What’s about Acid Chloride? Acyl Chloride

This blog will provide a piece of great information about the manufacturing, properties as well as reactivity of Acid Chloride. Acid Chloride is also called acyl chloride. Functional group -COCl present in any organic compound is referred to as an acid chloride or acyl chloride. After understanding the concept of acid chloride we will understand what does acyl group means??

Acyl group refers to the group containing RCO. In this general form, “R” may be any functional group that can be methyl, ethyl, propyl, etc. Acetyl chloride is the perfect example of acid chloride (CH3COCl). Acid chlorides are organic compounds that have no hydrogen bonding between them. Acid chlorides are used to prepare acid anhydrides, amides, and esters.

Acid Chloride
A general formula for Acid chloride

What are the properties that Acid Chloride possesses?

Acid Chloride may possess many unique properties. The addition of a more branched acyl group can cause a change in many properties:

  • Acid Chlorides are highly corrosive and flammable.
  • It is pale yellow and fuming liquid with a pungent smell.
  • Acid chlorides never form a hydrogen bond with other compounds.
  • This absence of a Hydrogen bond affects the melting point & boiling point of this compound. The bond formed between highly electropositive hydrogen and electronegative atom.
  • For example, the difference between the melting point and boiling point of an acyl chloride is less as compared to pure carboxylic acid. The boiling point of acetic acid is 118℃ but the boiling point of acetyl chloride is 51℃.
  • Some acyl chlorides did not stable at room temperature but can be prepared at -60℃.
  • Acyl chloride did not dissolve in water but when they add up in water they form carboxylic acid and HCl.

Is HCl an Acid Chloride?????

Basically, HCl is an Arrhenius acid. In an aqueous medium, HCl gives a proton. But acid chloride is basically an organic compound. It is a derivative of any weak organic acid. We cannot relate it with HCl. Acid chloride does not mean that the acid with any halogen group attached to it. By the definition of acid chloride, it is noticed that:

The acid chloride is also called acyl chloride and the acyl chloride group is -RCOCl group having a chloride with it.

HCl + H2O → H3O+ + Cl

So, it is a strong acid, not an acid chloride.

How can we synthesize Acyl Chloride??

Synthesizing acid chloride is important. We cannot prepare acid chloride with only one method. We can prepare it by many methods, i.e. industrial method, and laboratory method.

1. Industrial method:

  • In industry, we will prepare it by reaction of HCl with the acetic anhydride. Interaction of HCl with the weak organic anhydride will produce acid chloride.

(CH3CO)2O + HCl → CH3COCl + CH3COH2

  • Manufacturing or synthesis of acid chloride will also be possible when propanoic acid is chlorinated with phosgene. Their reaction causes to make the proponyl chloride (acid chloride).

CH3CH2CO2H + COCl2 → CH3CH2COCl + CO2 + HCl

  • Hydrolysis of Benzotrichloride will also synthesize acid chloride. If we react Benzotrichloride with water, benzoyl chloride is formed which is basically an acyl chloride. (Acid chloride).

C6H5CCl3 + H2O → C6H5COCl + 2HCl

Acid Chloride
Acyl Chloride

2. Laboratory methods:

In the laboratory, we can prepare acid chloride in many ways. However, we are going to explain the preparation of acid chloride from thionyl chloride and phosphorus chloride.

From Thionyl Chloride:

As we produce/synthesized the acid chloride in industry. We can produce it in the laboratory as well. When carboxylic acid get interacts with the thionyl chloride (SOCl2) and forms an acid chloride.

When this reaction the hydroxyl part of the carboxylic acid is replaced by the chlorosulfite intermediate. This will act as a good leaving group and chloride may act as an excellent nucleophile so it attacks chlorosulfite intermediate and forms acid chloride.

The general reaction to this preparation is given below:

Acid Chloride

Further, we will discuss its mechanism in detail: The mechanism of this process comprises the following five steps:

  1. Attack of chlorine nucleophile on thionyl chloride
  2. Formation of good leaving group and its removal.
  3. Attacking of nucleophile (Nucleus lover) on a carbonyl.
  4. Again removal of leaving group.
  5. Deprotonation from chlorosulfite.
Acid chloride
1. Attack on thionyl chloride
Acid chloride
2. Formation of good leaving group and its removal
Acid chloride
3. Attacking of nucleophile (Nucleus lover) on the carbonyl
Acid chloride
4. Again removal of leaving group.
Acid chloride
5. Deprotonation

Synthesis by Phosphorus chloride

Phosphorus trichloride and phosphorus pentachloride are used as a manufacturer of acid chloride. But the most common is phosphorus pentachloride. The reaction also occurs if carboxylic acid gets reacts with phosphorus pentachloride.

RCO2H + PCl3 → RCOCl + HCl

  • The formation of HCl in every reaction is not necessary. So, we can prepare acid chloride by using an Appel reaction.

RCO2H + Ph3P + CCl4 → RCOCl + Ph3PO + HCCl3

Reactivity of Acid Chlorides

Acid chlorides or acyl chloride generally give a nucleophilic reaction. The reaction in which a nucleophile may attack the atom in order to make a good leaving group and to form a complete reaction is called a Nucleophilic reaction.

Why Acid Chloride is more reactive than amides??

Acid Chlorides are extremely reactive. The Reactiveness of any compound can be measured by the number of the good leaving groups. An increase in the number of leaving groups causes vacant space in a compound so, nucleophile has a great chance to attack the surface of the compound.

The nature of leaving group may depend upon the electronegative atom. An increase in the electronegativity of an atom causes fewer chances for a leaving group to leave because an electronegative atom loves the electron and holds it tightly. So, the question of the answer is cleared by the following relation:

Reactivity ∝ No. of leaving groups ∝ 1/electronegative atom

By the following reactivity order:

Acid chloride

Hence, it is noticed that acyl chloride has greater reactivity due to fewer electronegative atoms. Therefore, we can explain all these compounds’ reactivity on the basis of the electronegativity parameters.

Risks of Acid Chlorides:

Inhalation of Acid chlorides can cause irritation in the nose and throat. Its exposure to the skin can cause damage to the skin. Less exposure to the lungs causes coughing and pain in the lungs but higher exposure can cause pulmonary edema. Acid chlorides are involved in high corrosiveness and are reactive. Handle it carefully as it acts as an explosion Hazard.

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