6A.1-6A.7 - Organic Formulae, Naming And Isomerism

Why One Molecule Can Have Several Formulae

A label such as $\ce{C4H10}$ gives the atoms present, but it does not tell you how the atoms are connected. That is a serious limitation in organic chemistry: the same molecular formula can describe more than one molecule. The first job is to choose the representation that carries the information the question needs.

$\ce{CH4}$, $\ce{C2H6}$, $\ce{C6H14}$ and $\ce{C2H4}$ are hydrocarbons; $\ce{CH3CH2OH}$ is not, because oxygen is present.

[DIAGRAM: asset_name: Organic Representation Motifs; asset_slug: edexcel_a_level_chemistry_l029_organic_representation_motifs; recommended_method: deterministic_chart; description: Butane shown as molecular, structural, displayed and skeletal formulae; each skeletal end or vertex represents carbon.]

Use these formula types with different expectations:

In skeletal formulae, each line end or vertex is a carbon atom unless another atom is written. Hydrogens attached to carbon are usually not shown; heteroatoms such as $\ce{O}$, $\ce{N}$, $\ce{Cl}$ and hydrogens attached to them must be shown. Double and triple bonds are still drawn because they change both bonding and formula.

Worked Example

Represent butane, molecular formula $\ce{C4H10}$, in several ways.

The chemical interpretation is that these are not five different substances. They are five levels of detail for the same substance, and a question may ask you to translate from one level to another.

Homologous Series, Functional Groups And Names

Organic chemistry becomes manageable because molecules fall into families.

Successive members differ by $\ce{CH2}$, so their physical properties such as boiling temperature change gradually as the carbon chain gets longer.

For example, a $\ce{C=C}$ bond gives alkene behaviour, -OH gives alcohol behaviour, and a halogen atom in a haloalkane changes the molecule's polarity and reactions. You do not need every later reaction yet; you do need to recognise that the functional group is the reactive handle.

For IUPAC-style naming in this specification, the carbon-chain prefixes up to $\ce{C10}$ are essential:

Use a naming route rather than guessing:

1. Find the longest carbon chain that contains the main functional group or multiple bond.
2. Choose the parent prefix from the number of carbons in that chain.
3. Choose the ending from the family, such as -ane for an alkane or -ene for an alkene.
4. Number the chain to give the important feature the lowest possible number.
5. Add side-chain names and position numbers, such as 2-methyl-. Repeated identical branches use di-, tri- and so on, with every locant shown: $\ce{CH3C(CH3)2CH3}$ is 2,2-dimethylpropane, because both methyl branches are on carbon 2 of a three-carbon parent. When several branches are present, use the lowest set of locants at the first point of difference; different branch names are listed alphabetically, ignoring di-/tri- for ordering.

Worked Example

Name $\ce{CH3CH(CH3)CH2CH3}$.

The longest continuous chain has four carbon atoms, so the parent is butane. There is a one-carbon branch, methyl, on carbon 2 when the chain is numbered from the nearer end. The name is 2-methylbutane.

The tempting wrong name is 3-methylbutane. That uses the same chain but numbers from the far end; IUPAC numbering gives the lowest possible locant to the branch.

Guided Practice

Name each structure.

Common-Error Contrast

But-1-ene and but-2-ene have the same molecular formula, but the number is not decorative. It tells you where the double bond is. In organic naming, a missing or wrong locant can describe a different molecule.

Classifying Organic Reaction Types

Before you learn detailed mechanisms, you need to classify what sort of change has happened. The classification comes from the structural change, not from the size of the equation or the memorised topic heading.