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17.4: Electrophilic Aromatic Substitution

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    213826
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    The first reaction in this sequence is an electrophilic aromatic substitution (EAS). This reaction is used to place the second group (SO2Cl) on the benzene ring. This is not intended to be a full discussion of EAS mechanisms. For a comprehensive treatment please consult your organic chemistry textbook.

    The most important steps in the mechanism are presented below.

    1. GENERATION OF THE ELECTROPHILE

    clipboard_e8d5c5ff647b3d8bddff5ce54ee1f4bb4.png

    2. POSSIBLE SITES FOR ELECTROPHILIC ATTACK ON THE BENZENE RING

    clipboard_ef572c01e1a03488cd1d0cc03e50b69e6.png

    3. FORMATION OF THE “ACTIVATED σ-COMPLEX,” A RESONANCE-STABILIZED REACTION INTERMEDIATE (structures I-IV).

    clipboard_e95a247cdb94b893ff18c3227299f2f01.png

    4. REGENERATION OF THE AROMATIC BENZENE RING

    clipboard_ec097afad856d8e9b62d66d120d07d636.png

    5. REACTION OF SULFONYL CHLORIDE GROUP WITH A NUCLEOPHILE (AMMONIA).

    clipboard_eb9c8b033e73eb8f8c525249f1aaa8db7.png

    6. HYDROLYSIS OF THE AMIDE GROUP

    clipboard_eb78d0aaada7b9baf777a2e2f86219417.png

    7. PROTECTING GROUP STRATEGY - Acetanilide is used as the starting material in these reactions, but at the end the amide group gets hydrolyzed into an amine group. Why not start the reaction sequence with aniline (below) rather than acetanilide, and save the last step? The answer is that the amino group in aniline is too reactive and it can react at several stages in the course of the reaction sequence.

    clipboard_e75e6d640f0c6bc3adfcfb7608c586094.png

    For example, the amine can react in step 5 (reaction of the sulfonyl chloride group with ammonia), just like ammonia does, since they are both nucleophiles of similar structure. At the beginning of the reaction, the free amino group would get protonated by the strong acid (chlorosulfonic acid) to become a meta-directing group, rather than a paradirecting group.

    clipboard_ed40adbde9ce75bc4b1830e3870508473.png

    By using the amide, instead of the free amine, the nucleophilic properties of the nitrogen get reduced by resonance stabilization of the lone pair of electrons. In addition, the bulk of the amide group in acetanilide favors para-substitution over ortho-substitution due to steric hindrance.

    clipboard_eb4663c31785c6c63197f7c466c427725.png

    the free amine is a strong nucleophile

     

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    resonance delocalization of the lone electron pair reduces the nucleophilic ability of nitrogen in the amide

    clipboard_e004f57dd7bc1e2bbba1151a457cbaff8.png


    This page titled 17.4: Electrophilic Aromatic Substitution is shared under a not declared license and was authored, remixed, and/or curated by Sergio Cortes.

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