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Predict the structures of the intermediates/products in the following reaction sequence:

image 1

In the first step, I know that aluminium chloride is playing some role, as otherwise hydronium ion would have reacted with succinic anhydride to give succinic acid.

I thought of the following two possibilities:

  • Friedel Crafts: But till date, I have only seen Friedel Crafts type reactions and have not seen aluminium chloride complex with either acid or anhydride. I could also not find it on Google.

  • Chloride formation: But I have also never seen aluminium chloride forming a chloride in reaction with anhydride. I am not sure if it does that, does it?

So, what is the reaction mechanism for the formation of (A)?

Source: Joint Entrance Exam (JEE) 1996 India

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Aluminium chloride ($\ce{AlCl_3}$) is mainly used for this Friedel-Crafts alkylation or acylation, where this compound acts as catalyst, and also controls the production of the desired compound.

For example, in this case, the reactants are anisole ($\ce{Ph -OMe}$) and succinic anhydride. So, the main reaction which drives these reactants towards product is the nucleophilic attack by the para carbon (w.r.t the $\ce{OMe}$ group) of the phenyl ring at the carbonyl carbon of the succinic anhydride, and an addition-elimination of the attached oxygen of the ester part in succinic anhydride.

For this, we need the state of the reactants in a proper way. We have to make sure that, the oxygen of $\ce{OMe}$ does not attack prior to the phenyl ring, and also, the electrophilicity of the carbonyl carbon needs to be increased to much extent, which is generally not so much due to present resonance in the compound.

$\ce{AlCl_3}$ plays a special role over controlling these two phenomena. Being a Lewis acid, it forms a Lewis acid-base interaction with the oxygen of $\ce{OMe}$ and also with the oxygen atom of $\ce{C=O}$ group in succinic anyhdride. Due to this interaction, a δ+ charge is introduced on both oxygen atoms described before. This increased formal charge helps to increase the electrophilicity of the carbon due to increased -I effect of the attached oxygen and also reduces the chances of attacking by $\ce{OMe}$ group. This catalyses to form our desired product with the proper reaction between the reactants.

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  • $\begingroup$ You claim that the reaction is initiated by a nucleophilic attack. In a medium where AlCl3 is available, what nucleophile would attack an acid anhydrite? The better approach is to initiate a reaction between the Anhydrite and the aluminum chloride, forming a carbocation like species, which undergoes EAS easily in the given conditions. $\endgroup$ – Shekhar Upadhyay Apr 27 '18 at 12:58
  • $\begingroup$ @ShekharUpadhyay can I not see EAS be seen as nucleophilic attack of benzene derivatives on the electrophile?? $\endgroup$ – Kavita Juneja Apr 27 '18 at 13:48
  • $\begingroup$ @ShekharUpadhyay I saw what you are saying in Peter Sykes. $\endgroup$ – Kavita Juneja Apr 27 '18 at 13:50
  • $\begingroup$ @KavitaJuneja no. There is a reason it's called Electrophilic Aromatic Substitution. The initiator is an Electrophile. Benzene, in itself, is not a system that would want to show nucleophilic activity. When it's electrons are disturbed, it loses its aromaticity. It won't spontaneously do this on it's own. An extremely strong Electrophile is necessary for the initiation of this reaction. $\endgroup$ – Shekhar Upadhyay Apr 27 '18 at 13:51
  • $\begingroup$ @ShekharUpadhyay Any source of your claim? $\endgroup$ – Kavita Juneja Apr 27 '18 at 14:42

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