Arenes - Electrophilic Substitution Reaction

Arenes are a special type of hydrocarbons. Despite, that it contains a pi electron ring, which implies that like alkenes, it is highly susceptible to attack by electrophiles, arenes are basically less reactive than alkenes. In our syllabus, our attention is on benzene, one of the arenes.

The more inert nature of arenes must be attributed to the delocalisation of pi electrons, which resulted in an aromatic stability to be attained. Aromaticity is resulted when the pi electron clouds are able to be delocalised in a ring. In addition, the number of pi electrons must satisfy the 4n+2 rule (where n is an interger).

This stability is so significant, such that instead of undergoing electrophilic addition reactions, benzene would rather undergo electrophilic substitution reactions. Hence, the illustration below depicts the mechanism.

In order to aid your understanding of the mechanism, the video clip below describes the mechanism. In addition, there is explanation to the structure of benzene and the structure of the intermediate.



The intermediate is significant in deciding on the location of the second electrophilic substitution as shown below.

The following are factors that influence the location of the second electrophilic substitution.

(1) Steric replusion between the electron cloud of the first substituent and the electron cloud of the second substituent.

(2) The stability of the carboncation - Hence, when the first substituent is electron withdrawing, the second substituent prefers to attack on the C3 position. As the intermediate is the most stable.

Lastly, the rate of the second electrophilic substitution is affected by the type of substituent found on benzene. If the substituent is electron donating, it enriches the electron ring, hence making it more susceptible to attacks by electrophile. The result? Lower temperature or less severe conditions are required.

Naturally, the converse is true too.

^{-- -- -- -- --}

^{Article written by Kwok YL 2009.}

^{Disclaimer and remarks:}

• ^{If you would like to use this source, kindly drop me a note by leaving behind a comment with your name and institution. I am all for sharing as the materials on this blog is actually meant for the education purpose of my students.}
• ^{This material is entirely written by the author and my sincere thanks will be given to anyone who is kind, generous and gracious to point out any errors.}