CH₃CHO — Acetaldehyde

Acetaldehyde (CH₃CHO) is one of the simplest and most important aldehydes in organic chemistry. It is a colorless, volatile, and flammable liquid with a characteristic sharp, fruity odor. Acetaldehyde plays a crucial role as an intermediate in the synthesis of several organic compounds such as acetic acid, perfumes, dyes, plastics, and pharmaceuticals.

In nature, acetaldehyde occurs in small quantities in coffee, bread, ripe fruits, and even in human metabolism as a product of ethanol oxidation. However, it is also a known environmental pollutant found in cigarette smoke, vehicle exhaust, and industrial emissions. Industrially, acetaldehyde is a precursor to many important chemicals including pyridine derivatives, acetic anhydride, and peracetic acid.

Acetaldehyde consists of a carbonyl group (C=O) attached to a methyl group (–CH₃) and a hydrogen atom, giving the molecular structure:

The carbon atom of the carbonyl group is sp² hybridized, forming a trigonal planar structure with bond angles of approximately 120°. The oxygen atom possesses two lone pairs, contributing to the molecule’s polarity.

The carbonyl carbon is electrophilic, making acetaldehyde reactive towards nucleophiles. The presence of the polar C=O bond enables hydrogen bonding with water and other polar solvents, accounting for its complete miscibility with water. Due to its small molecular size and volatility, acetaldehyde easily evaporates and is found in atmospheric reactions contributing to smog formation.

Acetaldehyde can be prepared through various laboratory and industrial methods. Some of the most common include:

• 1. Oxidation of Ethanol: Acetaldehyde is formed by controlled oxidation of ethanol using copper or silver catalysts at high temperatures.

• 2. Wacker Process: The industrial production of acetaldehyde primarily employs the Wacker oxidation of ethylene using palladium chloride and copper chloride catalysts.

The palladium metal is reoxidized by copper chloride, completing the catalytic cycle:

• 3. Dehydrogenation of Ethanol: In this reaction, ethanol is passed over a heated catalyst such as copper or zinc oxide to remove hydrogen, forming acetaldehyde.

• 4. Hydration of Acetylene: Acetaldehyde can also be synthesized by adding water to acetylene in the presence of a mercury(II) salt catalyst.

Physical Properties:

• Acetaldehyde is a colorless, mobile liquid with a sharp, pungent odor similar to fruit or vinegar.
• It has a low boiling point (20.2°C), making it extremely volatile.
• Completely miscible with water, alcohol, and ether due to hydrogen bonding.
• Highly flammable and forms explosive mixtures with air.

Chemical Properties:

• 1. Oxidation: Acetaldehyde is easily oxidized to acetic acid by mild oxidizing agents.

\( CH_3CHO + [O] \rightarrow CH_3COOH \)

• 2. Reduction: It can be reduced to ethanol using hydrogen gas and a metal catalyst such as nickel.

\( CH_3CHO + H_2 \xrightarrow{Ni} CH_3CH_2OH \)

• 3. Polymerization: Acetaldehyde polymerizes upon standing or on exposure to acids to form paraldehyde (cyclic trimer) and metaldehyde (cyclic tetramer).

\( 3CH_3CHO \rightarrow (CH_3CHO)_3 \text{ (Paraldehyde)} \)

• 4. Addition Reactions: Acetaldehyde reacts with hydrogen cyanide (HCN) to form cyanohydrin, an important intermediate in organic synthesis.

\( CH_3CHO + HCN \rightarrow CH_3CH(OH)CN \)

• 5. Reaction with Ammonia: Forms acetaldimine and other condensation products.

\( CH_3CHO + NH_3 \rightarrow CH_3CH=NH + H_2O \)

Acetaldehyde is a vital chemical intermediate with diverse applications in the chemical and pharmaceutical industries:

• 1. Production of Acetic Acid: The primary industrial use of acetaldehyde is as a precursor for acetic acid through catalytic oxidation.
• 2. Manufacture of Pyridine and Peracetic Acid: Used in the synthesis of important organic compounds such as acetic anhydride, pentaerythritol, and pyridine derivatives.
• 3. Solvent and Intermediate: Acts as a solvent for resins, oils, and rubber, and as a key intermediate in manufacturing perfumes, dyes, and pharmaceuticals.
• 4. Laboratory Reagent: Used in organic synthesis, including aldol condensation reactions for producing larger carbon frameworks.
• 5. Agricultural Chemicals: Utilized in the production of herbicides, pesticides, and plant growth regulators.
• 6. Alcohol Metabolism: In the human body, acetaldehyde forms as an intermediate during ethanol metabolism before being converted into acetic acid by aldehyde dehydrogenase.

Acetaldehyde is a toxic and flammable compound that should be handled with great care. Prolonged exposure can irritate mucous membranes, respiratory pathways, and skin. It is classified as a Group 2B carcinogen (possibly carcinogenic to humans) by the International Agency for Research on Cancer (IARC).

Health Hazards:

• Inhalation may cause coughing, dizziness, and shortness of breath.
• Prolonged exposure leads to liver damage, similar to alcohol toxicity.
• Skin contact can cause burns or irritation.
• Eye exposure results in redness and tearing.

Safety Precautions:

• Handle in well-ventilated areas or under fume hoods.
• Use gloves, protective clothing, and goggles.
• Store in tightly sealed containers away from heat, flames, and oxidizing agents.
• Dispose of acetaldehyde waste according to hazardous material guidelines.