What is Acid Chloride?
Acid chloride has the molecular formula RCOCl, where R is a side chain. They are carboxylic acid reactive derivatives.
An acyl chloride (or acid chloride) is an organic compound composed of a chlorine atom attached to an acyl group in organic chemistry. An acyl group is a functional group defined by the molecular formula of RCO.
Thus, the acid chloride family is part of a larger organic family known as acyl halides.
Table of Contents
- Formation of Acid Chloride
- Naming Acid Chlorides
- Physical Properties
- Chemical Properties and Reactions
- Hazards
- Frequently Asked Questions- FAQs
Formation of Acid Chlorides
Laboratory method
Acid chlorides are formed when carboxylic acids react with thionyl chloride (SOCl2). The carboxylic acid’s hydroxyl group is converted to a chlorosulfite intermediate during the reaction, making it a better leaving group. The chloride anion formed as a result of the reaction acts as a nucleophile.
Industrial Method
- Acetic anhydride is reacted with hydrogen chloride to produce acetyl chloride in the industrial process.
(CH3CO)2O + HCl → CH3COCl + CH3CO2H
- Certain phosphorus chloride reagents, such as phosphorus trichloride or phosphorus pentachloride, can also be used to prepare acyl chlorides:
RCOOH + PCl5 → RCOCl + POCl3 + HCl
Naming Acid Chlorides
- Acyl chlorides are named by taking the name of the parent carboxylic acid and substituting -yl chloride for -ic acid where the acyl chloride substituent takes priority.
Thus:
| Carboxylic acid name | Acyl chloride name | Acyl chloride formula |
|---|---|---|
| ethanoic acid | ethanoyl chloride | CH3COCl |
| propanoic acid | propanoyl chloride | CH3CH2COCl |
| butanoic acid | butanoyl chloride | CH3CH2CH2COCl |
- When other functional groups predominate, acyl chlorides are referred to as prefixes- chlorocarbonyl.
For example- ClOCCH2COOH is named as (chlorocarbonyl)acetic acid
Physical Properties
1. Appearance- Lower acyl chlorides are colourless liquids with a strong odour.
2. Solubility- They dissolve in organic solvents like alcohol, ether, and chloroform.
3. Boiling points and Melting points- The boiling points and melting points of acyl chlorides are lower than those of the corresponding acids which is due to the lack of ability to form hydrogen bonds.
Chemical Properties and Reactions
Due to their high reactivity, acid halides can be easily converted into other acyl compounds via nucleophilic acyl substitution.
1. They are the most reactive of all carboxylic acid derivatives.
2. Through nucleophilic acyl substitution, acid chlorides react with carboxylic acids to form anhydrides.
3. Acyl chlorides can be easily hydrolyzed by water to produce the corresponding carboxylic acids.
4. Esters are formed when acid chlorides react with alcohol nucleophiles. This is the preferred method for producing esters. To remove the HCl produced, pyridine is frequently added to the reaction mixture.
5. Primary alcohols are produced by reducing acyl chlorides with lithium aluminium hydride and di-isobutyl aluminium hydride.
6. In the presence of palladium, they are reduced by hydrogen with BaSO4 to an aldehyde. This reaction is the Rosenmund Reaction.
7. Ammonia, 1° amines, and 2° amines react with acid chlorides to form 1°, 2°, and 3° amides respectively.
8. When Grignard reagents are added, acid halides are converted to 3° alcohols. During this reaction, the Grignard reagent adds to the carbonyl carbon twice.
9. When an acid chloride is reacted with a diorgano cuprate (Gillman) reagent (R2CuLi), an excellent yield of ketone product is produced.
Hazards
Since acyl chlorides are such reactive compounds, they are generally toxic and should be handled with extreme caution. They are lachrymatory chemicals because they can react with water on the eye’s surface, producing hydrochloric and organic acids that are irritating to the eye. Inhaling acyl chloride vapours can cause similar problems.
Uses of Acid chlorides
Acid chlorides are reactive chemical species by nature, and they are used as key building blocks in a wide range of downstream applications. Some of the uses are as follows-
- Reagents for chemistry
- Pharmaceutical manufacturing
- Synthesis of organic compounds
- Peroxides of organic origin
- Production of agrochemicals
- Pigments and plastics
Frequently Asked Questions on Acid Chloride
How is acid chloride formed?
Acid chlorides are formed when carboxylic acids react with thionyl chloride (SOCl2). The carboxylic acid’s hydroxyl group is converted to a chlorosulfite intermediate during the reaction, making it a better leaving group. The chloride anion formed during the reaction functions as a nucleophile.
What are examples of acid chlorides?
Some examples of acid chlorides are ethanoyl chloride- CH3COCl, propanoyl chloride- C2H5COCl, butanoyl chloride- C3H7COCl, benzoyl chloride, etc.
Are acid chlorides toxic?
Acyl chlorides are generally toxic, and special precautions must be taken when handling them. They can react with water on the eye’s surface, producing hydrochloric and organic acids that causes irritation to the eye.
Are acid chlorides acidic?
Since there is no H+ to donate, anhydrous acid chloride is neutral. However, in an aqueous medium, acid chloride can react with water to form carboxylic acid and HCl.
Is it more acidic to use acyl chlorides or carboxylic acids?
Since acyl chlorides have a good leaving group attached to the carbonyl carbon, they are more reactive than carboxylic acids or carboxylic esters.
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