(a)
Interpretation:
The mechanism for the given elimination reaction including carbocation rearrangement is to be drawn.
Concept introduction:
The E1 reaction proceeds with the formation of carbocation intermediate by elimination of the leaving group in the first step. This is the rate-determining step; thus, the stability of the carbocation formed increases the
Answer to Problem 8.65P
The E1 mechanism for the given reaction involving carbocation rearrangement is
Explanation of Solution
The given reaction equation is
In first step, the leaving group Br detaches from the carbon and forms the secondary carbocation.
The carbocation formed is rearranged by 1, 2- hydride shift and forms the more stable tertiary carbocation.
The water molecule acts as a base and abstracts a proton from the carbon adjacent to the carbocation, forming C=C bond.
The mechanism for the given elimination reaction is drawn to show the carbocation rearrangement by 1, 2- hydride shift.
(b)
Interpretation:
The mechanism for the given elimination reaction without carbocation rearrangement is to be drawn.
Concept introduction:
The E1 reaction proceeds with the formation of carbocation intermediate by elimination of the leaving group in the first step. This is the rate-determining step; thus, the stability of the carbocation formed increases the rate of reaction. The electron donating group (alkyl substituents) adjacent to the carbocation increases its stability. The order of stability of carbocation is CH3 < 1o < 2o < 3o. The carbocation can be rearranged by 1, 2- hydride or 1, 2- methyl shift to form a more stable carbocation. Sometimes the carbocation formed in the first step, after and before rearrangement, gives the same product. More substituted products are stable and hence form preferably.
Answer to Problem 8.65P
The E1 mechanism for the given reaction without carbocation rearrangement is
Explanation of Solution
The given reaction equation is
In the first step, the leaving group Br detaches from the carbon and forms secondary carbocation.
In the second step, without rearrangement, the proton is eliminated by the base H2O from the carbon adjacent to the carbocation, forming C=C bond. The more substituted double bond is more stable and hence is formed as a major product.
The mechanism for the given elimination reaction is drawn to without rearrangement step, indicating that the same product is formed with or without rearrangement.
(c)
Interpretation:
It is to be explained how the C13 isotope labeling is useful to determine whether the rearrangement occurred in the given E1 reaction.
Concept introduction:
The E1 reaction proceeds with the formation of carbocation intermediate by elimination of the leaving group in the first step. This is the rate-determining step; thus, the stability of the carbocation formed increases the rate of reaction. The electron donating group (alkyl substituents) adjacent to the carbocation increases its stability. The order of stability of carbocation is CH3 < 1o < 2o < 3o. The carbocation can be rearranged by 1, 2- hydride or 1, 2- methyl shift to form a more stable carbocation. Sometimes the carbocation formed in the first step, after and before rearrangement, gives the same product. More substituted products are stable and hence form preferably. Whether the reaction proceeds with or without rearrangement can be determined by labeling the carbon as C13 where the leaving group is bonded.
Answer to Problem 8.65P
The reaction with carbocation rearrangement gave two products, and the reaction without carbocation rearrangement gave only one product, as shown below, indicating that the C13 isotope labeling is useful to determine whether the rearrangement occurred in the given E1 reaction.
Reaction with rearrangement:
Reaction without rearrangement:
Explanation of Solution
The given reaction equation is
If the carbon bonded to the leaving group in the given substrate is labelled as C13 and rest carbon atoms are considered as C12, then two products are formed when the reaction occurs through carbocation rearrangement. The detailed mechanism is as follows:
In one product, one of the double bonded carbon is C13 labelled, and in another product, both the double bonded carbons are C12.
If the reaction proceeds without rearrangement, then only one product is formed where one of the double bonded carbon is C13. The detailed mechanism is as follows:
As the reaction with rearrangement of carbocation formed two products, and reaction without rearrangement formed only one product, it indicates that the E1 products depend on whether the rearrangement occurred.
It is explained that the E1 products depend on whether the reaction includes carbocation rearrangement occurring with C13 isotope labeling.
(d)
Interpretation:
How the deuterium isotope labeling is useful to determine whether the rearrangement is occurred in given E1 reaction, it is to be explained.
Concept introduction:
The E1 reaction proceeds with the formation of carbocation intermediate by elimination of the leaving group in the first step. This is the rate-determining step; thus, the stability of the carbocation formed increases the rate of reaction. The electron donating group (alkyl substituents) adjacent to the carbocation increases its stability. The order of stability of carbocation is CH3 < 1o < 2o < 3o. The carbocation can be rearranged by 1, 2- hydride or 1, 2- methyl shift to form a more stable carbocation. Sometimes the carbocation formed in the first step, after and before rearrangement, gives the same product. The more substituted products are stable and hence form preferably. Whether the reaction proceeds with or without rearrangement can be determined by replacing the hydrogen that proposed to undergo migration.
Answer to Problem 8.65P
The reaction with carbocation rearrangement by migration of deuterium gave two products, and the reaction without carbocation rearrangement gave only one product as shown below, indicating that the deuterium isotope labeling is useful to determine whether the rearrangement occurred in the given E1 reaction.
Reaction with rearrangement:
Reaction without rearrangement:
Explanation of Solution
The given reaction equation is
If the migrating hydrogen in the given substrate is replaced with deuterium, then two products are formed when the reaction occurred through carbocation rearrangement. The detailed mechanism is as follows:
One product is formed by elimination of hydrogen atom and another by elimination of deuterium atom.
If the reaction proceeds without rearrangement, only one product is formed by elimination of deuterium atom. The detailed mechanism is as follows:
As the reaction with rearrangement of carbocation formed two products and reaction without rearrangement formed only one product, it indicates the E1 products depend on whether the rearrangement occurs.
It is explained on the basis of formation of different products that deuterium isotope labeling is useful to determine whether the rearrangement occurred in the given E1 reaction.
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Chapter 8 Solutions
Organic Chemistry: Principles and Mechanisms (Second Edition)
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Organic Chemistry: A Guided InquiryChemistryISBN:9780618974122Author:Andrei StraumanisPublisher:Cengage Learning