Carboxylic Acids - Organic Chemistry - OCR 21C - BBC

In this guide

  1. Revise
  2. Test
  1. Alkanes
  2. Alkenes
  3. Reactions of alkanes and alkenes
  4. Alcohols
  5. Carboxylic acids

Carboxylic acids

The carboxylic acids form a homologous series. Like all homologous series, the carboxylic acids:

  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, such as their boiling points
  • have similar chemical properties

Functional group

The functional group in the carboxylic acids is the carboxyl group, –COOH.

Molecular structure of carboxyl

It is responsible for the typical reactions of carboxylic acids, which are weak acids. Vinegar is a dilute solution of ethanoic acid.

General formula

The general formula for the carboxylic acids is CnH2n+1COOH (where n is the number of carbon atoms in the molecule, minus 1).

Remember that the O atoms are not joined to each other:

Worked example

Decanoic acid is a carboxylic acid. Its molecules contain 10 carbon atoms. Predict the molecular formula of decanoic acid and explain your answer.

The formula will be C9H19COOH. This is because n = (10 – 1) = 9. So, 2n+ 1 = (2 × 9) = 18 + 1 = 19.

Question

Hexanoic acid is a carboxylic acid. Its molecules contain 6 carbon atoms. Predict the molecular formula of hexanoic acid.

Show answer

The formula will be C5H11COOH.

Structures

The table shows four carboxylic acids, their molecular, displayed and structural formulae.

Table displaying carboxylic acids and their molecular, displayed and structural formulae.

Acid properties

The carboxylic acids have the typical properties of acids. For example, they:

  • dissolve in water to form acidicsolutions with pH values lower than 7
  • react with metals to form a salt and hydrogen
  • react with bases to form a salt and water
  • react with carbonates to form a salt, water and carbon dioxide

These properties are due to the –COOH functional group.

Next upTest your understandingPrevious pageAlcohols

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