Nomenclature, Structure and Preparation of Carboxylic Acids: MDCAT Chemistry notes
Nomenclature, Structure and Preparation of Carboxylic Acids for MDCAT: common names, IUPAC rules, hydrogen bonding, nitrile hydrolysis.
The carboxyl group
Carboxylic acids contain the carboxyl group, $-\mathrm{COOH}$, a carbonyl and a hydroxyl on the same carbon. The carboxyl carbon is sp$^2$ and the group is planar. Saturated monocarboxylic acids have the general formula $\mathrm{C_nH_{2n+1}COOH}$ (or $\mathrm{C_nH_{2n}O_2}$).
Naming
IUPAC names replace the final -e of the parent alkane with -oic acid. The chain includes the carboxyl carbon, and the carboxyl carbon is always C1. The acid group outranks aldehydes, ketones and alcohols, so those become prefixes (oxo-, hydroxy-).
- $\mathrm{HOCH_2CH_2CH_2COOH}$ → 4-hydroxybutanoic acid (numbering starts from COOH).
- $\mathrm{CH_3COCH_2CH_2COOH}$ → 4-oxopentanoic acid (pattern: oxo…oic acid).
| Formula | Common name | IUPAC name | Note |
|---|---|---|---|
| HCOOH | Formic acid | Methanoic acid | Found in ant stings |
| $\mathrm{CH_3COOH}$ | Acetic acid | Ethanoic acid | First obtained from vinegar (Latin acetum); vinegar is its dilute solution |
| $\mathrm{CH_3CH_2COOH}$ | Propionic acid | Propanoic acid | |
| $\mathrm{CH_3(CH_2)_2COOH}$ | Butyric acid | Butanoic acid | Unpleasant smell of rancid butter |
| $\mathrm{CH_3CH(OH)COOH}$ | Lactic acid | 2-hydroxypropanoic acid | Sour milk; muscles |
| HOOC–COOH | Oxalic acid | Ethanedioic acid | Dicarboxylic |
| $\mathrm{HOOC(CH_2)_4COOH}$ | Adipic acid | Hexanedioic acid | Aliphatic, dicarboxylic |
| $\mathrm{C_6H_4(COOH)_2}$ | Phthalic acid | Benzene-1,2-dicarboxylic acid | Aromatic |
Tartaric acid is 2,3-dihydroxybutanedioic acid (in grapes and tamarind).
Physical properties
- Hydrogen bonding: two acid molecules join into a hydrogen-bonded dimer, so boiling points are higher than those of alcohols of similar mass.
- Solubility: the COOH group hydrogen bonds with water. The first four acids are miscible with water; solubility falls as the hydrocarbon chain grows. So ethanoic acid is more soluble than butanoic, pentanoic or hexanoic acid. Acetic acid also dissolves in alcohol and ether.
- Melting points do not rise smoothly: ethanoic acid (about 17$^\circ$C) melts higher than propanoic, butanoic and pentanoic acids, because its molecules pack well in the crystal.
- Dicarboxylic acids have higher melting points than monocarboxylic acids of similar molar mass, because two COOH groups form more hydrogen bonds and pack better; oxalic and adipic acids are solids.
Preparation
1. Hydrolysis of nitriles
$$\mathrm{R{-}C{\equiv}N + 2H_2O \xrightarrow{H^+} R{-}COOH + NH_3}$$
Heating with dilute mineral acid (or alkali, then acidification) gives the acid. Methyl cyanide ($\mathrm{CH_3CN}$) gives acetic acid. Since a nitrile is made from an alkyl halide and KCN, this route adds one carbon.
2. Oxidation of primary alcohols and aldehydes
Acidified $\mathrm{K_2Cr_2O_7}$ or $\mathrm{KMnO_4}$: $\mathrm{RCH_2OH \rightarrow RCHO \rightarrow RCOOH}$.
3. Grignard reagent + carbon dioxide
RMgX adds to $\mathrm{CO_2}$; acid hydrolysis gives RCOOH.
4. Hydrolysis of fats and oils
Fats and oils are esters of glycerol. Hydrolysis gives glycerol and long-chain aliphatic monocarboxylic acids, which is why these are called fatty acids.
5. Oxidation of alkylbenzenes
Toluene is oxidized by $\mathrm{KMnO_4}$ to benzoic acid.
Common MDCAT traps
- Nitriles hydrolyse to carboxylic acids, not aldehydes, ketones or esters.
- 2-hydroxypropanoic acid is lactic acid, not tartaric acid.
- Numbering always starts at the COOH carbon, even if OH or C=O is present.
- High boiling point is due to intermolecular hydrogen bonding, not molar mass.
- Phthalic acid is aromatic; adipic acid is aliphatic.
Quick revision
- Suffix -oic acid; COOH carbon = C1.
- Butyric acid smells of rancid butter.
- Smaller acids are more water soluble.
- Fats + water → glycerol + fatty acids.