5.6 - Carboxylic Acids

The carboxylic acids, formula and trends

• Carboxylic acids are a homologous series - a family built on the same pattern: they share one 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 general formula

Saturated, straight-chain carboxylic acids have the general formula:

C_nH_2n+1COOH

• n is the number of carbon atoms in the chain in front of the group, not counting the carbon of the COOH itself.
• In propanoic acid, there are three carbon atoms in total, so n = 2, giving C₂H₅COOH (or CH₃CH₂COOH).

The first four members

• Methanoic acid is the member with nothing in front of the group: n = 0, so a single hydrogen atom sits where the chain would be (HCOOH).
• The molecular formula gives the total numbers of each atom.
• The shortened structural formula shows the arrangement of atoms along the chain, keeping the carboxyl group as COOH.
• The full structural formula shows every atom and bond.

How the properties change along the series

• Mixing with water - Methanoic, ethanoic, propanoic and butanoic acid are miscible with water. Past these four, solubility decreases as the molecules get bigger.
• Melting and boiling points rise as the molecules get bigger, because the intermolecular forces become stronger and require more energy to break.

Where they turn up

• Carboxylic acids are used to make preservatives, soaps and medicines.
• Vinegar is a dilute solution of ethanoic acid, CH₃COOH, in water. It's used in household cleaning products because it's a non-toxic acid.

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.