Reactivity of Carboxylic Acids: MDCAT Chemistry notes
Reactivity of Carboxylic Acids for MDCAT: acid strength, carboxylate resonance, chloro substituent effects, carbonates and derivative order.
Why carboxylic acids are acidic
In water a carboxylic acid ionizes partly:
$$\mathrm{RCOOH + H_2O \rightleftharpoons RCOO^- + H_3O^+}$$
The carboxylate ion is stabilized by resonance: the negative charge is shared equally by the two oxygen atoms, and both C–O bonds become identical. This stabilization is greater than in the phenoxide ion (where the charge spreads onto carbon) and absent in an alkoxide ion. Hence:
$$\text{Carboxylic acid} > \text{Phenol} > \text{Water} > \text{Alcohol}$$
Weak acids
Carboxylic acids are weak acids: only a small fraction of the molecules ionize. Acetic acid has $K_a \approx 1.8\times10^{-5}$ ($\mathrm{p}K_a \approx 4.76$). Sulphuric acid is almost completely ionized. So acetic acid is weaker than sulphuric acid because of its lower degree of ionisation, not because of the $-\mathrm{COOH}$ group itself.
Effect of substituents
Anything that stabilizes the carboxylate ion increases acid strength.
- Electron-withdrawing groups (Cl, F, $\mathrm{NO_2}$) pull electron density away by the $-I$ effect, spread the negative charge and make the acid stronger.
- More such groups give a stronger acid; a group closer to COOH ($\alpha$-carbon) has more effect than one farther away ($\beta$-carbon).
- Alkyl groups are electron-releasing (+I) and make the acid weaker: formic > acetic > propanoic acid.
| Acid | Approx. p$K_a$ |
|---|---|
| Trichloroethanoic acid, $\mathrm{Cl_3CCOOH}$ | 0.7 (strongest) |
| Dichloroethanoic acid | 1.3 |
| Chloroethanoic acid | 2.9 |
| Ethanoic acid | 4.8 |
| Propanoic acid | 4.9 |
So $\mathrm{Cl_3CCOOH}$ is stronger than monochloro, monofluoro or mononitro ethanoic acid, and 2-chloropropanoic acid ($\mathrm{CH_3CHClCOOH}$) is stronger, with lower pH, than 3-chloropropanoic acid ($\mathrm{CH_2ClCH_2COOH}$).
Reactions as acids
- With active metals: $\mathrm{2RCOOH + 2Na \rightarrow 2RCOONa + H_2}$.
- With bases: $\mathrm{RCOOH + NaOH \rightarrow RCOONa + H_2O}$.
- With carbonates and bicarbonates: brisk effervescence of colourless $\mathrm{CO_2}$, e.g. $\mathrm{2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + CO_2 + H_2O}$. Alkenes, aldehydes, esters and phenols do not do this, so the reaction identifies a carboxylic acid.
- A carboxylate salt treated with a stronger mineral acid (HCl, $\mathrm{H_2SO_4}$) releases the free carboxylic acid.
Why the C=O of an acid is unreactive
The lone pair on the OH oxygen is delocalized into the C=O by resonance, reducing the positive charge on the carbonyl carbon. Carboxylic acids therefore do not give the characteristic carbonyl reactions (2,4-DNPH, $\mathrm{NaHSO_3}$, HCN addition) that aldehydes and ketones give.
Reactivity of acid derivatives
In nucleophilic acyl substitution, the better the leaving group, the more reactive the derivative:
$$\text{Acyl chloride} > \text{Acid anhydride} > \text{Ester} > \text{Amide}$$
Chloride is the best leaving group; the amide's $\mathrm{NH_2^-}$ is the poorest.
Use
Besides being the acid of vinegar and an important solvent, acetic acid is listed in the textbook as used in medicine as a local irritant.
Common MDCAT traps
- Least pH = strongest acid; look for the chlorine nearest to COOH or the most chlorines.
- Three chlorines outweigh one fluorine or one nitro group.
- Carbonates give $\mathrm{CO_2}$, not $\mathrm{H_2}$ or CO; metals give $\mathrm{H_2}$.
- Correct acidity order: RCOOH > phenol > water > alcohol.
- Acyl chloride is more reactive than anhydride, not the reverse.
Quick revision
- Carboxylate ion: charge shared equally by two O atoms.
- Electron-withdrawing groups strengthen acids.
- Acid + $\mathrm{NaHCO_3}$ → effervescence of $\mathrm{CO_2}$.
- Salt + mineral acid → free carboxylic acid.