Chapter 8 · Chemistry
Uses of Aldehydes and Ketones: From Formalin to Vanilla
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Where aldehydes and ketones turn up in daily life: formalin and Bakelite from formaldehyde, acetic acid from acetaldehyde, acetone as a solvent, and vanillin, cinnamaldehyde and benzaldehyde as smells and flavours, each explained from its C=O group.
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The Carboxyl Group: Structure and NamingWhat –COOH really is: a carbonyl and a hydroxyl on one sp² carbon, joined by resonance that makes the carbon less electrophilic than in aldehydes; and how carboxylic acids get their common and IUPAC names.Chemistry
Key points
- Formaldehyde (HCHO): formalin (~40% in water) preserves specimens; it also makes Bakelite and urea–formaldehyde glues.
- HCHO is the most reactive carbonyl compound: no alkyl group pushes electrons into, or crowds, its C=O carbon.
- Acetone and ethyl methyl ketone are solvents: a polar C=O (O δ⁻, C δ⁺) plus non-polar CH₃ groups.
- Acetaldehyde is a starting material: oxidation gives acetic acid; also ethyl acetate, polymers and drugs.
- Vanillin (vanilla), cinnamaldehyde (cinnamon) and benzaldehyde (almond; perfumes and dyes) each carry a benzene ring + CHO.
The uses at a glance
| Compound | Use | Why it works |
|---|---|---|
| Formaldehyde, HCHO | formalin (preserving specimens), Bakelite, urea–formaldehyde glues | the most reactive C=O: no alkyl groups |
| Acetaldehyde, CH₃CHO | acetic acid, ethyl acetate, vinyl acetate, polymers, drugs | –CHO is oxidised to –COOH |
| Acetone; ethyl methyl ketone | industrial solvents; nail-polish remover | polar C=O plus non-polar CH₃ groups |
| Benzaldehyde | perfumery and dyes (almond odour) | an aromatic aldehyde |
| Vanillin; cinnamaldehyde | vanilla and cinnamon flavours | benzene ring + CHO |
Why formalin preserves a specimen
Formaldehyde reacts with the –NH₂ groups of proteins and joins the chains through –CH₂– bridges. The tissue is fixed, and microbes cannot decompose it; formaldehyde also kills them.
Bakelite, step 1, arrow by arrow
- In base, phenol becomes phenoxide; a lone pair on O⁻ moves into the ring.
- The ring's C=C π bond at the ortho carbon attacks the carbon of HCHO.
- The C=O π pair of HCHO moves onto its oxygen.
- After proton transfers the ring carries –CH₂OH; repeated, –CH₂– bridges join rings into Bakelite, a hard cross-linked plastic.
Why acetone is a good solvent
The C=O group is polar (O δ⁻, C δ⁺), so acetone mixes with water. Its two CH₃ groups are non-polar, so it also dissolves oils, paints and grease: one molecule, both kinds of solvent power.
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