Conversion of Carboxylic Acids: MDCAT Chemistry notes
Conversion of Carboxylic Acids for MDCAT: acyl chlorides with SOCl2 and PCl5, Fischer esterification, amides, anhydrides and LiAlH4 reduction.
Overview
The $-\mathrm{OH}$ of a carboxyl group can be replaced by Cl, OR', $\mathrm{NH_2}$ or OCOR to give the acid derivatives, and the whole group can be reduced to $\mathrm{CH_2OH}$.
| Reagent | Product from $\mathrm{CH_3COOH}$ | By-products |
|---|---|---|
| $\mathrm{SOCl_2}$ (thionyl chloride) | $\mathrm{CH_3COCl}$ (acetyl/ethanoyl chloride) | $\mathrm{SO_2}$ + HCl (gases) |
| $\mathrm{PCl_5}$ | $\mathrm{CH_3COCl}$ | $\mathrm{POCl_3}$ + HCl |
| $\mathrm{PCl_3}$ | $\mathrm{CH_3COCl}$ | $\mathrm{H_3PO_3}$ |
| Alcohol + conc. $\mathrm{H_2SO_4}$ | Ester | $\mathrm{H_2O}$ |
| $\mathrm{NH_3}$, then heat | $\mathrm{CH_3CONH_2}$ (acetamide) | $\mathrm{H_2O}$ |
| $\mathrm{P_2O_5}$, heat (dehydration) | $\mathrm{(CH_3CO)_2O}$ (acetic anhydride) | $\mathrm{H_2O}$ |
| $\mathrm{LiAlH_4}$, then $\mathrm{H_3O^+}$ | $\mathrm{CH_3CH_2OH}$ (1$^\circ$ alcohol) |
Acyl chlorides
$$\mathrm{CH_3COOH + SOCl_2 \rightarrow CH_3COCl + SO_2\uparrow + HCl\uparrow}$$
Thionyl chloride is preferred because both by-products are gases and escape, leaving pure acyl chloride. Acyl chlorides are very reactive; water hydrolyses them back to the carboxylic acid and HCl.
Esterification (Fischer)
$$\mathrm{RCOOH + R'OH \underset{}{\overset{conc.\,H_2SO_4}{\rightleftharpoons}} RCOOR' + H_2O}$$
- It is a condensation reaction (two molecules join with loss of water).
- Concentrated $\mathrm{H_2SO_4}$ acts as catalyst and also absorbs the water.
- It is reversible (an equilibrium). Removing water or using excess alcohol drives it forward. Salt formation and reactions with $\mathrm{PCl_5}$ or $\mathrm{SOCl_2}$ are not reversible in this way.
- Isotope labelling with $^{18}$O shows the acid loses its OH and the alcohol loses only its H: the O–H bond of the alcohol breaks, and the alcohol's oxygen ends up in the ester.
- Examples: formic acid + ethanol → ethyl formate; 2,2-dimethylpropanoic acid + ethanol → ethyl 2,2-dimethylpropanoate. The partner for an ester is always an alcohol, not an aldehyde, ketone, alkane or another acid.
Ester hydrolysis
The reverse reaction: $\mathrm{CH_3COOC_2H_5 + H_2O \rightleftharpoons CH_3COOH + C_2H_5OH}$, giving back both the acid and the alcohol.
Esters as flavours
| Ester | Odour/flavour |
|---|---|
| Amyl butyrate | Apricot |
| Ethyl butyrate | Pineapple |
| Amyl (isoamyl) acetate | Banana |
| Octyl acetate | Orange |
| Benzyl acetate | Jasmine |
Amides
$$\mathrm{CH_3COOH + NH_3 \rightarrow CH_3COONH_4 \xrightarrow{heat} CH_3CONH_2 + H_2O}$$
The ammonium salt forms first; strong heating dehydrates it to the amide.
Acid anhydrides
Two molecules of acid lose one molecule of water (dehydration) on heating with $\mathrm{P_2O_5}$: $\mathrm{2CH_3COOH \rightarrow (CH_3CO)_2O + H_2O}$.
Reduction
Lithium aluminium hydride reduces a carboxylic acid right down to a primary alcohol. Oxidizing agents ($\mathrm{KMnO_4}$, $\mathrm{K_2Cr_2O_7}$) and $\mathrm{H_2SO_4}$ cannot do this.
Common MDCAT traps
- Thionyl chloride gives $\mathrm{CH_3COCl + SO_2 + HCl}$; check the balanced equation uses one $\mathrm{SOCl_2}$ and the correct acid.
- Esterification is condensation; the alcohol's O–H bond breaks, not its C–O bond.
- Acetic anhydride comes from dehydration, not oxidation or reduction.
- Heating the ammonium salt gives an amide, not an ester or alkane.
- The only reversible reaction among the common ones is esterification.
Quick revision
- Acid + $\mathrm{SOCl_2}$ or $\mathrm{PCl_5}$ → acyl chloride.
- Acyl chloride + water → carboxylic acid + HCl.
- Acid + alcohol (conc. $\mathrm{H_2SO_4}$) $\rightleftharpoons$ ester + water.
- Acid + $\mathrm{LiAlH_4}$ → 1$^\circ$ alcohol.