Carboxylic Acids

Carboxylic acids contain the carboxyl group, $\ce{-COOH}$: a C=O and an O–H on the same carbon. Ethanoic acid gives vinegar its sharp taste; methanoic acid is in ant stings. They are the only common organic compounds that behave as typical (if weak) acids, fizzing with carbonates and reacting with metals and alkalis to form salts. This note covers their structure and properties, the three ways the syllabus makes them (oxidation, hydrolysis of nitriles, hydrolysis of esters), and their five reactions: with reactive metals, alkalis and carbonates, esterification and reduction. Expect them in synthesis routes, in "identify the functional group from these tests" questions, and in titration calculations.

Structure and naming

The carboxyl carbon is sp² hybridised and the $\ce{-COOH}$ group is planar. It contains a polar C=O and a polar O–H.

The carboxyl carbon is always carbon 1, and it is counted in the stem:

formula	name
$\ce{HCOOH}$	methanoic acid
$\ce{CH3COOH}$	ethanoic acid
$\ce{CH3CH2COOH}$	propanoic acid
$\ce{CH3CH2CH2COOH}$	butanoic acid
$\ce{(CH3)2CHCOOH}$	2-methylpropanoic acid
$\ce{HOOCCOOH}$	ethanedioic acid

Salts are named after the metal and the acid with "-oate": $\ce{CH3COONa}$ is sodium ethanoate; $\ce{(CH3COO)2Mg}$ is magnesium ethanoate.

Physical properties

Carboxylic acids form strong hydrogen bonds: the O–H hydrogen of one molecule bonds to the C=O oxygen of another. In the pure liquid and in non-polar solvents, pairs of molecules hydrogen bond to each other twice, forming dimers.

compound	$M_r$	boiling point / $^\circ\text{C}$
propan-1-ol	60	97
ethanoic acid	60	118
propanal	58	48

• Carboxylic acids have higher boiling points than alcohols of similar $M_r$: each molecule has two hydrogen-bonding oxygens, and dimer formation effectively doubles the size of the particle that must be separated.
• Short-chain acids (up to about four carbons) are completely miscible with water, because they hydrogen bond to water molecules. Solubility falls as the hydrocarbon chain lengthens.

Carboxylic acids as weak acids

In water, a carboxylic acid partially dissociates:

$$\ce{CH3COOH(aq) <=> CH3COO^-(aq) + H+(aq)}$$

The position of equilibrium lies well to the left: only a small fraction of molecules donate their proton, so carboxylic acids are weak acids (see Acids and bases). A $0.1\ \text{mol dm}^{-3}$ solution of ethanoic acid has a pH of about 3, compared with 1 for hydrochloric acid of the same concentration.

Making carboxylic acids

Hydrolysis of nitriles

The $\ce{C#N}$ group is converted to $\ce{-COOH}$, and the nitrogen ends up as ammonium ions (in acid) or ammonia (in alkali).

With dilute acid:

$$\ce{CH3CH2CN + 2H2O + H+ -> CH3CH2COOH + NH4+}$$

With dilute alkali, the product is the carboxylate salt and ammonia gas is released:

$$\ce{CH3CH2CN + NaOH + H2O -> CH3CH2COONa + NH3}$$

Adding a strong acid afterwards converts the salt into the free acid:

$$\ce{CH3CH2COONa + HCl -> CH3CH2COOH + NaCl}$$

The nitrile carbon becomes the carboxyl carbon, so the acid has the same number of carbons as the nitrile. Because nitriles are made from halogenoalkanes (KCN) or carbonyl compounds (HCN), this is a route that lengthens the chain by one carbon: bromoethane → propanenitrile → propanoic acid.

Hydrolysis of esters

Esters are split by water into the acid and the alcohol they were made from. Acid hydrolysis is reversible. Alkaline hydrolysis goes to completion and gives the carboxylate salt, which must be acidified to release the free acid. Details are in Esters.

Reactions of carboxylic acids

Reactions as an acid

These are the same reactions as for any acid, but slower than with hydrochloric acid of the same concentration, because there are fewer $\ce{H+}$ ions.

• Metals: magnesium fizzes and dissolves, giving hydrogen (squeaky pop with a lighted splint). This is a redox reaction: Mg is oxidised (0 to $+2$), H is reduced ($+1$ to 0).
• Alkalis: neutralisation gives a salt and water. This is the basis of titrations of carboxylic acids with sodium hydroxide.
• Carbonates and hydrogencarbonates: effervescence of carbon dioxide (turns limewater milky).

$$\ce{CH3COOH + NaHCO3 -> CH3COONa + H2O + CO2}$$

Esterification

Heating a carboxylic acid with an alcohol and a few drops of concentrated sulfuric acid gives an ester:

$$\ce{CH3CH2COOH + CH3CH2OH <=> CH3CH2COOCH2CH3 + H2O}$$

The reaction is reversible and slow; the acid catalyst speeds it up. See Esters.

Reduction

Only the powerful reducing agent lithium tetrahydridoaluminate(III) reduces carboxylic acids, all the way to the primary alcohol (the aldehyde intermediate is reduced faster than it can be isolated). $\ce{NaBH4}$ does not reduce carboxylic acids.

$$\ce{CH3CH2COOH + 4[H] -> CH3CH2CH2OH + H2O}$$

Worked examples

Practice

Answers

1. (a) Methanoic acid. (b) Butanoic acid. (c) 2-methylpropanoic acid. (d) Sodium propanoate.
2. Both form hydrogen bonds, but ethanoic acid molecules hydrogen bond in pairs (dimers) through both the C=O and the O–H, so the hydrogen bonding is more extensive and the effective particle is larger. More energy is needed to separate the molecules.
3. (a) $\ce{2CH3COOH + Zn -> (CH3COO)2Zn + H2}$ (b) $\ce{2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2}$ (c) $\ce{CH3COOH + NH3 -> CH3COONH4}$
4. Excess acidified potassium dichromate(VI), heat under reflux. Under reflux the intermediate ethanal (and ethanol) vapour condenses and returns to the flask, so it stays in contact with the oxidising agent until it is fully oxidised to ethanoic acid, instead of escaping.
5. (a) $\ce{CH3CN + 2H2O + HCl -> CH3COOH + NH4Cl}$ (b) $\ce{CH3CN + NaOH + H2O -> CH3COONa + NH3}$
6. $\ce{CH3CH2COOH + 4[H] -> CH3CH2CH2OH + H2O}$; lithium tetrahydridoaluminate(III), $\ce{LiAlH4}$, in dry ether.
7. Sodium hydrogencarbonate: only propanoic acid fizzes (carbon dioxide turns limewater milky). Tollens' reagent, warm, on the other two: propanal gives a silver mirror; propan-1-ol does not. (Or 2,4-DNPH: orange precipitate with propanal only.)
8. $n(\ce{NaOH}) = 2.50 \times 10^{-3}\ \text{mol} = n(\text{acid})$; $M_r = 0.220 / 2.50 \times 10^{-3} = 88.0$. $\ce{C_{n}H_{2n+1}COOH}$: $14n + 46 = 88$, $n = 3$: $\ce{C3H7COOH}$. Butanoic acid, $\ce{CH3CH2CH2COOH}$, and 2-methylpropanoic acid, $\ce{(CH3)2CHCOOH}$.
9. $\ce{HOOCCH2COOH + 2NaHCO3 -> NaOOCCH2COONa + 2H2O + 2CO2}$. $M_r = 104.0$; $n = 1.04 / 104.0 = 0.0100\ \text{mol}$; $n(\ce{CO2}) = 0.0200\ \text{mol}$; volume $= 0.480\ \text{dm}^3$ ($480\ \text{cm}^3$).
10. Step 1: ethanol to bromoethane. KBr and concentrated $\ce{H2SO4}$, heat (or HBr); nucleophilic substitution. Step 2: bromoethane to propanenitrile. KCN in ethanol, heat under reflux; nucleophilic substitution (the chain gains one carbon). Step 3: propanenitrile to propanoic acid. Dilute HCl, heat under reflux (or NaOH(aq), reflux, then acidify); hydrolysis.