13.8 - 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.

The salts these acids make, and how they are named

• Reactions of carboxylic acids produce a salt, where the acidic hydrogen is replaced by a metal ion:
  • The first part of the name is the metal.
  • The second part comes from the acid, with -oic acid replaced by -oate (e.g. ethanoic acid makes ethanoates).
• The salt is an ionic compound containing a carboxylate ion, such as the ethanoate ion, CH₃COO^-:
  • Sodium ethanoate is CH₃COONa (or NaCH₃COO) - one Na⁺ balances one CH₃COO^-.
  • Magnesium ethanoate is (CH₃COO)₂Mg - One Mg²⁺ requires two CH₃COO^- ions.

Reaction with a metal

Adding a metal to a carboxylic acid solution produces bubbles of gas and an increase in temperature:

carboxylic acid + metal → a salt + hydrogen

ethanoic acid + magnesium → magnesium ethanoate, (CH₃COO)₂Mg, + hydrogen

The hydrogen gas burns with a squeaky pop.

Reaction with a carbonate

Adding a carbonate to a carboxylic acid solution produces fizzing as the solid dissolves:

carboxylic acid + carbonate → a salt + water + carbon dioxide

ethanoic acid + sodium carbonate → sodium ethanoate, CH₃COONa, + water + carbon dioxide

The carbon dioxide gas turns limewater milky.

Reaction with a base

Neutralising a carboxylic acid with a metal oxide or metal hydroxide releases heat without producing gas:

carboxylic acid + metal oxide (or hydroxide) → a salt + water

ethanoic acid + sodium hydroxide → sodium ethanoate, CH₃COONa, + water

What the three reactions have in common

• A metal gives a salt and hydrogen. A base gives a salt and water. A carbonate gives a salt, water and carbon dioxide.
• These are the standard reactions of any acid.
• Carboxylic acids are weak acids.

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.

Writing an equation and naming the salt

A reaction of a carboxylic acid can be written as a balanced symbol equation using six steps, shown here for ethanoic acid with sodium carbonate.

Telling the organic families apart

Four organic families can be distinguished by chemical tests: alkanes, alkenes, alcohols and carboxylic acids.

The three tests

• Bromine water tests for alkenes (decolourised by C=C double bonds).
• Warm acidified potassium dichromate tests for alcohols (orange turns green as the alcohol is oxidised).
• Sodium carbonate tests for carboxylic acids (releases carbon dioxide, which turns limewater milky).
• Alkanes give a negative result in all three tests.

Other evidence

• State at room temperature - Small alkanes and alkenes are gases; small alcohols and carboxylic acids are liquids.
• Solubility in water - Small alcohols and carboxylic acids dissolve completely in water; liquid alkanes and alkenes are immiscible and form two layers.
• Combustion - Alcohols burn with a clean, blue flame; alkanes and alkenes burn with more orange, smoky flames; carboxylic acids do not burn easily.

Two worked identifications

Liquid 1: colourless, melting point -114 °C, boiling point 79 °C; burns with a clean blue flame; turns acidified potassium dichromate from orange to green.

• Room temperature lies between its melting point and its boiling point, so it is a liquid at room temperature.
• Clean blue flame and positive potassium dichromate test confirm it is an alcohol.

Liquid 2: colourless; leaves bromine water orange/brown; leaves acidified potassium dichromate orange; fizzes with sodium carbonate to produce a gas that turns limewater milky.

• Negative bromine water test rules out an alkene; negative dichromate test rules out an alcohol.
• Fizzing with sodium carbonate producing CO₂ confirms it is a carboxylic acid.

Worked example - Acid with a metal

Write the balanced symbol equation for the reaction of ethanoic acid with magnesium.