Concept explainers
Suggest reasonable explanations for each of the following observations:
The first-order rate constant for solvolysis of
After a solution of
in aqueous sulfuric acid had been allowed to stand for
and
Treatment of
with hydrogen bromide gave the same mixture of
Treatment of
The major product in parts (c) and (d) was
Interpretation:
The reasonable explanations for each of the given observations are to be suggested.
Concept introduction:
The allyl group contains the unit
In substitution reactions, allylic halides react faster than the corresponding alkyl halides.
The primary or secondary allylic carbocations are less stable than tertiary allylic carbocations.
In allylic carbocations that are not symmetrically substituted, the two resonance structures are not equivalent and do not contribute equally to the resonance hybrid. A more stable resonance form contributes more to the resonance hybrid.
Two non-equivalent resonance forms of the allylic carbocation yield two different products.
Reactions that occur via
The regioselectivity of an
A more substituted alkene is stable and forms more readily than a less substituted alkene.
Answer to Problem 52P
Solution:
a) Resonance stabilization of the carbocation formed by the removal of chloride ion from
b) The resonance stabilization of the carbocation formed by the reaction of
c) The resonance stabilization of the carbocation formed by the reaction of
d) The resonance stabilization of the carbocation formed by the reaction of both
e)
Explanation of Solution
a) The comparison of rate constant of solvolysis of the given allylic chlorides.
The expanded structures for the two given allylic chlorides are shown below:
Both the allylic chlorides follow the
b) Reaction of
The expanded structure for
When a solution of
c) Treatment of the given compound with hydrogen bromide.
The structure for
When
resonance stabilized. However, the two resonance forms for this allylic carbocation are not equivalent. Each allylic carbocation will undergo nucleophilic substitution reaction to form two different allylic bromides as shown below.
d) Treatment of
The structure for
When
e) Major products obtained in part (c) and (d).
The products obtained in part (c) and (d) are as follows:
This is because this reaction follows
The major product formed in the reaction is governed by the fact that a more substituted alkene is stable and forms readily than a less substituted alkene. The bromine atom bonds to the carbon that carries the positive charge, giving
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