O LevelChemistryCambridge (CIE)Revision Notes11. Organic Chemistry11.1 Formulae, Functional Groups & TerminologyOrganic Formulae

Organic Formulae (Cambridge (CIE) O Level Chemistry): Revision Note

Exam code: 5070

Caroline Carroll

Written by: Caroline Carroll

Reviewed by: Lucy Kirkham

Updated on

Displayed Formulae

  • Organic Chemistry is the scientific study of the structure, properties and reactions of organic compounds

  • Organic compounds are those which contain carbon

  • For conventional reasons metal carbonates, carbon dioxide and carbon monoxide are not included in organic compounds

  • Many of the structures you will be drawing are hydrocarbons

  • A hydrocarbon is a compound that contains only hydrogen and carbon atoms

  • Organic compounds can be represented in a number of ways:

    • Displayed Formulae

    • General Formulae

    • Structural Formulae

  • The displayed formula shows the spatial arrangement of all the atoms and bonds in a molecule

  • For example:

The displayed formula of 2-methylbutane. A horizontal chain of four carbon atoms, each written as C and joined to the next by a single line. The first carbon carries three hydrogen atoms, one above, one to the left and one below. The second carries one hydrogen above and one below. The third carries one hydrogen above and, below it, a single line down to a fifth carbon atom which carries three hydrogen atoms, one to the left, one to the right and one below. The fourth carbon carries three hydrogen atoms, one above, one to the right and one below. There are five carbon atoms and twelve hydrogen atoms in total, and every bond is drawn as a single line.

  • This displayed formula tells us several things about the compound

    • It has 5 carbon atoms

    • It has 12 hydrogen atoms

    • It has only single bonds 

Structural Formulae

  • In structural formulae, enough information is shown to make the structure clear, but most of the actual covalent bonds are omitted

  • Only the important bonds are shown, such as double and triple bonds

  • Identical groups can be bracketed together

  • Side groups are also shown using brackets

  • Straight chain alkanes are shown as follows:

    • E.g. pentane is written as CH3CH2CH2CH2CH3, or as CH3(CH2)3CH3 with the identical CH2 groups bracketed together

On the left, the displayed formula of pentane: a horizontal chain of five carbon atoms joined by single lines, with one hydrogen atom above and one below each carbon, and one further hydrogen atom at each end of the chain. To the right, the text reads: is represented as CH3CH2CH2CH2CH3, or CH3(CH2)3CH3.
On the left, the displayed formula of ethene: two carbon atoms joined by a double bond drawn as two parallel lines, with each carbon also joined by a single line to one hydrogen atom above it and one hydrogen atom below it. To the right, the text reads: is represented as CH2=CH2.

Structural Isomers

  • Structural isomers are compounds that have the same molecular formula but different structural formulae

  • The molecular formula is the actual number of atoms of each element in a compound

  • Compounds with the same molecular formula can have different structural formulae due to the different arrangement of their atoms

  • Two examples of structural isomers are shown below 

Table showing Structural Isomerism in C4H10

A table comparing the two isomers of C4H10 in three rows. The molecular formula row gives C4H10 for both. The structural formula row gives CH3CH2CH2CH3 for the first and CH3CH(CH3)CH3 for the second. The displayed formula row draws each one. The first is an unbranched horizontal chain of four carbon atoms, each carrying a hydrogen above and below, with one further hydrogen at each end of the chain. The second is a horizontal chain of three carbon atoms with a fourth carbon attached above the middle carbon; that fourth carbon carries three hydrogen atoms, and the three chain carbons carry the remaining seven.

Table showing Structural Isomerism in C4H8

A table comparing the two isomers of C4H8 in three rows. The molecular formula row gives C4H8 for both. The structural formula row gives CH3CH2CH=CH2 for the first and CH3CH=CHCH3 for the second. The displayed formula row draws each one as a horizontal chain of four carbon atoms. In the first, the double bond is drawn as two parallel lines between the third and fourth carbon atoms, at the end of the chain. In the second, the double bond is between the second and third carbon atoms, in the middle of the chain.

Examiner Tips and Tricks

Remember: All the atoms are shown in a structural formula, but of the bonds, only double and triple bonds are drawn. Single bonds are left out.

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Author
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Caroline Carroll

Author: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.

Lucy Kirkham

Reviewer: Lucy Kirkham

Expertise: Content Creator

Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.