Acid Chloride Functional Group

Core Concepts

This article serves as a guide to the acid chloride functional group in organic chemistry. After reading this article, you will be able to understand and describe the naming conventions of acid chlorides, how they are synthesized, the chemical reactions they undergo, and their uses and applications.

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Acid Chlorides Overview

The acid chloride (or acyl chloride) functional group in organic chemistry is derived from the carboxylic acid functional group. Known for their versatility and unique reactivity, acid chlorides are essential for various chemical reactions and industrial applications.

It is also worth noting that acid chlorides are very similar to other common functional groups such as acid anhydrides, aldehydes, and ketones. For example, all of them contain a carbonyl group. However, properties such as reactivity are different due to the chlorine atom of acid chlorides. Chlorine is highly electronegative which makes the carbonyl carbon attached to the chlorine atom strongly positive. This means that acid chlorides are more open to nucleophilic attack and therefore more reactive than other carbonyl containing functional groups.

Quick Facts on Acid Chlorides

Structure: Almost a carboxylic acid group, but with Cl instead of OH
General formula: Acid chlorides have the general formula of RCOCl where R is a side chain.
Acidity: Neutral for anhydrous acid chlorides since there are no H⁺ ions to donate.
Solubility: Soluble in organic solvents such as alcohols and ethers but insoluble in water.
IR Spectroscopy: Acid chlorides show a strong C=O band at 1775-1810 cm^-1 and a C-Cl stretch from 550-730 cm^-1.

Acid Chloride Naming Conventions

Acid chlorides are named based on the specific carboxylic acids that they are derived from. This is specifically done by substituting the -ic acid prefix of the parent carboxylic acid with -yl chloride. For example, the name of the acid chloride derived from butanoic acid would be butanonyl chloride.

Basic rules for naming acid chlorides:

Select the longest carbon chain to begin naming.
Number from the carbonyl (C=O) carbon.
Add the suffix of -yl chloride to the parent carbon chain name.

Synthesis of Acid Chlorides

Acid chlorides are most commonly prepared by reacting carboxylic acids with thionyl chloride (SOCl₂). The hydroxyl group (OH) of the carboxylic acid is replaced with a chlorine atom (Cl) to form an acid chloride. The reaction mechanism starts with carboxylic acid acting as a nucleophile which attacks the thionyl chloride. Next, the chlorine atom (Cl) is removed as a leaving group which forms a chlorosulfite intermediate. The chlorine atom acts as leaving group and becomes a chloride anion (Cl^–) that then performs a nucleophilic attack on the carbonyl group. This is then followed by the removal of the thionyl chloride as a leaving group. The final step is deprotonation which forms the acid chloride product and hydrochloric acid (HCl) as a byproduct.

Additionally, it is worth noting that acid chlorides can also be prepared by reacting carboxylic acids with phosphorus chloride reagents such as phosphorus trichloride (PCl₃) and phosphorus pentachloride (PCl₅).

Reactions of Acid Chlorides

Acid chlorides are highly reactive organic compounds and therefore readily undergo a variety of reactions such as nucleophilic acyl substitution, Friedel-Crafts acylation, and esterification.

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution can convert acid chlorides into many other acyl compounds such as carboxylic acids, esters, ketones, aldehydes, alcohols, amides, and acid anhydrides. Examples of nucleophiles that react with acid chlorides include water, alcohols/phenols, and ammonia/amines.

The reaction mechanism for acid chloride nucleophilic acyl substitution involves nucleophilic attack, leaving group removal, and deprotonation.

Water and acid chlorides react to produce carboxylic acids:

Alcohols and acid chlorides react to produce esters:

Ammonia/primary amines/secondary amines and acid chlorides react to form amides:

Carboxylic acids and acid chlorides react to form acid anhydrides:

Friedel-Crafts Acylation

Acid chlorides react with benzene in Friedel-Craft acylation reactions. These reactions occur in the presence of an aluminum chloride catalyst (AlCl₃) and result in the production of aromatic ketones, comprised of a phenyl group attached to a carbonyl group. This reaction is important in organic chemistry since it is an effective way to attach hydrocarbon groups to benzene.

Esterification

Similar to carboxylic acids, acid chlorides can undergo esterification reactions, reacting with alcohols to form esters. A significant difference is that esterification with the use of acid chlorides makes the reaction irreversible.

The reaction mechanism for acid chloride esterification begins with nucleophilic addition where the alcohol acts as a nucleophile and attacks the carbonyl group of the acid chloride. This is then followed by elimination which involves leaving group removal and deprotonation.

Uses of Acid Chlorides

In organic chemistry, acid chlorides have the general use of synthesizing other organic compounds, as previously mentioned. Acid chlorides have several valuable industrial applications due to their versatility as an organic reagent. Examples include the synthesis of pharmaceuticals, agrochemicals, fragrances, pigments, and plastics.