MYP Chemistry Structural Formulae – Carboxylic Acids and Esters Notes

Carboxylic Acids – Structure, Formulae and Properties

The Carboxyl Group (–COOH)

1. Carboxylic acids are organic compounds that contain the carboxyl functional group, written as –COOH.
2. The carboxyl group is made of:
   1. A carbonyl group: C=O
   2. A hydroxyl group: –OH
   3. joined to the same carbon atom.
3. So the functional group looks like: $$–C(=O)OH–C(=O)OH$$
4. General formula (simple carboxylic acids / alkanoic acids): $$C_nH_{2n+1}COOH$$ (or often written simply as R–COOH, where R is an alkyl group.)

Drawing Structural and Displayed Formulae

Example

Ethanoic Acid (CH₃COOH)

• Molecular formula: CH₃COOH or C₂H₄O₂
• Condensed structural formula: CH₃COOH or CH₃–COOH

Physical Properties of Carboxylic Acids

Because of –COOH, carboxylic acids:

1. Form hydrogen bonds with each other → relatively high boiling points
2. Form hydrogen bonds with water → small acids (like methanoic and ethanoic acid) are soluble in water
3. Are weak acids, partially ionising in water: $$\mathrm{CH}_3 \mathrm{COOH}_{(\mathrm{aq})} \rightleftharpoons \mathrm{CH}_3 \mathrm{COO}_{(\mathrm{aq})}^{-}+\mathrm{H}_{(\mathrm{aq})}^{+}$$​

Typical Reactions of Carboxylic Acids

1. With reactive metals (e.g. Mg, Zn)
   → Salt + hydrogen gas $$2 \mathrm{CH}_3 \mathrm{COOH}+\mathrm{Mg} \rightarrow\left(\mathrm{CH}_3 \mathrm{COO}\right)_2 \mathrm{Mg}+\mathrm{H}_2$$
2. With bases (alkalis)
   → Salt + water $$\mathrm{CH}_3 \mathrm{COOH}+\mathrm{NaOH} \rightarrow \mathrm{CH}_3 \mathrm{COONa}+\mathrm{H}_2 \mathrm{O}$$ The salts are called carboxylates (e.g. sodium ethanoate).
3. With alcohols
   → Esters + water (this is esterification, see below).

Uses of Carboxylic Acids

1. Ethanoic acid (acetic acid):
   1. Main acid in vinegar, used as a food preservative.
2. Citric acid:
   1. Used in cleaning products and as a food additive (sour taste).
3. Used as starting materials in making polymers, pharmaceuticals and dyes.

Esters – Structure, Formulae and Their Relationship to Carboxylic Acids

Ester Functional Group and General Structure

1. Esters are derivatives of carboxylic acids.