CH₂O — Formaldehyde

Formaldehyde (CH₂O) is the simplest and most fundamental member of the aldehyde family. It consists of a carbonyl group (C=O) bonded to two hydrogen atoms. At room temperature, formaldehyde exists as a gas but is commonly used in a 37% aqueous solution called formalin.

It is a highly reactive compound with a characteristic pungent odor. Because of its strong disinfectant and preservative properties, formaldehyde is widely used in producing plastics, adhesives, coatings, and as a biological preservative in laboratories and hospitals. It is also an important intermediate in the synthesis of many organic compounds.

Formaldehyde has the chemical formula \(CH_2O\). The molecule consists of a carbon atom double-bonded to an oxygen atom and single-bonded to two hydrogen atoms, forming a planar trigonal geometry. The carbon is sp² hybridized, and the molecule exhibits a polar character due to the high electronegativity of oxygen.

The carbonyl carbon has a partial positive charge (\(δ^+\)), making it susceptible to nucleophilic attack, while the oxygen bears a partial negative charge (\(δ^-\)), allowing hydrogen bonding and intermolecular attractions. These interactions explain its high solubility in water and reactivity toward many chemical reagents.

Formaldehyde can be prepared by several industrial and laboratory methods:

1. Catalytic Oxidation of Methanol

This is the most common industrial method. Methanol is oxidized in the presence of air and a metal catalyst such as copper or silver at high temperature:

2. Partial Oxidation of Methane

Formaldehyde can also be formed by the partial oxidation of methane using oxygen over molybdenum oxide or iron oxide catalysts:

3. From Methanol and Air in Laboratory

In the lab, formaldehyde can be produced by passing methanol vapor mixed with air over hot platinum or copper.

4. From Paraformaldehyde

Formaldehyde gas can also be obtained by heating paraformaldehyde (a polymerized solid form of formaldehyde):

Physical Properties:

• Colorless gas with a strong, pungent odor.
• Soluble in water, forming formalin (37% solution).
• Boiling point: -19°C; Melting point: -92°C.
• Highly flammable and reactive.

Chemical Properties:

• 1. Oxidation: Formaldehyde is easily oxidized to formic acid by mild oxidizing agents.

\(CH_2O + [O] \rightarrow HCOOH\)

• 2. Reduction: It can be reduced to methanol using reducing agents like hydrogen or sodium amalgam.

\(CH_2O + H_2 \rightarrow CH_3OH\)

• 3. Polymerization: Formaldehyde readily polymerizes to form paraformaldehyde and trioxane under suitable conditions.

\(nCH_2O \rightarrow (CH_2O)_n\)

• 4. Reaction with Ammonia: When mixed with ammonia, formaldehyde forms hexamethylenetetramine (urotropine), a solid used as a urinary antiseptic and in resins.

\(6CH_2O + 4NH_3 \rightarrow (CH_2)_6N_4 + 6H_2O\)

• 5. Reaction with Phenol: Phenol reacts with formaldehyde to form phenol-formaldehyde resin (Bakelite), one of the first synthetic plastics.

\(C_6H_5OH + CH_2O \rightarrow \, \text{Phenol-formaldehyde resin}\)

• 6. Cannizzaro Reaction: In the absence of alpha-hydrogen, formaldehyde undergoes the Cannizzaro reaction with a base to form methanol and formic acid.

\(2CH_2O + NaOH \rightarrow CH_3OH + HCOONa\)

• 1. Resin and Plastic Production: Used to make phenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyde resins for laminates, adhesives, and molded products.
• 2. Disinfectant and Preservative: Used in formalin to disinfect biological specimens and preserve tissues in laboratories.
• 3. Textile Industry: Applied to fabrics for crease resistance and durability improvement.
• 4. Chemical Synthesis: Acts as a key intermediate in producing formic acid, pentaerythritol, and other organic compounds.
• 5. Agriculture: Used in fungicides and germicides for seed treatment and soil sterilization.
• 6. Medicine: Used in vaccine preparation and sterilization (though limited due to toxicity).

Formaldehyde is toxic and a known human carcinogen. Prolonged exposure can cause irritation of the eyes, skin, and respiratory tract. Inhalation of its vapors may lead to coughing, throat irritation, and even cancer with chronic exposure.

Safety Measures:

• Handle formaldehyde only in well-ventilated areas or fume hoods.
• Wear protective gloves, goggles, and masks.
• Avoid direct skin or eye contact.
• Store in tightly sealed containers away from heat and sunlight.
• Dispose of waste through approved hazardous waste systems.