What is the Difference Between Acetyl Chloride and Acetic Anhydride

The main difference between acetyl chloride and acetic anhydride is that acetyl chloride has a single chlorine atom, while acetic anhydride has an oxygen atom connecting two acetyl groups.

Acetyl chloride and acetic anhydride are organic compounds extensively used in various chemical processes and industries. These compounds have distinct characteristics and play important roles in organic synthesis.

Key Areas Covered

1. What is Acetyl Chloride  
     – Definition, Composition, Features
2. What is Acetic Anhydride
     – Definition, Composition, Features
3. Similarities Between Acetyl Chloride and Acetic Anhydride
     – Outline of Common Features
4. Difference Between Acetyl Chloride and Acetic Anhydride
    – Comparison of Key Differences

Key Terms

Acetyl Chloride, Acetic Anhydride

What is Acetyl Chloride

Acetyl chloride is an organic compound with the chemical formula CH₃COCl. It is a colorless liquid with a pungent odor. Acetyl chloride is an important reagent in organic synthesis. Acetyl chloride is a highly reactive compound due to the presence of a reactive acyl chloride functional group. It has a boiling point of around 51 degrees Celsius and a density of approximately 1.10 g/mL. Moreover, acetyl chloride is soluble in many organic solvents but reacts violently with water, undergoing hydrolysis to form acetic acid (CH3COOH) and hydrogen chloride gas (HCl). This hydrolysis reaction is exothermic and releases a significant amount of heat.

Acetyl chloride can be synthesized by the reaction of acetic acid (CH₃COOH) with thionyl chloride (SOCl₂). The reaction proceeds as follows:

CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl

This reaction occurs under controlled conditions, such as in a fume hood or with proper ventilation, due to the release of corrosive gases.

Acetyl chloride has wide applications in organic synthesis, particularly as an acylating agent. It readily reacts with nucleophiles, such as alcohols, amines, and thiols, to form acyl derivatives. The acylation reaction involves the substitution of a hydrogen atom with an acetyl group, resulting in the formation of esters, amides, or thioesters. Acetyl chloride is also commonly used in the pharmaceutical industry to synthesize various drugs and pharmaceutical intermediates. It also involves in the production of acetyl derivatives of organic compounds, which can modify their physical and chemical properties.

Acetyl chloride is also helpful in producing dyes, perfumes, and flavoring agents. It is used to introduce acetyl groups into aromatic compounds, which can alter their odor or taste characteristics.

 What is Acetic Anhydride

Acetic anhydride is an organic compound with the chemical formula (CH₃CO)₂O. It is a colorless liquid with a strong, pungent odor. Acetic anhydride is derived from acetic acid, and its molecular structure consists of two acetyl groups (-COCH3) connected by an oxygen atom (-O-). Acetic anhydride is a highly reactive compound with a boiling point of approximately 139 degrees Celsius. It has a density of around 1.08 g/mL and is miscible with a wide range of organic solvents. Acetic anhydride is hygroscopic. That is, it readily absorbs moisture from the surrounding environment.

Acetic anhydride can be synthesized through the reaction of acetic acid (CH₃COOH) with various compounds, such as ketene (CH2=C=O) or acetic acid anhydride (CH₃COOH). The reaction proceeds as follows:

CH₃COOH + CH₂=C=O → (CH₃CO)₂O

This process is typically carried out under carefully controlled conditions, such as in the presence of a catalyst or at specific temperatures, to ensure efficient conversion.

Acetic anhydride has a wide range of applications in various industries, including pharmaceuticals, plastics, textiles, and chemicals.

 Similarities Between Acetyl Chloride and Acetic Anhydride

• Acetyl chloride and acetic anhydride contain the acetyl group (-COCH3) as their primary functional group.
• Both compounds are commonly used as acylating agents in organic synthesis.
• Both are colorless, pungent liquids.
• Acetyl chloride and acetic anhydride participate in similar chemical reactions due to their acetylating properties.

Difference Between Acetyl Chloride and Acetic Anhydride

Definition

Acetyl chloride (CH₃COCl) is an organic compound belonging to the acyl halide family, while acetic anhydride (C₄H₆O₃) is an organic compound in the acid anhydride class.

Reactivity

Acetyl chloride is highly reactive and readily hydrolyzes in the presence of moisture, while acetic anhydride is less reactive than acetyl chloride.

Chemical Composition

While acetyl chloride has a single chlorine atom, acetic anhydride has an oxygen atom connecting two acetyl groups.

Boiling and Melting Points

Acetyl chloride has a lower boiling point of 51.8 °C and a lower melting point of -112 °C than acetic anhydride, which has a higher boiling point of 139.5 °C and a higher melting point of -73.1 °C.

Applications

Acetyl chloride has wide applications in organic synthesis, pharmaceuticals, and dye production, whereas acetic anhydride has a wide range of applications in various industries, including pharmaceuticals, plastics, textiles, and chemicals.

Conclusion

The main difference between acetyl chloride and acetic anhydride is that acetyl chloride has a single chlorine atom, while acetic anhydride has an oxygen atom connecting two acetyl groups. Moreover, acetyl chloride is highly reactive and readily hydrolyzes in the presence of moisture, while acetic anhydride is less reactive than acetyl chloride.

Reference:

1. “Acetyl chloride | CH3COCl.” PubChem. National Library of Medicine.
2. “Acetic Anhydride | (CH3CO)2O.” PubChem. National Library of Medicine.

Image Courtesy:

1. “Acetyl-chloride skeletal” By Hbf878 – Own work (CC0) via Commons Wikimedia
2. “Acetic anhydride2DACS” By Fuse809 – Own work (Public Domain) via Commons Wikimedia