11.8 - 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.