9.29C - Formulae and structures of carboxylic acids
Use carbon-count prefixes to learn the first four carboxylic acids. Build their displayed structures by showing the complete acid group and checking every atom’s bonds.
Names and formulae
The prefixes meth-, eth-, prop- and but- count all carbon atoms, including the carbon in COOH.
| Acid | Molecular formula | Condensed structure |
|---|---|---|
| Methanoic acid | CH2O2 | HCOOH |
| Ethanoic acid | C2H4O2 | CH3COOH |
| Propanoic acid | C3H6O2 | CH3CH2COOH |
| Butanoic acid | C4H8O2 | CH3CH2CH2COOH |
A molecular formula gives total atom counts; a condensed structure also groups atoms to show their connections. For butanoic acid, count 4 C, 3 + 2 + 2 + 1 = 8 H, and 2 O. Both C4H8O2 and CH3CH2CH2COOH describe it, but use the requested type of formula.
Drawing the acid group
A displayed structure shows every atom and bond. Each carbon has four bond lines in total, each oxygen two and each hydrogen one. A double bond is two shared pairs of electrons, shown by two parallel lines.
The acid end is C(=O)–O–H: one oxygen is double bonded to carbon; the other is single bonded to carbon and to hydrogen. The two oxygens are not bonded to each other.
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