Understanding Organic Molecules (DP IB Chemistry: SL)

• 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

Answer:

Step 1: Draw the structural formula of the compound

Step 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

Answer:

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!

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