Carboxylic acid

A carboxylic acid is an organic compound that contains at least one carboxyl group (-COOH) or (-C(=O)OH). The general formula of a carboxylic acid is R–COOH, with R being a substituent, usually organic.

Types

Carboxylic acids can be classified under several categories:

• Monocarboxylic acids: only have one -COOH group. Most are liquid at room temperature and have strong odor. Ex: formic acid, acetic acid;
• Dicarboxylic acids: have two -COOH groups. Solid and poorly volatile at standard conditions. Ex: oxalic acid, succinic acid;
• Polycarboxylic acids: can have more than three -COOH groups. Solid and non-volatile. Ex: citric acid;
• Organic: The R substituent is an organic group. Ex: acetic acid, benzoic acid.
• Inorganic: The R substituent is an inorganic group (exception for formic acid, where R is H and is organic due to the C-H bond). Ex: carbonic acid, trifluoroacetic acid;
• Aminoacids: Organic carboxylic acids that also contain an amino (-NH₂) group. Ex: glycine;

Salts and esters of carboxylic acids are called carboxylates. The carboxylate anion R–COO⁻ is usually named with the suffix -ate (ex: formic acid - formate).

General properties

Monocarboxylic acids are liquid (except for aromatic), volatile and have a strong poignant smell. Polycarboxylic acids on the other hand are all solids and non-volatile. Carboxylic acids are polar compounds.

Carboxylic acids are Brønsted–Lowry acids because they are proton (H⁺) donors. Generally they are weak acids.

Availability

Concentrated formic acid is sold by various beekeeping stores, as is acetic and oxalic acids.

Citric acid can be purchased as lemon salt from most stores, while diluted acetic acid is available as vinegar.

Benzoic acid can be made by acidifying sodium benzoate, which is sold as food preservative. Tartaric acid can be easily made from potassium tartrate, in the same way.

Preparation

Oxidation of primary alcohols, such as methanol or ethanol with potassium permanganate in the presence of sulfuric acid will give their respective carboxylic acids. Jones oxidation is another route.

CH₃OH + [O] → HCOOH + H₂O

CH₃CH₂OH + [O] → CH₃COOH + H₂O

Carbonation of a Grignard and organolithium reagents, followed by acidification:

RLi + CO₂ → RCO₂Li

RCO₂Li + HCl → RCO₂H + LiCl

Carboxylates are the side products from the haloform reaction. The following ketones/alcohols will give the following carboxylates:

• Ethanol/acetaldehyde - Formate
• Acetone/isopropanol - Acetate
• Methyl ethyl ketone/sec-Butanol - Propanoate
• Acetylacetone - Acetate
• Acetophenone - Benzoate
• Phenylacetone - Phenylacetate

Safety

Concentrated carboxylic acids, such as formic or acetic acids are corrosive to skin and have a strong irritant smell. Wear proper protection when working with them.

Polycarboxylic acids are less corrosive and volatile, though conc. aqueous solutions are irritant.

References

Relevant Sciencemadness threads

• Test for carboxylic acids
• Carboxylic acid -> Alkan
• Reduction of carboxylic acid to alcohol!