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Arrange the following alkyl halides in d...

Arrange the following alkyl halides in decreasing order of the rate of `beta`-elimination reaction with alcoholic KOH.
(I) `CH_(3)-underset(CH_(3))underset(|)overset(H)overset(|)(C)-CH_(3)Br`
(II) `CH_(3)-CH_(2)-Br`
(III) `CH_(3)-CH_(2)-CH_(2)-Br`

A

I>II>III

B

III>II>I

C

II>III>I

D

I>III>II

Text Solution

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The correct Answer is:
To arrange the given alkyl halides in decreasing order of the rate of beta-elimination reaction with alcoholic KOH, we will analyze the structure of each compound and the stability of the resulting alkenes after elimination. ### Step-by-Step Solution: 1. **Identify the Compounds:** - (I) `CH₃-CH(CH₃)-CH₃Br` (2-bromopropane) - (II) `CH₃-CH₂-Br` (bromethane) - (III) `CH₃-CH₂-CH₂-Br` (1-bromopropane) 2. **Understanding Beta-Elimination:** - Beta-elimination (E2 mechanism) involves the removal of a hydrogen atom from the beta carbon (the carbon adjacent to the carbon bonded to the halogen) and the halogen itself, forming a double bond (alkene). - The stability of the alkene formed is crucial; more substituted alkenes are generally more stable. 3. **Analyze Each Compound:** - **Compound (I):** - The structure has a tertiary carbon (the carbon attached to Br is bonded to two other carbons). - The beta carbon has two alkyl groups (from the two methyl groups), leading to a more stable alkene (more substituted). - **Compound (II):** - This is a primary alkyl halide (the carbon attached to Br is bonded to one other carbon). - The beta carbon has only one alkyl group, leading to a less stable alkene (less substituted). - **Compound (III):** - This is also a primary alkyl halide, but it has a straight-chain structure. - Similar to compound (II), the beta carbon has only one alkyl group, leading to a less stable alkene. 4. **Order of Reactivity:** - The reactivity of alkyl halides in beta-elimination is influenced by the stability of the alkene formed: - (I) forms a more stable, more substituted alkene. - (III) and (II) form less stable alkenes, but (III) has a slightly more stable structure than (II) due to the longer carbon chain. - Therefore, the order of reactivity in decreasing order is: - (I) > (III) > (II) ### Final Answer: The order of the alkyl halides in decreasing order of the rate of beta-elimination reaction with alcoholic KOH is: **(I) > (III) > (II)**
To arrange the given alkyl halides in decreasing order of the rate of beta-elimination reaction with alcoholic KOH, we will analyze the structure of each compound and the stability of the resulting alkenes after elimination. ### Step-by-Step Solution: 1. **Identify the Compounds:** - (I) `CH₃-CH(CH₃)-CH₃Br` (2-bromopropane) - (II) `CH₃-CH₂-Br` (bromethane) - (III) `CH₃-CH₂-CH₂-Br` (1-bromopropane) ...
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