13.8 - Carboxylic Acids

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.

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.