Structure and formulae of alkenes (chem only)

Inside an alkene

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Propene's full displayed formula. Tap each carbon and count its bonds, or hide the hydrogens to see the skeleton underneath.

The displayed formula of propene: every atom drawn, every bond shown as a line.

The first four alkenes

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Ethene, C₂H₄: two carbons, both in the double bond, each with two hydrogens. It's the smallest alkene, because a double bond needs two carbon atoms.

Fully displayed structural formulae of the first four alkenes. Draw each one yourself before you tap it.

Alkene or alkane?

Sort the hydrocarbons

Every molecule here is a hydrocarbon with its carbon atoms in one chain. Is each one an alkene or an alkane?

8 of 8 still to sort.

Use either test: look for a C=C, or check whether the hydrogens are exactly twice the carbons.

What you need to know

• Alkenes are hydrocarbons with a double bond between two carbon atoms. This C=C double bond is the functional group of the alkenes.
• Every carbon atom makes four bonds in total, and a double bond counts as two of them.
• Alkenes are unsaturated: they have two fewer hydrogen atoms than the alkane with the same number of carbon atoms.
• The general formula for the homologous series of alkenes is CₙH₂ₙ.
• The first four alkenes are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈) and pentene (C₅H₁₀).
• You can show an alkene by its molecular formula (such as C₃H₆) or by its fully displayed structural formula, which shows every atom and every bond. You should be able to draw the displayed formulae of all four.

The big picture

Alkenes are hydrocarbons with a double bond between two carbon atoms. That C=C is their functional group. Every carbon atom makes four bonds, and the double bond uses two of them, so the carbons in it have less room for hydrogen. An alkene has two fewer hydrogen atoms than the alkane with the same number of carbons, which is what 'unsaturated' means, and it gives the general formula CₙH₂ₙ. The first four alkenes are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈) and pentene (C₅H₁₀). A molecule can be shown by its molecular formula, like C₃H₆, or by a fully displayed structural formula that draws every atom and every bond.

⚠ Watch out

Treating the double bond as an extra bond added to a full alkane, so the hydrogens stay the same: writing propene as C₃H₈. Each carbon still makes four bonds, and the C=C uses two of them, so an alkene has two fewer hydrogens.

Memory hook

The double bond costs two hydrogens. Alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ.

Check yourself

Cover the page. Draw the displayed formulae of ethene and butene, check that every carbon has four bonds, and explain in one sentence why alkenes are unsaturated.