Why do we prefer Friedel craft acylation to Friedel craft alkylation in preparation of alkylbenzene?

Why do we prefer Friedel craft acylation to Friedel craft alkylation in preparation of alkylbenzene?


3 Answers

Vikas TU
14149 Points
7 years ago
Friedel craft acylation  inviles always the double bonded Oxygen with R or other group which leads to decrease in the density of the electrons.
.Where as alkyl group -R shows  +I effect which leads to increase the elctron density on Benzene.
dolly bhatia
54 Points
7 years ago
There are many electrophilic substitution reactions associated with benzene and benzene derivatives. However, very few of them involve formation of a carbon-carbon bond, one of the primary challenges in organic chemistry. In principle, reactions could be carried out with benzene in presence of a sufficiently electrophilic carbon-based electrophile. There are two such transformations., called the Friedel-Crafts reactions. Secret behind success of both processes is use of a Lewis acid, usually aluminum chloride. In presence of this reagent, halo-alkanes attack benzene to form alkyl-benzenes.
In 1877, Friedel and crafts discovered that a haloalkane reacts with benzene in presence of an aluminum halide. Resulting products are the alkyl-benzene and hydrogen halide. This reaction, which can be carried out in presence of other Lewis Acid catalysis, is called Friedel-Crafts alkylation of benzene. 
Reactivity of haloalkane increases with polarity of C-X bond in the order RI
With primary halides, reaction begins with coordination of Lewis Acid to halogen of the haloalkane. This coordination places a partial positive charge on halogen-bearing carbon, rendering it more electrophilic. Attack on benzene ring is followed by proton loss giving observed product.
With secondary and tertiary halides, free carbocations are formed as intermediates.
Intramolecular alkylations can be used to fuse a new ring onto benzene nucleus.
Friedel-Crafts alkylations can be carried out with any starting material like an alcohol or alkene which acts as a precursor to a carbocation.
In summary, the Friedel-Crafts alkylation produces carbocations capable of electrophilic aromatic substitution by formation of aryl-carbon bonds. Haloalkanes, alkenes and alcohols can be used to achieve aromatic alkylation in presence of a Lewis Acid.

Limitations of Friedel-Crafts Alkylation:
Above reaction is accompanied by two important side reactions, one is polyalkylation (more than one alkyl group attaches itself to the ring); other is carbocation rearrangement (intermediate carbocation rearranges and so a different product is formed). This leads to a decrease in useful yield and a mixture of products which are difficult to separate.
Because of these limitations, Friedel-Crafts alkylations are used rarely in synthetic chemistry. However, a second reaction helped to fix this problem. It involves an electrophilic carbon species which does not rearrange and one that deactivates ring to prevent further substitution (polyalkylation).
Alkanoylation (Acylation):
This reaction proceeds through intermediacy of acylium cations. Those ions readily attack benzene to form ketones.
Benzene reacts with alkanoyl (acyl) halides in presence of an aluminum halide to give 1-phenylketones (phenyl ketones). An example is preparation of 1-pheylethanone (acetophenone) from benzene and acetyl chloride, by using aluminum chloride as Lewis acid. 
Alkanoyl (acyl) chlorides are reactive derivatives of carboxylic acids. They react with Lewis acids to produce acylium cations. Acylium ion is sufficiently electrophilic to attack benzene by usual aromatic substitution mechanism. Because this ion (now a substituent of benzene ring) is electron-withdrawing, it deactivates the ring and protects it form further substitution. Effect is accentuated by formation of strong complex between aluminum chloride catalyst and carbonyl function of product ketone.
This complexation removes aluminum chloride from reaction mixture. Aqueous work-up is necessary to liberate ketone from its aluminum chloride complex.
In summary, problems encountered with Friedel-Crafts alkylation are avoided in Friedel-Crafts alkanoylation, in which an alkanoyl halide or carboxylic acid is reaction partner, in presence of a Lewis acid. Intermediate acylium cations undergo electrophilic aromatic substitution to obtain resultant aromatic ketones.
A diagram summarizing both reactions:-
The Lewis Acid:
Aluminum chloride has a central aluminum atom which has only 6 electrons; like boron in acidic BF3, it is very electron-deficient. It is one of the stronger and more common Lewis acids. In presence of alkyl halides or acyl halides, it can remove halide to generate a carbocation and AlCl4-. Carbocation will react with whatever is available, like an aromatic ring. With carbons that are less able to support a positive charge, aluminum chloride loosens the C-Cl bond enough to make carbon positive enough to react with aromatic ring (and to rearrange as if it were a carbocation). Aluminum chloride is powerful dehydrating agent; partially hydrated (to reduce its acidity) it works well as dehydrating agent – the antiperspirant aluminum chlorhydrate.
Ferric chloride is the catalyst of choice for halogenating benzene rings. Reaction proceeds like that of alkyl halides with aluminum chloride, namely polarization to remove a halide to form FeCL4- and generating Cl+. A way to do this reaction is just to add powdered iron to reaction and an excess of halogen; halogen oxidizes Fe to FeX3 which then catalyzes reaction.
Pravesh Jain
12 Points
6 years ago
Because during alkylation polysubstituted are also formed. These additional products decrease the yield of our desired product. Moreover , carbo cation rearrangement also takes place.l, which further decreases the yield. During acylation, the electron density on ring is decreased and hence polysubstituted products are not possible. And since rearrangement is also not possible , acylation results in much better yield.

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