Reactions of Benzynes Answer the following questions to the best of your ability. Once you have completed the assignment, be sure to check your answers agains the answer key
1. Benzynes are very reactive intermediates. Please explain why they are so reactive, and the best way to trap those reactive intermediates.
Answer: Benzyne is an neutral reactive intermediate that can be formed by removing two ortho substituents from a benzene ring leaving the p-orbitals with free electrons. Benzyne is an extremely reactive species because of the presence of triple bonds. Triple bonds in alkynes usually result in a linear geometry to facilitate orbital overlap. In benzyne, however, the p-orbitals are distorted to accommodate the triple bond within the ring system, reducing their effective overlap. This lack of overlap results in the high reactivity of benzynes. This intermediate is so reactive that its existence was hard to showcase. One way to illustrate the existence of benzyne as an intermediate in nucleophilic addition is to trap it via a diels-alder reaction.
2. Explain why benzyne is one of the intermediates in the following reactions:
Answer: In a typical electrophilic aromatic substitution reaction mechanism, a methyl group is will direct the substitution on the carbons in the ortho and para positions. Substitution of a chloro group doesn’t happen easily. The fact that both the meta and the para substituted products were obtained in equal amounts strongly suggest that the reaction occurs via an elimination/addition mechanism with benzyne as the intermediate. Since benzyne is planar and achiral, the amino group has equal change of bonding to the meta and para carbons, resulting in the formation of both products in a 1:1 ratio.
3. Describe four methods for the preparation of benzynes?
Answer: a. Dehydrohalogenation
b. Elimination from organolithium or organomagnesium precursors
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c. Loss of neutral leaving groups
d. Photolytic and/or Pyrolytic methods
4. Show how benzynes can be used in electrophilic addiction reactions.
Answer: Benzyne can act as a nucleophile in electrophilic addition reaction. The reaction is as follows:
5. What is the product of dimerization of benzyne? Show the mechanism.
Answer: Dimerization of benzyne is a 2+2 cycloaddition reaction which occurs via a concerted mechanism. The product is byphenylene. The mechanism is as follows:
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