiary, very stable)

- For Alkene II (CH₃CH=CH₂):
- When H⁺ attacks, a secondary carbocation is formed.
- Carbocation: CH₃CH⁺CH₃ (secondary, moderately stable)

- For Alkene III (CH₂=CH₂):
- When H⁺ attacks, a primary carbocation is formed.
- Carbocation: CH₃CH₂⁺ (primary, least stable)

4. **Order of Stability of Carbocations**:
- Tertiary carbocation (from Alkene I) > Secondary carbocation (from Alkene II) > Primary carbocation (from Alkene III).
- Therefore, the order of reactivity based on carbocation stability is:
- Alkene I > Alkene II > Alkene III

5. **Final Answer**:
- The order of reactivity of the alkenes in acid-catalyzed hydration is:
- I > II > III