FRIEDEL CRAFT REACTION

Friedel-Crafts alkylation

The Friedel-Crafts reactions, discovered by French alkaloid chemist Charles Friedel and his American partner, James Crafts, in 1877, is either the alkylation or acylation of aromatic compounds catalyzed by a Lewis acid. They are very useful in the lab for formation of carbon-carbon bonds between an aromatic nucleus and a side chain.

Source of electrophile

Friedel-Crafts alkylation is an example of electrophilic substitution in aromatic compounds. The electrophile is formed in the reaction of an alkyl halide with a Lewis acid. The Lewis acid polarizes the alkyl halide molecule, causing the hydrocarbon part of it to bear a positive charge and thus become more electrophilic.

CH₃—Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻

or

CH₃Cl + AlCl₃ → CH₃^δ+Cl⁺Al⁻Cl₃

(The carbon atom has a slight excess of positive charge, as the electronegative chlorine atom draws electron density towards itself. The chlorine atom has a positive charge, as it has formed a sub-ordinate bond with the aluminium atom. In effect, the Cl atom has lost an electron, while the Al atom has gained an electron. Therefore, the Al atom has a negative charge.)

Mechanism of alkylation

The polarized, electrophilic molecule then seeks to saturate its electron deficiency and forms a π-complex with the aromatic compound that is rich in π-electrons. Formation a π-complex does not lead to loss of aromaticity. The aromaticity is lost however in the σ-complex that is the next stage of reaction. The positive charge in the σ-complex is evenly distributed across the benzene ring.

C₆H₆ + CH₃⁺ → C₆H₆⁺Br → C₆H₅Br + H⁺

The σ-complex C₆H₆⁺Br can be separated (it is stable at low temperatures), while the π-complex can not.

RestrictionS

• Deactivating functional groups, such as nitro (-NO₂), usually prevent the reaction from occurring at any appreciable rate, so it is possible to use solvents such as nitrobenzene for Friedel-Crafts alkylation.

• Primary and secondary carbocations are much less stable than tertiary cations, so rearrangement typically occurs when one attempts to introduce primary and secondary alkyl groups onto the ring. Hence, Friedel-Crafts alkylation using n-butyl chloride generates the n-butylium cation, which rearranges to the t-butyl cation, which is far more stable, and the product is exclusively the t-butyl derivative. This may, in some cases, be circumvented through use of a weaker Lewis acid.

• The Friedel-Crafts reaction can not be used to alkylate compounds which are sensitive to acids, including many heterocycles.

• Another factor that restricts the use of Friedel-Crafts alkylation is polyalkylation. Since alkyl groups have an activating influence, substituted aromatic compounds alkylate more easily than the original compounds, so that the attempted methylation of benzene to give toluene often gives significant amounts of xylene and mesitylene. The usual workaround is to acylate first (see the following sections) and then reduce the carbonyl group to an alkyl group.

Friedel-Crafts acylation

Friedel-Crafts acylation, like Friedel-Crafts alkylation, is a classic example of electrophilic substitution.

Source of electrophile]Reacting with Lewis acids, anhydrides and chloranhydrides of acids become strongly polarized and often form acylium cations.

RCOCl + AlCl₃ → RC⁺O + AlCl₄^-

Mechanism of acylation                                                                                                                      The mechanism of acylation is very similar to that of alkylation.

C₆H₆ + RC⁺O → C₆H₆—CO—R + H⁺

The ketone that is formed then forms a complex with aluminum chloride, reducing its catalytic activity.

C₆H₆—CO—R + AlCl₃ → C₆H₆—C⁺(R)—O—Al⁻Cl₃

Therefore, a much greater amount of catalyst is required for acylation than for alkylation.

QUESTIONS  RELATED WITH TOPIC :

Which is most reactive in electrophilic substitution?

a) 

b) 

c) 

d) 

Question 2

Which is least reactive in electrophilic substitution?

a) 

b) 

c) 

d) 

Question 3

Which gives a meta nitro compound as the main product upon nitration with a nitric acid-sulfuric acid mixture?

a) 

b) 

c) 

d) 

Question 4

Which is most reactive in electrophilic substitution?

a) 

b) 

c) 

d) 

Question 5

Which is least reactive in electrophilic substitution?

a) 

b) 

c) 

d) 

Question 6

Which gives a meta nitro compound as the main product upon nitration with a nitric acid-sulfuric acid mixture?

a) 

b) 

c) 

d) 

Question 7

Which will be the main product upon chlorination of m-nitrotoluene with Cl2/AlCl3?

a) 

b) 

c) 

d) 

Question 8

Which is obtained as the main mononitration product upon reaction of m-t-butylanisole (1-t-butyl-3-methoxybenzene) with HNO3-H2SO4?

a) 

b) 

c) 

d) 

Question 9

Which of the following statements regarding Friedel-Crafts reactions is wrong?

a) Alkylation of benzene with an alkyl chloride requires only a catalytic amount of a Lewis acid such as aluminum chloride.

b) Alkylation of benzene with an alcohol requires only a catalytic amount of a Brønsted acid such as phosphoric acid.

c) Acetylation of benzene with acetyl chloride requires only a catalytic amount of a Lewis acid.

d) Acetylation of benzene with acetic anhydride requires more than one equivalent of a Lewis acid.

Question 10

Which of the following statements regarding electrophilic aromatic substitution is wrong?

a) Acetyl and cyano substituents are both deactivating and m-directing.

b) Alkyl groups are activating and o,p-directing.

c) Ammonio groups are m-directing but amino groups are and o,p-directing.

d) Chloro and methoxy substituents are both deactivating and o,p-directing.

Question 11

Which is most reactive in electrophilic substitution?

a) 

b) 

c) 

d) 

Question 12

Which is least reactive in electrophilic substitution?

a) 

b) 

c) 

d) 

Question 13

Which gives a para nitro compound as the main product upon nitration with a nitric acid-sulfuric acid mixture?

a) 

b) 

c) 

d) 

Question 14

Which gives a meta nitro compound as the main product upon nitration with a nitric acid-sulfuric acid mixture?

a) 

b) 

c) 

d) 

Question 15

Which of (a)-(d) does not give isopropylbenzene as a product upon reaction with benzene?

a) (CH3)2CHCl/AlCl3

b) CH3CH2CH2Cl/AlCl3

c) CH3CH=CH2/H3PO4

d) (CH3)2C=CH2/H3PO4

Question 16

Which combination of reagents used in the indicated order with benzene will give m-nitropropylbenzene?

a) 1) HNO3/H2SO4, 2) CH3CH2CH2Cl/AlCl3

b) 1) CH3CH2CH2Cl/AlCl3, 2) HNO3/H2SO4

c) 1) CH3CH2COCl/AlCl3, 2) HNO3/H2SO4, 3) H2NNH2/NaOH

d) 1) HNO3/H2SO4, 2) CH3CH2COCl/AlCl3, 3) H2NNH2/NaOH

Question 17

Which pair of compounds are the most probable main products of the following reaction?
 

a) 

b) 

c) 

d) 

Question 18

Which pair of compounds are the most probable main products of the following reaction?
 

a) 

b) 

c) 

d) 

Question 19

Which will be the main product upon bromination of phenyl benzoate with Br2/AlBr3?

a) 

b) 

c) 

d) 

Question 20

Which of the following statements regarding electrophilic aromatic substitution is wrong?

a) Sulfonation of toluene is reversible.

b) Friedel-Crafts alkylation of benzene can be reversible.

c) Friedel-Crafts alkylation with primary alkyl chloride may involve rearrangement.

d) Friedel-Crafts acylation of nitrobenzene readily gives a meta substitution product.