Acetaldehyde

What is Acetaldehyde?

Acetaldehyde (also called ethanal) is a colurless, pungent-smelling organic compound with the molecular formula CH₃CHO or C₂H₄O. It is prepared industrially by the dehydrogenation or air oxidation of ethanol in the presence of a silver catalyst at 300 °C. Acetaldehyde is used in the preparation of several chemical compounds and synthetic resins like acetic acid, ethanol, rubber accelerator, phenolic resins, synthetic drugs, etc. The structure and hybridization of the acetaldehyde molecule are given below the picture,

Acetaldehyde occurs naturally in several foods like coffee, bread, and ripe fruit. It is also produced by the partial oxidation of ethanol by the liver enzyme dehydrogenase.

Structure of Acetaldehyde

An aldehyde (-CHO) functional group is bound to a methyl group (-CH₃) in an acetaldehyde molecule. Therefore, it is the second simplest aldehyde after formaldehyde.

The chemical structure of acetaldehyde shows that one carbon atom present in the aldehyde functional group is sp² hybridized while another carbon atom present in the methyl group is sp³ hybridized. Therefore, the CH₃CHO molecule has a planar-trigonal together with tetrahedral geometry.

Preparation of Acetaldehyde

• Acetaldehyde can be prepared industrially by the dehydrogenation or air oxidation of alcohol in the presence of a silver catalyst at 300 °C.
  CH₃CH₂OH → CH₃CHO
• When acetylene passes into dilute sulfuric acid at 60 °C in the presence of a mercuric sulfate catalyst, acetylene adds one molecule of water to form acetaldehyde.
  CH ≡ CH + H₂O → [CH₂ = CHOH] → CH₃CHO
• It can be prepared by passing a mixture of ethylene and oxygen under pressure into an aqueous solution of palladium and cupric chloride at 50 °C.
  CH₂ = CH₂ + PdCl₂ + H₂O → CH₃CHO + Pd + 2HCl
• It can be also prepared by hydroformylation of methanol with chemical catalysts like cobalt, nickel, or iron salts.

Chemical Reactions

Oxidation reactions

Acetaldehyde can easily be oxidized by acid dichromate and permanganate to form acetic acid.

CH₃CHO + [O] → CH₃COOH

Reduction Reactions

Acetaldehyde is very easily oxidized. Therefore it is a powerful reducing agent. It reduces Fehling’s solution (an alkaline solution containing a complex of copper tartrate) to red curious oxide.

CH₃CHO + 2 Cu⁺² + 5 OH⁻ → RCOO⁻ + Cu₂O + 3 H₂O

It also reduces Tollens reagent or ammonical silver nitrate solution to form metallic silver.

CH₃CHO + 2 Ag(NH₃)₂OH → RCO₂NH₄ + 2 Ag + 3 NH₃ + H₂O

Condensation Reactions

Acetaldehyde undergoes condensation in the presence of dilute sodium hydroxide, potassium carbonate, or hydrochloric acid to form a syrupy liquid. It is known as aldol.

2 CH₃CHO → CH₃CH(OH)CH₂CHO

On heating, aldol eliminates water to form unsaturated compounds like crotonaldehyde.

CH₃CH(OH)CH₂CHO → CH₃CH = CHCHO + H₂O

In Strecker synthesis, CH₃CHO condenses with ammonia and hydrogen cyanide to give an amino-nitrile compound. It is further hydrolyzed to give the amino acid alanine.

Polymerization Reactions

When acetaldehyde is treated with a few drops of concentrated sulfuric acid, a vigorous reaction takes place by the formation of a polymer like paraldehyde (CH₃CHO)₃. It is a pungent-smelling liquid with a boiling point of 128 °C. Paraldehyde is used in medicine as a hypnotic.

When acetaldehyde is treated with a few drops of concentrated sulfuric acid at 0°C, the tetramer metaldehyde (CH₃CHO)₄ is formed. It is a white solid with a melting point of 246 °C. It regenerates CH₃CHO by distilling sulfuric acid.

Uses of Acetaldehyde

Acetaldehyde has a wide range of applications. It is used as a starting material for the production of several compounds like acetic acid, acetic anhydride, ethyl acetate, n-butanol, etc.

• It is a raw material for manufacturing several household products like paint binders, plasticizers, and baby nappies.
• It is also used to manufacture various types of building materials, fire protection paints, lubricants, and explosives.
• In the pharmaceutical industry, paraldehyde formed from the polymerization of acetaldehyde uses as a sleeping medicine.
• It is used for the production of several types of dyes.
• Metaldehyde is formed by polymerization processes used as a fuel.
• It is used for manufacturing several types of disinfectants and perfumes.
• It can be used for the production of plastics and synthetic rubber.
• In the food industry, it is used for the production of preservatives and flavorings agents. It occurs naturally in several fruit and fruit juices.
• A low level of acetaldehyde is found in several foods like milk products, soy products, pickled vegetables, and non-alcoholic beverages.