Friedel-Crafts Reaction

Imagine standing amidst a running industrial site where perfume, plastic, and pharmaceutical manufacturing is underway. The chemistry behind the making of these very ordinary products involves extremely complicated reactions of simple molecules into complex ones. One such basic reaction in Organic Chemistry is the Friedel–Crafts reaction. It is named after the French chemist Charles Friedel and the American chemist James Crafts.

This really forms the backbone of many industrial processes and hence has acquired immense importance both academically and practically. In this article, we are going to get a closer look at the details regarding the Friedel-Crafts reaction. There are two major categories: Friedel-Crafts Alkylation and Friedel-Crafts Acylation. We are going to take a look at crosswise features and uses of Friedel-Crafts Acylation and notice how often it crowns up to many features.

Knowing and Understanding the Friedel Crafts Reaction

Friedel-Crafts reaction is basically a common reaction in organic chemistry and attaches substituents onto an aromatic ring. There are two major forms this reaction can take: Friedel-Crafts Alkylation and Friedel-Crafts Acylation. That is, the general mechanism involves the formation of an extremely reactive electrophilic reagent, which reacts with the aromatic ring to give a substitution for one of the hydrogen atoms by the electrophile, as indicated above.

In the Friedel-Crafts alkylation process, any alkyl halide inserts an alkyl group into the ring of the aromatic compound. The usual catalyst used for such reactions involves a strong Lewis acid, usually aluminum chloride, AlCl3. Such alkylation makes it possible to build up carbon-carbon bonds and has been quite useful in the synthesis of a myriad of organic compounds.

Instead, the Friedel-Crafts acylation is the process by which an atom group is incorporated into the aromatic ring. The reaction makes use of an acyl chloride and a Lewis acid catalyst is necessary in its reaction. Since this acylation does not happen through carbocation rearrangement, most of the dilemmas faced by the Friedel-Crafts alkylation reaction are not suffered by this acylation process and as a result, it is way more controlled and certain.

This reaction allowed for the formation of alkyl benzenes from alkyl halides. The reactivity of haloalkanes increases as we move up the periodic table and increase polarity. This means that an RF haloalkane is most reactive followed by RCl then RBr and finally RI. This means that the Lewis acids used as catalysts in Friedel-Crafts Alkylation reactions tend have similar halogen combinations such as$\mathrm{BF}_3, \mathrm{SbCl}_5, \mathrm{AlCl}_3, \mathrm{SbCl}_5$, and $\mathrm{AlBr}_3$, all of which are commonly used in these reactions.

For example:

Mechanism

1. In this step a carbocation is formed that acts as the electrophile in the reaction. This step activates the haloalkane. Secondary and tertiary halides only form the free carbocation in this step.

2. In this step, an electrophilic attack on the benzene occurs which results in multiple resonance forms. The halogen reacts with the intermediate and picks up the hydrogen to eliminate the positive charge. The aromaticity is regained by the substrate after this step.

Aspects of Friedel-Crafts Acylation

Such synthetic processes are favored for the specificity and stability of the products formed. Generally, the scheme of acylation occurs through the formation of an acylium ion, RCO+, which is formed through the reaction of the acyl chloride with the Lewis acid catalyst. This acylium ion will further work as a very strong electrophile that is able to attack the aromatic ring to give a ketone product through substitution.

One of the most important factors of Friedel-Crafts acylation is the preparation of aromatic ketones, very important and versatile ideal products in syntheses of a chemical nature. Acetophenone forms when acetic chloride is acylated by benzene.

Therefore, conditions are mutable during the reaction so that different substrates are accepted by the reaction and different products are made. For instance, different acyl chlorides can be used so that the different functional groups can be attached to the aromatic ring by it; thus, it was found to be one of the most versatile reactions. In addition, higher selectivity in the process of acylation can be realized through optimization in terms of appropriate solvents and temperatures for a better yield.

Relevance and Applications of Friedel–Crafts Acylation

The Friedel-Crafts acylation reaction has been a very important reaction both in the industrial setting and academic situation. Some aromatic ketones synthesized via acylation take pivotal positions in the pharmaceutical field for the manufacture of drugs. Indeed, the synthesis of paracetamol begins with an initial step of Friedel-Crafts acylation to form one of its major intermediates; thus, the present reaction becomes very essential in drug processing. The Friedel-Crafts acylation of aromatic polymers has placed materials science at the forefront of modifications. Most of the aromatics find applications in advanced materials, with diverse functional groups that are towards increasing solubility, enhancing thermal stability, and improving mechanical strength.

It is an academically important reaction in the sense that it forms part of the example lesson covered in all courses in organic chemistry, usually one used to explain different principles behind the electrophilic substitution of aromatic compounds. In most cases, the Friedel-Crafts acylation reaction provides a background of many other reactions or processes.

For example:

Recommended topic video on (Friedel-Crafts Reaction)

Some Solved Examples

Example 1
Question: In the Friedel-Crafts reaction, benzene is:
1. Alkylated
2. Hydrolysed
3. Hydrogenated
4. Dehydrogenated

Solution: In the Friedel-Crafts reaction, benzene is either alkylated or acylated depending upon the reagent. The correct answer is option (1) Alkylated.

Example 2
Question: Which of these will produce the highest yield in Friedel Crafts reaction?

1)

2) (correct)

3)

4)

Solution:

As we have learned,

Phenol does not undergo Friedel Craft Alkylation or Acylation. This is because oxygen's lone pair in phenol makes the coordinate bond with AlCl₃ (A Lewis acid) hence blocking it. The same is the case with Aniline. These are not preferred as substrates for Friedel Crafts Reactions.

Haloarenes undergo the usual electrophilic reactions of the benzene ring such as Friedel-Crafts reactions. Halogen atom besides being slightly deactivating is o, p-directing; therefore, further substitution occurs at the ortho- and para-positions concerning the halogen atom.

Hence, the option number (2) is correct.

Example 3
Question: The increasing order of reactivity of the following compounds towards aromatic electrophilic substitution reaction is :

1) $D<A<C<B$
2) $B<C<A<D$
3) $A<B<C<D$
4) $D<B<A<C$

Solution

Order of strength for electrophilic substitution

$
+M>+I>-I>-M
$

So, $D<A<C<B$

Summary

The Friedel-Crafts processes include alkylation and acylation processes, among others. We will concentrate herewith on Friedel-Crafts acylation, which is a quite selective and diversified process using the introduction of an acyl group on the aromatic ring to produce highly stable aromatic ketones, widely enduring, and esteemed in many ways. This reaction has enormous significance in lots of areas, from the manufacturing of pharmaceuticals to materials science, and is of prime pedagogical value for courses on organic chemistry.