The key difference between Friedel Crafts acylation and alkylation is that the Friedel Crafts Acylation involves the acylation of an aromatic ring whereas the Friedel Crafts Alkylation involves the alkylation of an aromatic ring. Furthermore, multiple reactions and carbocation rearrangements do not occur in acylation reactions whereas they may occur in alkylation reactions.
Friedel Crafts reactions are a set of reactions that we can use to attach substituent groups to aromatic rings and to remove these substituent groups from aromatic rings. The scientists Charles Friedel and James Crafts developed these reactions in 1877. There are two main types of this reaction, and they are Friedel Crafts acylation reaction and Friedel Crafts alkylation reaction. Both these are electrophilic substitution reactions.
CONTENTS
1. Overview and Key Difference
2. What is Friedel Crafts Acylation
3. What is Friedel Crafts Alkylation
4. Side by Side Comparison – Friedel Crafts Acylation vs Alkylation in Tabular Form
5. Summary
What is Friedel Crafts Acylation?
Friedel Crafts acylation involves the attachment of an acyl group to an aromatic ring. This addition of an acyl group is “acylation” in organic chemistry. The typical acylating agents used for this reaction are acyl chlorides. Further, the catalysts enhance the progression of these reactions. The common catalysts include acids and aluminum trichloride.
Figure 01: Benzene Friedel Crafts Acylation
Moreover, we can do this reaction using acid anhydrides. There are advantages of this reaction over alkylation. One is, multiple acylations does not occur at the same time. It is because the ketone product of the reaction is always less reactive than the original molecule due to the electron-withdrawing effect of the carbonyl group. Moreover, there are no carbocation rearrangements during the progression of the reaction. The carbonium ion that forms during the reaction stabilizes by resonance structures.
What is Friedel Crafts Alkylation?
Friedel Crafts alkylation involves the attachment of alkyl groups to an aromatic ring. It is an electrophilic substitution reaction. We can use alkyl halides for this reaction along with a strong Lewis acid catalyst. If we use ferric chloride (FeCl3) as the catalyst, it attaches to the chloride ion of alkyl chloride and forms the alkyl cation. This alkyl cation can then attach to the aromatic ring. The major disadvantage of this reaction is that the product of the reaction is more nucleophilic than the reactant. Therefore, this may cause overalkylation.
Figure 02: Benzene Friedel Crafts Alkylation
Moreover, the reaction is suitable for tertiary carbon in the aromatic ring. Otherwise, the carbocation may undergo rearrangements. There is a hypothesis that the Friedel Crafts alkylation reactions are reversible. We call it Friedel Crafts de-alkylation.
What is the Difference Between Friedel Crafts Acylation and Alkylation?
Friedel Crafts Acylation involves the attachment of an acyl group to an aromatic ring. There are no multiple acylation reactions because the ketone product of the reaction is always less reactive than the original molecule due to the electron-withdrawing effect of the carbonyl group. Moreover, there are no carbocation rearrangements. In contrast, multiple alkylation reactions take place during Friedel Crafts alkylation which involves the attachment of alkyl groups to an aromatic ring. If the substitution takes place at a primary or secondary carbon, there are carbocation rearrangements as well.
Summary – Friedel Crafts Acylation vs Alkylation
Friedel Crafts reaction is a set of organic reactions that involves acylation and alkylation reactions. The difference between Friedel Crafts acylation and alkylation is that Friedel Crafts acylation reaction involves the acylation of an aromatic ring whereas Friedel Crafts alkylation involves the alkylation of an aromatic ring.
Reference:
1. “Friedel–Crafts Reaction.” Wikipedia, Wikimedia Foundation, 27 June 2018. Available here
2. “Friedel-Crafts Alkylation.” Organic Chemistry. Available here
Image Courtesy:
1.’Friedel-Crafts-acylation-overview’By Benjah-bmm27 – Own work, (Public Domain) via Commons Wikimedia
2.’Benzene Friedel-Crafts alkylation’By Krishnavedala – Own work, (Public Domain) via Commons Wikimedia