Acylation, also sometimes called alkanoylation, is a chemical process wherein an acyl group is added to a compound or molecule, to be more precise. The acyl group is provided by a compound which is called the acylating agent. In this reaction, acyl halides are mainly used as the acylating agents. This is because acyl halides form a strong electrophile when they are treated with some metal catalysts.
In any case, the two most powerful reactions in Organic Chemistry are the alkylation reaction and acylation reaction. Here, we will be dealing with the topic of “acylation reaction” and understand a few things about it.
The mechanism of the Acylation reaction is based on the electrophilic aromatic substitution. As stated above, in Organic Chemistry, acylation is the process or mechanism of adding an acyl group (RCO-) to a particular compound. The reaction basically involves substitution by a nucleophile (electron donor) at the electrophilic carbonyl group (C = O) of a carboxylic acid derivative. The substitution usually proceeds by an addition-elimination reaction.
While there are several acylating reagents the two common reagents that bring about acylation are acid halides (X = Cl, Br) and anhydrides (X = OCOR). The nucleophiles in the acylation reaction are alcohols (ROH) or phenols (ArOH). Both of them give rise to esters, ammonia or amines. In most cases, acyl halides are widely used as acylating agents since these form strong electrophiles when added with some metal catalysts.
Friedel-Crafts Acylation Reaction
Let us take the example of the Friedel-Crafts acylation. This reaction uses the acetyl chloride, CH3COCl, as the acylating agent and the compound called aluminium chloride, AlCl3 as the catalyst. In Friedel-Crafts acylation, there is an addition of an acyl group to an aromatic ring or in simple terms ethanol or the acetyl group is added to the benzene. The following reaction shows the mechanism.
In a Friedel-Crafts acylation reaction, the aromatic ring is transformed into a ketone.
An acyl group is the alkyl group that is attached to the carbon-oxygen double bond as shown in the below figure. If the letter “R” represents the alkyl group, the acyl group will have the formula RCO-. The process of Acylation is to substitute an acyl group into the reaction. The most widely used acyl group is the CH3CO-. It is called the ethanoyl group.
The acyl halides and the anhydrides of the carboxylic acids are commonly used as the acylating agents for acylating amines forming amides or also acylate alcohols which form the esters. The alcohols and amines are nucleophiles; this mechanism is a kind of nucleophilic acyl substitution reaction. Succinic acid is commonly used in various types of acylation reactions. Acylation occurs when two or more salts of succinic acid add to form a single compound.
The acylation process is used in preventing the rearrangement reactions which occur in the process of alkylation. To achieve this, the acylation reaction is usually performed. Later the carbonyl group is removed by the Clemmensen reduction reaction or some similar process.
Read More: Friedel-Crafts Acylation and Alkylation
Acylation Reaction Example
A simple kind of Acylation reaction example is given below;
The acetic acid or the CH3COOH, vinegar, which is 5 per cent of the acetic acid in water are present in the form of their chloride. This is the acetyl chloride, having a chemical structure CH3-COCl.
The process of Acylating the benzene with such a substance in the presence of the catalyst aluminium chloride gives this reaction. It is as shown below.
C6H6 + CH3-COCl + catalyst ? C6H5-CO-CH3 + HCl
The above sequence of reactions explains that one molecule of benzene and one molecule of acetyl chloride in the presence of some aluminium chloride catalyst is said to produce one molecule of the methyl phenyl ketone and one molecule of the hydrogen chloride gas. The ketones are of great importance as compounds offering a variety of reaction paths for the additional synthesis process.
Alternatively, acylation is a very important reaction for primary and secondary amines. In this case, a hydrogen atom is replaced by an acyl group. Reagents that are used in this case are basically anhydrides ((RCO)2O), acid chlorides (RCOC1, RSO2C1) or even esters (RCOOR′). The products obtained are mainly amides of the corresponding acids.