Why Vinegar Fizzes With Baking Soda but Alcohol Does Not

Vinegar, the sourness of tamarind and the sting of an ant bite all come from carboxylic acids. Their –COOH group combines a carbonyl and a hydroxyl, and together the two behave very differently from either on its own.

This part covers naming and the carboxyl group, preparation methods, acidity, and the reactions of carboxylic acids.

**Carboxylic acids are named with the ending -oic acid, and in the carboxylate ion formed on losing H${}^{+}$ the negative charge is shared equally by two oxygen atoms through resonance, so both C–O bonds become the same length.

Naming:

- HCOOH — formic acid; IUPAC methanoic acid**
- CH${}_3$COOH — acetic acid; ethanoic acid
- CH${}_3$CH${}_2$CH${}_2$COOH — butyric acid; butanoic acid
- HOOC–COOH — oxalic acid; ethanedioic acid

Structure. The carboxyl carbon is $s{p}^2$ and planar. The –OH lone pair is partly delocalised into C=O, so the carbonyl carbon is less electrophilic than in aldehydes and ketones.

Resonance in the carboxylate ion. Two equivalent structures share the charge over both oxygens, so both C–O bonds are identical, about $127$ pm — between a C=O double bond, $123$ pm, and a C–O single bond, $136$ pm.

Worked example. For HOOCCH${}_2$CH(CH${}_3$)COOH, the chain includes both –COOH carbons, four in all, with a methyl on C2: 2-methylbutanedioic acid.

An everyday example. The sting of a red ant comes from methanoic acid.

The substance. Carboxylic acids do not give typical carbonyl reactions such as forming oximes, because resonance with –OH dampens the carbonyl carbon's reactivity.

Carboxylic acids are made by oxidising primary alcohols and aldehydes, oxidising alkylbenzenes to benzoic acid, hydrolysing nitriles and amides, adding Grignard reagents to dry ice, or hydrolysing acyl halides, anhydrides and esters.

The routes:

- Primary alcohols and aldehydes — KMnO${}_4$ or acidified K${}_2$Cr${}_2$O${}_7$
- Alkylbenzenes — hot alkaline KMnO${}_4$ oxidises a side chain with a benzylic hydrogen right down to –COOH, giving benzoic acid
- Nitriles and amides — acid or base hydrolysis turns RCN into RCONH${}_2$ and then RCOOH
- Grignard reagents — add to dry ice, then acidify: $\text{RMgX}+{\text{CO}}_2\to \text{RCOOMgX}\stackrel{{\text{H}}_3{\text{O}}^{+}}{\to }\text{RCOOH}$
- Acyl halides and anhydrides hydrolyse with water; esters with acid or base

Worked example. Both nitrile hydrolysis and Grignard carbonation add one carbon to an alkyl halide. From bromoethane, KCN gives CH${}_3$CH${}_2$CN, which hydrolyses to propanoic acid; CH${}_3$CH${}_2$MgBr with CO${}_2$ gives propanoic acid too. Each turns a two-carbon halide into a three-carbon acid.

An everyday example. Vinegar is made by bacteria oxidising the ethanol in fermented juice — the same alcohol-to-acid oxidation, carried out biologically.

The substance. **Hot KMnO${}_4$ cuts any such side chain down to one carbon**, so ethylbenzene and propylbenzene both give benzoic acid.

Carboxylic acids are stronger acids than phenols and alcohols because the carboxylate ion has two equivalent resonance structures placing the charge on two oxygens; electron-withdrawing groups make them stronger, and electron-releasing groups weaker.

Order: carboxylic acid > phenol > water > alcohol

- Carboxylic acids react with **NaHCO${}_3$**, releasing CO${}_2$; phenols do not
- In phenoxide, the charge spreads onto less electronegative ring carbons — weaker stabilisation

Substituent effects:

- Electron-withdrawing groups — halogens, –NO${}_2$, –CN — increase acidity
- More withdrawing groups increase it further: CF${}_3$COOH > CCl${}_3$COOH > CHCl${}_2$COOH > ClCH${}_2$COOH > CH${}_3$COOH
- Distance matters — the effect fades as the group moves away from –COOH
- Electron-releasing alkyl groups decrease acidity, so methanoic acid is stronger than ethanoic acid

Worked example. With p$K_a$ $4.76$ for ethanoic acid and $2.87$ for chloroethanoic acid:

$$\frac{K_a({\text{ClCH}}_2\text{COOH})}{K_a({\text{CH}}_3\text{COOH})}=10^{4.76-2.87}=10^{1.89}\approx 78$$

One chlorine atom makes the acid about $78$ times stronger.

An everyday example. Baking soda fizzes with vinegar or lemon juice, releasing carbon dioxide — the same reaction that separates carboxylic acids from phenols in the laboratory.

The substance. Benzoic acid is stronger than ethanoic acid, because the $s{p}^2$ ring carbon attached to –COOH withdraws electrons more than a methyl group does.

**The –OH of a carboxylic acid can be replaced to form anhydrides, esters, acid chlorides and amides; LiAlH${}_4$ reduces the acid to a primary alcohol; soda lime removes CO${}_2$ in decarboxylation; the Hell-Volhard-Zelinsky reaction halogenates the $\alpha$-carbon; and –COOH directs ring substitution to the meta position.

Replacing –OH:

- Anhydride** — heating with P${}_2$O${}_5$ or conc. H${}_2$SO${}_4$
- Ester — with an alcohol and conc. H${}_2$SO${}_4$, a reversible reaction
- Acid chloride — with PCl${}_5$, PCl${}_3$ or SOCl${}_2$
- Amide — the ammonium salt loses water on heating

Other reactions:

- Reduction — LiAlH${}_4$ or B${}_2$H${}_6$ gives a primary alcohol
- Decarboxylation — a sodium salt heated with soda lime gives an alkane with one carbon fewer, plus Na${}_2$CO${}_3$
- Hell-Volhard-Zelinsky — Cl${}_2$ or Br${}_2$ with red phosphorus replaces an $\alpha$-hydrogen by halogen
- Ring substitution — –COOH is deactivating and meta-directing, and benzoic acid does not undergo Friedel-Crafts reactions

Uses: methanoic acid in rubber and textile processing, ethanoic acid as vinegar and a solvent, benzoate esters in perfumery, sodium benzoate as a food preservative, and higher fatty acids in soaps.

Worked example. Heating $8.2$ g of sodium ethanoate ($M=82$ g mol${}^{-1}$) with soda lime gives methane:

$$n=\frac{8.2}{82}=0.10\text{ mol},V=0.10\times 22.7=2.27\text{ L at STP}$$

An everyday example. Sodium benzoate appears as a preservative on many bottled sauces and pickles.

The substance. Decarboxylation removes one carbon, the reverse of the Grignard carbonation from the previous section.