Naming organic compounds

O Level / IGCSE Chemistry · Organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The name of an organic compound has two parts. The stem tells you how many carbon atoms are in the chain: meth- 1, eth- 2, prop- 3, but- 4, pent- 5. The ending tells you the family: -ane for an alkane, -ene for an alkene, -ol for an alcohol and -oic acid for a carboxylic acid.

A number in the name shows the position of the double bond or the –OH group. Count from the end of the chain that gives the smaller number. In a carboxylic acid the carbon of –COOH is counted as part of the chain.

An ester has a two-word name. The first word comes from the alcohol and the second from the acid.

Rules to remember

Common mistake

Students forget to count the carbon atom of –COOH, so they call CH3CH2COOH ethanoic acid. It has three carbon atoms, so it is propanoic acid.

Worked example

Name the ester CH3CH2CH2COOCH3.

  1. Split the formula at COO: the acid part is CH3CH2CH2COO and the alcohol part is CH3.
  2. The acid part has 4 carbon atoms (3 + the C of COO), so it comes from butanoic acid and gives 'butanoate'.
  3. The alcohol part has 1 carbon atom, so it comes from methanol and gives 'methyl'.
  4. Write the alcohol part first, then the acid part.

Answer: methyl butanoate

Practice questions

Try each one, then open the answer.

1. What is the structural formula of ethanol?

  1. A
    CH3CH2CH2OH
  2. B
    CH3COOH
  3. C
    CH3CH2OH
  4. D
    CH2=CH2
Show answer

Answer: C. Ethanol has two carbons (eth-) and an –OH group (-ol): CH3CH2OH. CH3CH2CH2OH is propan-1-ol, CH3COOH is ethanoic acid and CH2=CH2 is ethene.

2. What is the molecular formula of the alkene that has five carbon atoms in its molecule?

  1. A
    C5H12
  2. B
    C5H11OH
  3. C
    C5H10
  4. D
    C5H8
Show answer

Answer: C. Alkenes have the general formula CnH2n, so with n = 5 the formula is C5H10 (pentene). C5H12 is the alkane and C5H11OH is an alcohol.

3. What is the name of CH3CH=CHCH3?

  1. A
    but-1-ene
  2. B
    butane
  3. C
    propene
  4. D
    but-2-ene
Show answer

Answer: D. Four carbons with the double bond starting at carbon 2: but-2-ene.

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