Carboxylic Acids: Acidity and Synthesis

Carboxylic acids contain the carboxyl group (–COOH) and are weak acids that ionize to give a carboxylate ion; they are made by the oxidation of primary alcohols or aldehydes, the hydrolysis of nitriles or esters, or the reaction of a Grignard reagent with CO2. In brief: ethanoic (acetic) acid has Ka = 1.8 × 10−5, so a 0.10 M solution has [H+] = 1.3 × 10−3 M and pH 2.87, while trichloroethanoic acid is more than 10,000 times stronger.

Carboxylic acids are found in vinegar, citrus fruits, sour milk and all living cells, in the form of fatty acids and amino acids. They are the starting material for esters, amides and many polymers. This article explains the structure and properties of carboxylic acids, why they are acidic, how substituents change their strength, their synthesis and their typical reactions, with worked examples.

What is the structure of a carboxylic acid?

The carboxyl group combines a carbonyl group (C=O) and a hydroxyl group (–OH) on the same carbon. The group is planar, with bond angles of about 120°. The names use the suffix -oic acid, with the carboxyl carbon as C1: methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid, butanoic acid. Many have traditional names: formic, acetic, propionic and butyric acids. Aromatic acids are named as benzoic acid derivatives. Acids with two carboxyl groups are dioic acids, such as ethanedioic (oxalic) acid.

What are their physical properties?

Carboxylic acids form strong hydrogen bonds, and in the pure liquid and even in the vapor they exist partly as hydrogen-bonded dimers, with two O–H···O=C bonds between two molecules. As a result their boiling points are higher than those of alcohols of similar molar mass: ethanoic acid (60 g/mol) boils at 118 °C and propan-1-ol (60 g/mol) at 97 °C. The smaller acids (up to four carbons) are miscible with water, while the longer ones are poorly soluble. Pure ethanoic acid freezes at 16.6 °C, which is the reason why it is called “glacial”. The lower acids have sharp or unpleasant odors: butanoic acid smells of rancid butter.

R C O O H R C O O H

Why are they acidic?

When a carboxylic acid ionizes, RCOOH ⇌ RCOO− + H+, the negative charge of the carboxylate ion is shared equally by the two oxygen atoms through resonance. Both C–O bonds in the carboxylate ion have the same length (about 127 pm), between a single and a double bond. This delocalization stabilizes the anion, so carboxylic acids are much stronger acids than alcohols (pKa about 16), although they are weak acids compared with strong mineral acids: they ionize only partly in water.

R C O O− R C O− O R C O½− O½−

Electron-withdrawing substituents next to the carboxyl group stabilize the anion further, through the inductive effect, and increase the acid strength. Electron-donating alkyl groups decrease it slightly.

Acid strengths of carboxylic acids

Approximate pKa values at 25 °C
Acid Formula pKa Ka
Methanoic acidHCOOH3.751.8 × 10−4
Ethanoic acidCH3COOH4.761.7 × 10−5
Propanoic acidCH3CH2COOH4.871.3 × 10−5
Benzoic acidC6H5COOH4.206.3 × 10−5
Chloroethanoic acidClCH2COOH2.861.4 × 10−3
Dichloroethanoic acidCl2CHCOOH1.295.1 × 10−2
Trichloroethanoic acidCl3CCOOH0.650.22

Each chlorine atom makes the acid stronger: the first increases Ka about 80-fold, the second about 40-fold and the third about 4-fold. The effect decreases with distance: 4-chlorobutanoic acid is hardly stronger than butanoic acid.

How are they made?

  1. Oxidation of primary alcohols or aldehydes with acidified dichromate or permanganate: CH3CH2OH → CH3COOH. The alcohol is heated under reflux with an excess of oxidant.
  2. Hydrolysis of nitriles: RCN + 2H2O + H+ → RCOOH + NH4+. Combined with the preparation of the nitrile from a haloalkane and cyanide, this lengthens the chain by one carbon.
  3. Grignard reagent and carbon dioxide: RMgBr + CO2 gives RCOOMgBr, and acidification gives RCOOH.
  4. Hydrolysis of esters, amides and acid derivatives.
  5. Oxidation of alkylbenzenes: toluene with hot permanganate gives benzoic acid.
  6. Fermentation: acetic acid bacteria oxidize ethanol in wine or cider to vinegar.

What are the typical reactions?

Worked examples

Example 1: pH of acetic acid

Find the pH of 0.10 M ethanoic acid (Ka = 1.8 × 10−5). Ka = x2 / (0.10 − x) ≈ x2 / 0.10, so x = √(1.8 × 10−6) = 1.34 × 10−3 M and pH = −log(1.34 × 10−3) = 2.87. The percent ionization is 1.34 %.

Example 2: analyzing vinegar

A 10.00 mL sample of vinegar needs 33.3 mL of 0.250 M NaOH for neutralization. n(NaOH) = 0.0333 L × 0.250 = 8.33 mmol = n(CH3COOH), so the concentration is 8.33 mmol / 10.00 mL = 0.833 M. The mass per liter is 0.833 × 60.05 = 50.0 g/L, which is a 5.0 % acetic acid solution (w/v), typical for table vinegar.

Example 3: building a longer acid

To make butanoic acid from 1-bromopropane: (1) 1-bromopropane + KCN in ethanol gives butanenitrile, C3H7CN; (2) hydrolysis with dilute acid under reflux gives butanoic acid, C3H7COOH. The chain has grown by one carbon.

Example 4: comparing acid strengths

Which is the stronger acid, CH3COOH or FCH2COOH? The fluorine atom withdraws electrons and stabilizes the carboxylate ion, so fluoroethanoic acid (pKa about 2.6) is considerably stronger than ethanoic acid (4.76).

How were they discovered?

Vinegar has been used since antiquity. The alchemist Jabir ibn Hayyan is credited with concentrating acetic acid from vinegar in the 8th century, and its composition was later studied by Lavoisier. Formic acid was obtained from ants by John Ray in 1671, and the name comes from the Latin formica. Friedrich Wöhler’s synthesis of oxalic acid from cyanogen in 1824, and Hermann Kolbe’s synthesis of acetic acid from the elements in 1845, helped to disprove the idea that organic compounds can only come from living matter.

Which mistakes are common?