9.12C - Formulae and structures of alkenes

Names and molecular formulae

The alkenes here are hydrocarbons with one carbon–carbon double bond. A molecular formula counts atoms; it does not show their arrangement.

For these molecules there are twice as many hydrogens as carbons. The two butenes have the same molecular formula, so $\ce{C4H8}$ alone cannot distinguish them. Their double bonds are in different positions.

Counting bonds around each carbon

A displayed structure shows every atom and covalent bond. A single bond is one line; a double bond is two parallel lines. Count a double bond as two when checking that carbon has four bond lines. Hydrogen always has one.

[DIAGRAM: asset_name: alkene_displayed_structures: Displayed structures of ethene, propene, but-1-ene and but-2-ene - diagram 1; asset_slug: 216_9_12c_formulae_and_structures_of_alkenes_diagram1; file: 216_9_12c_formulae_and_structures_of_alkenes_diagram1_imagegen_v1.png; recommended_method: deterministic_drawn; description: Exact monochrome displayed-structure summary for Edexcel 9.12C. The diagram shows ethene C2H4 as CH2=CH2, propene C3H6 as CH2=CH-CH3, but-1-ene C4H8 as CH2=CH-CH2-CH3, and but-2-ene C4H8 as CH3-CH=CH-CH3. Every C-H, C-C and C=C covalent bond is drawn with atom labels, and the butene panels state where the C=C starts. Deterministic drawing is required because the formulae, names, bond counts and double-bond positions are assessed details.]

Ethene has two carbons joined by a double bond. Each carbon uses two bond lines for C=C, leaving two for hydrogen: $\ce{CH2=CH2}$.