Why Formic Acid Is Stronger Than Acetic Acid

Vinegar, the sourness of tamarind and the sting of an ant bite all come from acids that carry the -COOH group. Carboxylic acids are more acidic than alcohols and phenols, form fragrant esters, and can shed carbon dioxide under the right conditions.

This lesson covers naming and preparing carboxylic acids, their acidic strength and the effect of substituents, and their key reactions, including esterification and decarboxylation.

Carboxylic acids are named with the suffix -oic acid, and they are prepared by oxidising primary alcohols, aldehydes or alkylbenzenes, by hydrolysing nitriles and acid derivatives, and by treating Grignard reagents with carbon dioxide.

Naming:

- $\mathrm{HCOOH}$ is methanoic acid (formic acid) and $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}$ is ethanoic acid (acetic acid)
- ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}$ is benzoic acid, and $\mathrm{HOOC}-\mathrm{COOH}$ is ethanedioic acid (oxalic acid)
- The carboxyl carbon is carbon 1, so $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{C}{\mathrm{H}}_3)\mathrm{COOH}$ is 2-methylpropanoic acid

Methods of preparation:

- Oxidation of primary alcohols and aldehydes with acidified $\mathrm{KMn}{\mathrm{O}}_4$ or ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$: $\mathrm{RC}{\mathrm{H}}_2\mathrm{OH}\to \mathrm{RCHO}\to \mathrm{RCOOH}$
- Oxidation of alkylbenzenes with alkaline $\mathrm{KMn}{\mathrm{O}}_4$ — the whole side chain, whatever its length, becomes -COOH, so toluene gives benzoic acid
- Hydrolysis of nitriles with acid or alkali, passing through an amide: $\mathrm{RCN}\to \mathrm{RCON}{\mathrm{H}}_2\to \mathrm{RCOOH}$
- Grignard reagents with dry ice, then acid: $\mathrm{RMgX}+\mathrm{C}{\mathrm{O}}_2\to \mathrm{RCOOMgX}\stackrel{{\mathrm{H}}_3{\mathrm{O}}^{+}}{\to }\mathrm{RCOOH}$
- Hydrolysis of esters, acid chlorides, anhydrides and amides

Worked example — two routes from bromoethane to propanoic acid:

- ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Br}+\mathrm{KCN}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{CN}$, then hydrolysis gives ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{COOH}$
- ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Br}+\mathrm{Mg}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{MgBr}$, then carbon dioxide and acid give ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{COOH}$

An everyday example. Vinegar used in Indian pickles is a dilute solution of ethanoic acid, formed when bacteria oxidise the ethanol in fermented sugarcane juice or fruit.

The substance. Nitrile hydrolysis and the Grignard route both add one carbon — which makes them the standard ways to turn a haloalkane into the next higher acid.

Carboxylic acids are acidic because the carboxylate ion formed on losing a proton is stabilised by resonance, with the negative charge shared equally by two oxygen atoms; electron-withdrawing groups make them stronger, and electron-donating groups make them weaker.

Resonance in the carboxylate ion:

- Both C-O bonds become identical, and the charge spreads over two electronegative oxygen atoms
- In a phenoxide ion, the charge spreads onto less electronegative carbon atoms, so it is less stabilised
- Carboxylic acids therefore release carbon dioxide from sodium hydrogencarbonate, while phenols do not

Acidity order. Carboxylic acids > phenols > water > alcohols.

Effect of substituents:

- Electron-withdrawing groups such as -Cl, $-\mathrm{N}{\mathrm{O}}_2$ and -CN stabilise the carboxylate ion and increase acidity
- The effect grows with electronegativity and with the number of groups: $\mathrm{FC}{\mathrm{H}}_2\mathrm{COOH}>\mathrm{ClC}{\mathrm{H}}_2\mathrm{COOH}>\mathrm{BrC}{\mathrm{H}}_2\mathrm{COOH}$
- The effect fades with distance, so 2-chlorobutanoic acid is stronger than 3-chlorobutanoic acid
- Electron-donating alkyl groups destabilise the carboxylate ion: $\mathrm{HCOOH}>\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}>\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{COOH}$
- Benzoic acid, with a $\mathrm{p}{K}_a$ of about 4.2, is stronger than ethanoic acid, at about 4.8, and a 4-nitro group makes it stronger still

Worked example. Methanoic acid has a $\mathrm{p}{K}_a$ of about 3.8 and ethanoic acid about 4.8, so methanoic acid is about $10^{4.8-3.8}=10$ times stronger — the methyl group's electron donation weakens ethanoic acid.

An everyday example. The painful sting of red weaver ants in Indian mango orchards comes from methanoic acid, which the ants spray onto their attackers.

The substance. Distance matters as much as electronegativity — a chlorine atom three carbons away from the -COOH group barely changes the acid's strength.

Carboxylic acids react through their acidic O-H bond with metals and bases, through their -OH group in esterification and in forming acid chlorides and amides, and through the whole carboxyl group when decarboxylation removes carbon dioxide.

Reactions as acids:

- With active metals: $2C{\mathrm{H}}_3\mathrm{COOH}+2Na\to 2C{\mathrm{H}}_3\mathrm{COONa}+{\mathrm{H}}_2$
- With hydrogencarbonates: $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+\mathrm{NaHC}{\mathrm{O}}_3\to \mathrm{C}{\mathrm{H}}_3\mathrm{COONa}+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$ — brisk effervescence is a test for carboxylic acids

Reactions of the -OH group:

- Esterification with an alcohol and a little concentrated sulphuric acid: $\mathrm{C}{\mathrm{H}}_3\mathrm{COOH}+{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\rightleftharpoons \mathrm{C}{\mathrm{H}}_3\mathrm{COO}{\mathrm{C}}_2{\mathrm{H}}_5+{\mathrm{H}}_2\mathrm{O}$
- Acid chlorides with $\mathrm{SOC}{\mathrm{l}}_2$, $\mathrm{PC}{\mathrm{l}}_5$ or $\mathrm{PC}{\mathrm{l}}_3$: $\mathrm{RCOOH}+\mathrm{SOC}{\mathrm{l}}_2\to \mathrm{RCOCl}+\mathrm{S}{\mathrm{O}}_2+\mathrm{HCl}$
- Amides by heating the ammonium salt: $\mathrm{C}{\mathrm{H}}_3\mathrm{COON}{\mathrm{H}}_4\to \mathrm{C}{\mathrm{H}}_3\mathrm{CON}{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}$

Decarboxylation:

- A sodium carboxylate heated with soda lime loses carbon dioxide, giving an alkane with one carbon fewer: $\mathrm{C}{\mathrm{H}}_3\mathrm{COONa}+\mathrm{NaOH}\stackrel{\mathrm{CaO}}{\to }\mathrm{C}{\mathrm{H}}_4+\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3$
- Kolbe electrolysis of a concentrated sodium carboxylate solution joins two alkyl groups: $2C{\mathrm{H}}_3\mathrm{COONa}+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_2{\mathrm{H}}_6+2C{\mathrm{O}}_2+{\mathrm{H}}_2+2NaOH$

Other reactions:

- Hell-Volhard-Zelinsky reaction — chlorine or bromine with red phosphorus replaces an alpha hydrogen, giving $\mathrm{ClC}{\mathrm{H}}_2\mathrm{COOH}$ from ethanoic acid
- Reduction with $\mathrm{LiAl}{\mathrm{H}}_4$ gives primary alcohols
- On a benzene ring, the -COOH group deactivates the ring and directs substitution to the meta position

Worked example. Ethanoic acid with $\mathrm{SOC}{\mathrm{l}}_2$ gives ethanoyl chloride, which with ammonia gives ethanamide, while sodium ethanoate heated with soda lime gives methane.

An everyday example. Small soap-making units across India boil fats and oils with sodium hydroxide, hydrolysing their esters into sodium salts of long-chain carboxylic acids.

The substance. Carboxylic acids do not give the usual carbonyl tests — resonance with the -OH group makes their C=O far less reactive towards nucleophiles than an aldehyde's.