Aldehydes

Aldehydes are used as preservatives and disinfectants, as raw materials for plastics, resins and medicines, and as flavours and fragrances. This lesson explains these uses and links each one to the chemistry of the aldehyde group.

Methanal (formaldehyde) is sold as formalin, a 40% solution in water, and is used to preserve biological specimens, to disinfect, and to make Bakelite, urea–formaldehyde and melamine–formaldehyde resins, and urotropine. Ethanal (acetaldehyde) is a starting material for ethanoic acid, ethyl ethanoate and other chemicals, and gives paraldehyde, once used as a sleep-inducing drug. Benzaldehyde and natural aldehydes such as vanillin, cinnamaldehyde and citral flavour and scent foods and perfumes, while the reducing power of aldehydes is put to work in silvering mirrors and in tests for sugar. The lesson ends with the hazards of formaldehyde and how to handle it safely.

Uses of Aldehydes

Key Points Covered

1. Methanal (formaldehyde), HCHO
   • A colourless gas with a sharp smell, sold as formalin, a 40% solution in water.
   • Formalin preserves biological specimens and is used as a disinfectant and antiseptic. It works by reacting with proteins and cross-linking them, which hardens tissue and kills the microbes that cause decay.
   • Methanal condenses with phenol to give Bakelite, a hard thermosetting plastic long used for electrical switches and handles; with urea it gives urea–formaldehyde resins, used as glues for plywood and particle board; and with melamine it gives melamine–formaldehyde resins, used for laminates and unbreakable tableware.
   • With ammonia it forms urotropine (hexamethylenetetramine): 6HCHO + 4NH3 → (CH2)6N4 + 6H2O. Urotropine is used in medicine as a urinary antiseptic.
2. Ethanal (acetaldehyde), CH3CHO
   • A starting material for making ethanoic acid, ethyl ethanoate and many other chemicals, polymers and drugs.
   • Three ethanal molecules join to form paraldehyde, a cyclic trimer that was once widely used as a hypnotic (a sleep-inducing drug).
3. Benzaldehyde, C6H5CHO
   • Known as oil of bitter almonds; used as an almond flavouring, in perfumery and in making dyes.
4. Natural aldehydes
   • Vanillin gives vanilla its flavour, cinnamaldehyde gives cinnamon its smell and taste, and citral gives lemongrass and lemon peel their lemony scent.
   • In living things, retinal, an aldehyde made from vitamin A, is the light-sensitive molecule in the eye, and glucose is an aldose: its open-chain form has an aldehyde group.
5. Uses of the reducing power of aldehydes
   • Aldehydes reduce the silver ions in Tollens' reagent to metallic silver; reducing agents such as glucose and formaldehyde are used to deposit thin silver layers when making mirrors.
   • Because glucose has an aldehyde group, it reduces blue Benedict's or Fehling's solution to a red precipitate of copper(I) oxide. This is the basis of the traditional test for sugar in urine.
6. Safety
   • Formaldehyde vapour irritates the eyes, nose and throat, can cause skin allergies, and is classed as a carcinogen (a cancer-causing substance). Formalin must be handled with gloves and eye protection in a well-ventilated room, and specimen jars must be kept closed.

Each use follows from a property of the –CHO group: its high reactivity lets it link proteins and build resins, its ease of oxidation makes it a useful reducing agent, and the distinctive smells of many aldehydes make them valuable in flavours and perfumes.