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Halogenoalkanes react with NaOH(aq) either by an S ext{ }_{N}1 mechanism or by an S ext{ }_{N}2 mechanism - CIE - A-Level Chemistry - Question 24 - 2018 - Paper 1
Question 24
Halogenoalkanes react with NaOH(aq) either by an S ext{ }_{N}1 mechanism or by an S ext{ }_{N}2 mechanism. The mechanism followed by the reaction depends on the stru... show full transcript
Worked Solution & Example Answer:Halogenoalkanes react with NaOH(aq) either by an S ext{ }_{N}1 mechanism or by an S ext{ }_{N}2 mechanism - CIE - A-Level Chemistry - Question 24 - 2018 - Paper 1
Step 1
Which statement is correct?
Answer
To determine the correct mechanism for the reaction of 3-bromo-3-ethylpentane with NaOH(aq), we need to analyze the stability of the intermediates formed during the reaction.
In this case, 3-bromo-3-ethylpentane is a tertiary halogenoalkane. The primary distinction between the S ext{ }{N}1 and S ext{ }{N}2 mechanisms lies in how the reaction proceeds:
- S ext{ }_{N}1 Mechanism: This mechanism involves the formation of a carbocation intermediate. In the case of tertiary halogenoalkanes, the stability of the carbocation is enhanced by the presence of alkyl groups, which stabilize the positive charge through hyperconjugation and inductive effects. Therefore, the reaction will favor the S ext{ }_{N}1 mechanism.
- S ext{ }_{N}2 Mechanism: This mechanism involves a one-step process where the nucleophile attacks the substrate as the leaving group is expelled. Tertiary substrates are typically not favorable for this mechanism because steric hindrance prevents the nucleophile from effectively approaching the carbon attached to the leaving group.
Considering these aspects, the correct choice is:
B: The mechanism is S ext{ }_{N}1, due to the stabilisation of an intermediate cation by three alkyl groups.