3.15 - Carboxylic Acids

The carboxylic acids

• Carboxylic acids are a homologous series - a family built on the same pattern: they share a general formula, each member differs from the next by a CH₂ unit, and they all have similar chemical properties.
• They all share the functional group -COOH, the carboxyl group:
  • A carbon atom joined to two oxygen atoms - one by a double bond, the other by a single bond.
  • That second oxygen carries a hydrogen atom on a single bond.
  • The carbon uses its fourth bond to join to the rest of the chain.
• It's the hydrogen atom on that oxygen that makes the compound an acid.

Naming carboxylic acids

• All names end in -oic acid, and the prefix says how many carbon atoms the molecule has: meth- (1), eth- (2), prop- (3), but- (4).
• The carbon atom in the carboxyl group is counted in - Ethanoic acid has two carbon atoms in all, one of them the carbon of the COOH.

The first four members

• Written on one line, the four atoms of the group are kept together at the end as COOH, so the group can be seen at a glance.
• The molecular formula gives the totals and hides the group. For example, C₂H₅COOH and C₃H₆O₂ are the same molecule written two different ways.
• All four acids are liquids at room temperature and pressure.

Where they turn up

• Vinegar is a dilute solution of ethanoic acid in water, which gives it its sharp taste and smell.
• Carboxylic acids are used in foods (as flavours and preservatives) and in personal care products such as soaps.

What carboxylic acids do

• Carboxylic acids are a family of organic acids. A solution of one behaves the way any other acid solution behaves.
• When dissolved in water, a carboxylic acid forms an acidic solution.
• It's acidic because the acid releases hydrogen ions, H⁺, into the water.
• Because it contains hydrogen ions, the solution has a pH below 7 and turns universal indicator red, orange or yellow.
• When a carboxylic acid reacts, it reacts as an acid: its acidic hydrogen is replaced by a metal (or other positive ion) to form a salt.

Carboxylic acids are weak acids

An acid releases hydrogen ions, H⁺, when dissolved in water by ionising.

Strong versus weak

• Strong acid - Ionises completely in water. Every molecule splits up to release its H⁺ ion.
• Weak acid - Ionises only partially in water. Only a small fraction of the molecules release their H⁺ ions, forming an equilibrium mixture of ions and un-ionised molecules.

Carboxylic acids are weak acids because they only partially ionise in aqueous solution.

Why this changes the pH

• pH depends on the concentration of hydrogen ions: the higher the concentration of H⁺, the lower the pH.
• At the same concentration, a weak acid has fewer free H⁺ ions in solution than a strong acid.
• Therefore, a weak acid has a higher pH and reacts more slowly than a strong acid of the same concentration.

Strength and concentration are different things

• Concentration is the amount of acid dissolved per unit volume.
• Strength is the fraction of acid molecules that ionise in water.