IB DP Chemistry: SL复习笔记10.1.2 Understanding Organic Molecules

Representing Formulae

  • Organic compounds can be represented in a number of ways:
    • Empirical Formulae
    • Molecular Formulae
    • Structural Formulae
    • Condensed Structural Formulae
  • The empirical formula shows the simplest possible ratio of the atoms in a molecule
  • For example:
    • Hydrogen peroxide is H2O2 but the empirical formula is HO
  • The molecular formula shows the actual number of atoms in a molecule
  • For example:


The molecular formulae of butane and butene

  • The structural formula shows the spatial arrangement of all the atoms and bonds in a molecule
  • This is also known as the displayed formula or graphical formula.
  • For example:


The structural formula of 2-methylbutane

  • In a condensed structural formulae enough information is shown to make the structure clear, but most of the actual covalent bonds are omitted
  • Only important bonds are always 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:


Representing condensed structural formulae of straight chains

  • Branched alkanes are shown as follows:


Representing condensed structural formulae of branched alkanes

  • Alkenes are shown as follows:


Representing condensed structural formulae of alkenes


  • Structural isomers are compounds that have the same molecular formula but different structural formulae
    • Eg. propene and cyclopropane


Both propene and cyclopropane are made up of 3 carbon and 6 hydrogen atoms but the structure of the two molecules differs

  • There are three different types of structural isomerism:
    • Branch-Chain isomerism
    • Positional isomerism
    • Functional group isomerism

Branch-Chain isomerism

  • Branch-Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same
  • This is caused by branching
    • Eg. pentane and 2,2-dimethylpropane


Both compounds are made up of the same atoms however the longest carbon chain in pentane is 5 and in 2,2-dimethylpropane it is 3 (with two methyl branches)

Positional isomerism

  • Positional isomers arise from differences in the position of a functional group in each isomer
    • The functional group can be located on different carbons
    • For example, butan-1-ol and butan-2-ol


Both compounds have an alcohol group and are made up of 4 carbon, 10 hydrogen and one oxygen atom however in butan-1-ol the functional group is located on the first carbon and in butan-2-ol on the second carbon

Functional group isomerism

  • When different functional groups result in the same molecular formula, functional group isomers arise
  • The isomers have very different chemical properties as they have different functional groups
    • For example, butanol and ethoxyethane


Both compounds have the same molecular formula however butan-1-ol contains an alcohol functional group and ethoxyethane an ether functional group

  • You should be able to deduce all possible isomers for organic compounds knowing their molecular formula

Worked Example

How many isomers are there of, C3H6Br?


Step 1: Draw the structural formula of the compound

3.1-An-Introduction-to-AS-Level-Organic-Chemistry-Step-1-Isomers-of-dibromopropaneStep 2: Determine whether there is functional group, branch-chain or positional isomerism

    • Functional group? No, as Br is the only functional group possible
    • Branch-chain? No, as the longest chain can only be 3
    • Positional? Yes, as the two bromine atoms can be bonded to different carbon atoms



Worked Example

How many isomers are there of the compound with molecular formula C4H10 ?


Step 1: Draw the structural formula of the compound


Step 2: Determine whether it is a functional group, chain or positional isomerism

    • Functional group? No, as there are no functional groups
    • Positional? No, as there are no functional groups which can be positioned on different carbon atoms
    • Chain? yes!


Exam Tip

Don't be fooled by molecules by bending and turning through 90 degrees - that does not make them isomers. The best test is to try and name them - isomers will have a different name.

Saturated & Unsaturated

Saturated & unsaturated hydrocarbons

  • Saturated hydrocarbons are hydrocarbons which contain single bonds only resulting in the maximum number of hydrogen atoms in the molecule
  • Unsaturated hydrocarbons are hydrocarbons which contain carbon-carbon double or triple bonds


The diagram shows saturated hydrocarbons which contain single bonds only and unsaturated hydrocarbons which contain double/triple bonds as well