Arrange the following alkyl halides in decreasing order of the rate of...
Explanation:
The rate of beta-elimination reaction with alcoholic KOH depends on the stability of the carbocation intermediate formed during the reaction. The stability of the carbocation can be determined by considering the following factors:
1. Hyperconjugation: Hyperconjugation is the stabilizing interaction between a filled sigma orbital (usually C-H or C-C bond) and an empty or partially filled orbital (usually an empty p orbital). The more hyperconjugative structures a carbocation can form, the more stable it is.
2. Inductive effect: The inductive effect is the electron-donating or electron-withdrawing effect of a substituent group. Electron donating groups stabilize carbocations, while electron-withdrawing groups destabilize them.
3. Resonance effect: Resonance stabilization occurs when a carbocation can delocalize its positive charge through pi bonds or lone pairs. The more resonance structures a carbocation can form, the more stable it is.
Based on these factors, we can determine the order of stability and therefore the rate of beta-elimination reaction for the given alkyl halides:
1. CH3CH2CH2Br:
- There are no hyperconjugative structures, as there are no adjacent hydrogen atoms to donate electrons.
- The inductive effect of the ethyl group (+CH2CH3) is weakly electron-donating, but it does not significantly stabilize the carbocation.
- There are no resonance structures, as the carbocation is primary and cannot delocalize its positive charge.
- Therefore, the carbocation formed from CH3CH2CH2Br is the least stable among the given alkyl halides, leading to the slowest rate of beta-elimination reaction.
2. CH3CH(CH3)CH2Br:
- There are three hyperconjugative structures, as there are three adjacent hydrogen atoms to donate electrons. This increases the stability of the carbocation.
- The inductive effect of the methyl groups (+CH3) is weakly electron-donating, further stabilizing the carbocation.
- There are no resonance structures, as the carbocation is secondary and cannot delocalize its positive charge.
- Therefore, the carbocation formed from CH3CH(CH3)CH2Br is more stable than the one formed from CH3CH2CH2Br, leading to a faster rate of beta-elimination reaction.
3. CH3CH2CH(Br)CH3:
- There are two hyperconjugative structures, as there are two adjacent hydrogen atoms to donate electrons. This provides some stability to the carbocation.
- The inductive effect of the ethyl group (+CH2CH3) is weakly electron-donating, further stabilizing the carbocation.
- There are no resonance structures, as the carbocation is primary and cannot delocalize its positive charge.
- Therefore, the carbocation formed from CH3CH2CH(Br)CH3 is less stable than the one formed from CH3CH(CH3)CH2Br but more stable than the one formed from CH3CH2CH2Br. As a result, the rate of beta-elimination reaction for CH3CH2CH(Br)CH3 falls between the other two alkyl halides.
Conclusion:
Based on the stability of the carbocation intermediates formed, the correct order of the rate of beta-
Arrange the following alkyl halides in decreasing order of the rate of...
Order tertiary greater than secondary greater than primary
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