Friedel Crafts Acylation Alkylation

Introduction

Friedel-Crafts reaction is where an alkyl or acyl group swaps out the hydrogen atom of an aromatic molecule to create a hydrocarbon or ketone. An acid catalyst, including

$$\mathrm{AlCl_3, BF_3, ZnCl_2, or\: FCl_3, is}$$

present when the aromatic molecule is alkylated or acylated. An alkyl or acyl cation, the attacking particle, is produced by the catalyst. J. Crafts and C. Friedel invented this reaction in 1877–1878 and used it to link substituents to aromatic rings.

It should be noted that the hydrogen atom that was initially linked to the aromatic ring is exchanged with an electrophile during the alkylation and acylation procedures. Aluminium trichloride is the most widely utilised catalyst because it serves as an acid (Lewis) by combining with chlorine to create a powerful electrophile.

What is Friedel crafts reaction?

Friedel-Crafts reactions are organic coupling reactions that involve an electrophilic aromatic substitution, to attach substituents to aromatic rings. A new C-C bond forms during the typical Friedel–Crafts reaction. Friedel-Crafts processes can be classified as either acylation or alkylation. The benzene ring is added through an alkylation process along with a short carbon chain. Aryl ketones are produced when an acyl group is added during acylation.

What is its type

The two main categories of Friedel-Crafts processes are −

• Friedel craft acylation

• Friedel craft alkylation.

Explain Friedel craft alkylation

When an aromatic ring is present, Friedel Crafts alkylation involves treating an alkyl halide with a Lewis acid. While the C-H link is being broken, the alkyl group joins the ring and forms a C-C bond. In this reaction, a Lewis acid catalyst, such as FeCl₃ or AlCl₃, is used to speed up the elimination of the halide and create a carbocation. Before beginning the alkylation reaction, the resultant carbocation passes through a rearrangement. Alkenyl, alkynyl, or aryl halides should never be utilised; only an alkyl halogen (Cl,Br,or I) should be used; otherwise, the reaction will not succeed. These species' carbocations are challenging to make and very unstable.

High-octane fuels, surfactants, fragrances, antioxidants, etc., as well as other important goods like cumene and thymol are primarily produced in the industry using this method.

Mechanism of Friedel craft alkylation

STEP 1 − An electrophilic carbocation is created as a result of the Lewis acid catalysts (AlCl₃) interaction with the alkyl halide.

STEP 2 − To assault the aromatic ring, the carbocation first forms a cyclohexadienyl cation. Because of the broken C-C double bond, the arena briefly loses its aromaticity.

STEP 3 − The molecule regains its aromaticity as a result of the intermediate's deprotonation, which causes the C-C double bond to rebuild. The AlCl₃ The catalyst is renewed when this proton moves on to create HCl

Explain Friedel craft acylation

This synthesis of monoacylated compounds results from this aromatic electrophilic substitution reaction that involves arenes and acyl halogen or anhydrides when a Lewis acid catalyst like AlCl₃ is present. The acyl halide's halogen joins the Lewis acid in a complex to develop a superior electrophilic acylium cation (RCO⁺), which becomes stable through resonance. Ketones are the only products that this process can create.

An essential procedure for business is the Friedel Crafts acylation. Fine chemicals, synthetic intermediates, and chemical feedstock are all made with it.

Mechanism of Friedel craft acylation

STEP 1 − A Lewis acid catalyst and the acyl halide interact, causing a complex to develop and the acyl halide to lose some of its halide ion. Resonance causes the formation of an acylium ion, which becomes stable.

STEP 2 − The acylium ion (RCO⁺) then attacks the aromatic ring electrolytically. The aromaticity of the ring temporarily disappears as a complex form.

STEP 3 − The aromaticity of the ring has been recovered upon deprotonation of the intermediate complex. Using this proton and the chloride ion from the complex Lewis acid, HCl is created. It has now been possible to renew the AlCl₃ catalyst.

Conclusion

The process of replacing an aromatic proton with an alkyl group is known as "Friedel-Crafts alkylation." This is achieved by conducting an electrophilic attack on the aromatic ring using a carbocation. The Friedel-Crafts alkylation method, which results in alkylbenzenes, uses alkyl halides as reactants. Aryl ketones are produced when an acyl group is added during Friedel Crafts acylation.

There are a number of benefits to Friedel Crafts acylation above Friedel Craft alkylation. The ability to better regulate reaction products is one of these benefits, as is the acylium cation's stability through resonance, which eliminates the possibility of rearrangement. The produced ketones can be converted to alkyl groups using the Clemmensen reduction process.

FAQs

1. What constitutes Friedel Craft alkylation's primary flaw?

Since electron-donating substituents are formed during Friedel-Crafts alkylation, the resulting alkyl groups are more vulnerable to electrophilic attack than the starting material. This is a major letdown in terms of synthetic applications.

2. Why isn't aniline subject to Friedel-Crafts RXN?

AlCl₃, a Lewis acid catalyst used in the Friedel-Crafts process, and aniline combine to create salt. The positive charge that aniline's nitrogen gets makes it a potent deactivate group for subsequent reactions.

3. Why does the Friedel-Crafts process need anhydrous AlCl₃?

Since AlCl₃ is an electron-deficient molecule, the Friedel-Crafts procedure employs anhydrous AlCl₃. It's a Lewis acid. AlCl₃ absorbs Cl and changes into AlCl₄^-. The substance that Cl is accepted from develops into an electrophile.

4. The Friedel-Crafts reaction does phenol produce?

Phenols are capable of being Friedel-Crafts alkylation. The finest reagents are those that can produce the electrophile without needing Lewis acids. For Friedel-Crafts acylation on phenols, more difficult conditions, including high temperature, are needed. With AlCl₃ The phenol forms a compound, which reduces its activity.

5. Does AlCl₃ abide by the octet rule?

Drawing the covalent structure of AlCl₃ reveals that Al has 6 valence electrons instead of 8, making it an electron-deficient compound that cannot adhere to the octet rule. Additionally, it is a lewis acid

6. In a Friedel-Crafts acylation, which ion is produced?

In a Friedel-Crafts acylation Acylium ion is produced. A complex between both the Lewis acid and the acid chloride's chlorine atom is formed during the Friedel-Crafts acylation reaction. The complex's C-Cl bond breaks down to produce an acylium ion. Positively charged and resonance stabilised, the acylium ion has a carbon atom.

7. Is FeCl₃ compatible with the Friedel-Crafts reaction?

Anhydrous FeCl₃ and FeCl₃ −graphite catalysts have both been studied in the Friedel-Crafts alkylation of benzene with tert-butyl chloride. The reaction rate is slowed down by the FeCl₃ graphite. Due to this, the transformation becomes more selective, which has resulted in a decline in disubstitution.