Question

Complete the mechanism for the generation of the electrophile used for Friedel-Crafts acylation with the following acyl halide. Add curved arrows, bonds, electron pairs, and charges where indicated.

Answer 1

Concepts and reason

The concept used to solve this question is to write the mechanism of formation of electrophile in Friedel-Crafts acylation reaction. In Friedel-Crafts acylation reaction, the electrophile is generated by the reaction of acyl halide with Lewis acids like ${\rm{Al}}{{\rm{X}}_{\rm{3}}}$ , ${\rm{Fe}}{{\rm{X}}_{\rm{3}}}$ and so on. The mechanism of the reaction is indicated by the curved arrows which represent the moment of electrons from electron-rich species to electron-poor species.

Fundamentals

Friedel-Crafts acylation: This is one of the aromatic electrophilic substitution reactions. In this reaction, the acylation of benzene ring takes by the treatment of acyl halide in presence of Lewis acids. The electrophile for the reaction is generated by the reaction of acyl halide with Lewis acids like ${\rm{Al}}{{\rm{X}}_{\rm{3}}}$ , ${\rm{Fe}}{{\rm{X}}_{\rm{3}}}$ and so on. For example, consider the acetylation of benzene ring, which is shown as follows:

In the above reaction, the electrophile acetylium ion is generated by the reaction of acetyl chloride with ${\rm{FeC}}{{\rm{l}}_{\rm{3}}}$ .

Mechanism of a reaction: A mechanism of the reaction shows how a reaction takes place, in which curved arrows are used to show the flow of electrons. In a curved arrow, the tail shows where the electrons are coming from and the head shows where the electrons are going.

The tail of the curved arrow must be placed on either a lone pair or a bond and the head of the arrow must be placed on the electron deficient atom. It can be simply understood that the curved arrows represent the moment of electrons from electron-rich species to electron-poor species.

Given,

The structure of the acid chloride (butyryl chloride) is as follows:

And, the Lewis acid of the reaction is ${\rm{FeC}}{{\rm{l}}_{\rm{3}}}$ .

In the first step of the mechanism the curved arrow starts from electron rich more electronegative chlorine atom to electron deficient Fe atom of ${\rm{FeC}}{{\rm{l}}_{\rm{3}}}$ . The curved arrow for the first step of the mechanism is shown below.

The first step of the mechanism can be written by the putting the curved arrow from chlorine atom to Fe atom. In this step, the lone pair of electrons from chlorine atom is moved to empty orbital of Fe atom of ${\rm{FeC}}{{\rm{l}}_{\rm{3}}}$ to give zwitter ionic species.

The curved arrow for the second step of the mechanism is also drawn from bond pair electrons of C-Cl bond to electron deficient Cl atom which is shown in the following equation

The second step of the mechanism can be written by drawing a curved arrow from bond pair electrons of C-Cl bond to electron deficient Cl atom.

As a result, the carbonyl carbon becomes electron deficient to give acylium ion and ${\rm{FeC}}{{\rm{l}}_{\rm{4}}}^{\rm{ - }}$ ion as final species.

The second step of the mechanism can be written as follows:

Ans: Part A

The curved arrow for the first step of the mechanism is shown below:

Part B

The structure of species formed in the first step of the mechanism along with the curved arrow for the second step of the mechanism is shown below.

Part C

The structures of the species formed in the second step of the mechanism are as follows: