carboxylic acids

Chemistry only

Carboxylic acids

Carboxylic acids are another example of a homologous series of organic compounds. Carboxylic acids are a family of weak acids which contain the carboxyl group (-COOH) functional group. You may have met some of these carboxylic acids before and not realised it at the time; for example ant bites and wasp stings contain the simplest carboxylic acid; methanoic acid while ethanoic acid is the main ingredient in vinegar and many esters. Ethanoic acid is also used to marinated meat and fish and to pickle vegetables.

However as well as common uses the carboxylic acids have many industrial uses and they are used in the manufacture of many important industrial and biological processes e.g. they are used in the manufacture of solvents, adhesives, soaps, rubber, dyes, perfumes and many types of drugs.

Lets start by looking at the structure and formula for the first four carboxylic acid. Now all carboxlyic acids contain the carboxyl function group (-COOH) and their general formula is CnH2n+1COOH. The name of any carboxylic acid molecule is based on the corresponding alkane and replacing the -e ending in the alkane with the letters -oic acid e.g.

alkane carboxylic acid
methane methanoic acid
ethane ethanoic acid
propane propanoic acid
butane butanoic acid

Ball and stick models as well as the structural formula and displayed formula for the first 4 carboxylic acids are shown below

3d models, molecular formula and displayed formula for the first 4 carboxylic acids.

As before when we draw out the formula for these compounds we always draw them with the carboxyl functional group shown e.g. Methanoic acid is HCOOH and never CO2H. Ethanoic acid is CH3COOH and never C2H402.

Reactions and properties of carboxylic acids

Students working on a chemistry experiment in the lab

Carboxylic acids with less than 6 carbon atoms are soluble in water; they dissolve to form weak acids. If you remember a weak acid forms when a solid or liquid acidic compound only partly ionises when it is added to water. The dissociation of a carboxylic acid in water is shown below. Here the carboxylic acid molecule ionises to form a carboxylate anion (RCOO-) and a hydrogen ion (H+). It is the presence of the hydrogen ion (H+) which gives the molecule its acidic properties. However very few of the carboxylic acid molecules actually ionise, most remain intact. This means that the position of equilibrium lies very much to the left hand side; that is the reactants; hence the reason why carboxylic acids are weak acids with fairly high pH values of between 3-5.

Equation to show carboxylic acid dissociation.

As an example consider the simplest carboxylic acid; methanoic acid (HCOOH). When methanoic acid is added to water a few of molecules will ionise to form methanoate ions and hydrogen ions. Most of the methanoic acid molecules stay intact and do not dissociate or break down in water, we can show this as:

Dissociation of methanoic acid to form methanoate ions and hydrogen ions

similarly with ethanoic acid we have:
Dissociation of methanoic acid to form methanoate ions and hydrogen ions

In the symbolic equations below the water has been omitted to simplify the equations.

CH3COOHCH3COO- + H+
and with propanoic acid we have:
C2H5COOH C2H5COO- + H+
When the carboxylic acid molecule dissolves or reacts with other substances it loses its acidic hydrogen atom just as you would expect for an acid. The names of the ions formed in these reactions maybe new to you but simply add -ate for -ic on the end of the acid e.g.

carboxylic acid molecular formula ion formed structural formula of ion
methanoic HCOOH methanoate HCOO-
ethanoic CH3COOH ethanoate CH3COO-
propanoic C2H5COOH propanoote C2H5COO-
butanoic C3H7COOH butanoate C3H7COO-

Reactions of carboxylic acids with metal carbonates

Apparatus diagram showing the reaction of calcium carbonate with a carboxylic acid.  The gas carbon dioxide can be tested for by bubbling it through limewater.

A reaction you have seen before is when hydrochloric acid reacts with a metal carbonate such as sodium carbonate or calcium carbonate to form a salt, water and carbon dioxide gas. Equations for these reactions are shown below:

metal carbonate(s) + acid(aq) → salt(aq) + water(l) + carbon dioxide(g)
For example:
calcium carbonate(s) + hydrochloric acid(aq) → calcium chloride(aq) + water(l) + carbon dioxide(g)
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
Example2:
sodium carbonate(s) + hydrochloric acid(aq) → sodium chloride(aq) + water(l) + carbon dioxide(g)
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)

swapping the hydrochloric acid; a strong acid for a weak carboxylic acid will produce a similar reaction but it will be much slower e.g.

Example 3:
sodium carbonate(s) + methanoic acid(aq) → sodium methanoate(aq) + water(l) + carbon dioxide(g)
Na2CO3(s) + 2HCOOH(aq) → 2COONa(aq) + H2O(l) + CO2(g)
Example 4:
calcium carbonate(s) + ethanoic acid(aq) → calcium ethanoate(aq) + water(l) + carbon dioxide(g)
CaCO3(s) + 2CH3COOH(aq) → (CH3COO)2Ca(aq) + H2O(l) + CO2(g)

The apparatus to compare the rate of reaction of a weak carboxylic acid and a strong acid with chalk (calcium carbonate) is shown below. Using this simple set-up we can compare the rate of reaction of a strong and a weak acid with calcium carbonate (chalk). All we have to do is count the number of bubbles or modify the apparatus to include a gas syringe and measure the volume of carbon dioxide gas released in a given time.

Key Points

Practice questions

Check your understanding - Questions on carboxylic acids

Check your understanding - Quick Quiz on carboxylic acids.

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