Formula for Alkenes

An alkene is a type of hydrocarbon — a compound made of only hydrogen and carbon atoms.

What makes an alkene different from an alkane is one special bond: a carbon–carbon double bond, written C=C.

• A single bond (C–C) is one shared pair of electrons.
• A double bond (C=C) is two shared pairs of electrons.

Because an alkene has a C=C double bond, it is described as UNSATURATED.

• Unsaturated = the molecule contains a C=C double bond (alkenes).
• Saturated = the molecule has only single bonds (alkanes) — it holds the maximum number of hydrogen atoms.

The C=C double bond is the functional group of alkenes — the part of the molecule that gives the whole family its characteristic chemistry. (You will use it later to test for alkenes with bromine water and to make polymers.)

In one sentence: an alkene is an unsaturated hydrocarbon that contains a carbon–carbon double bond (C=C). Learn that definition word-for-word — it is a frequent 1–2 mark question.

Every alkene fits a single pattern called the general formula:

$C_n H_{2n}$

Here n is the number of carbon atoms in the molecule. The number of hydrogen atoms is always 2 × n — that is, twice the number of carbons.

How to use it. Choose the number of carbons (n), then double it for the hydrogens:

n (carbons)	2n (hydrogens)	Formula	Name
2	4	C₂H₄	ethene
3	6	C₃H₆	propene
4	8	C₄H₈	butene
5	10	C₅H₁₀	pentene

Why does the smallest alkene have 2 carbons?

You need two carbon atoms to make a carbon–carbon double bond — a single carbon has nothing to double-bond to. So:

• There is no 'methene' (a one-carbon alkene is impossible).
• Alkenes start at ethene (n = 2).

Spot the difference from alkanes. Alkanes are CₙH₂ₙ₊₂ (two extra H atoms). An alkene has 2 fewer hydrogen atoms than the alkane with the same number of carbons, because the C=C double bond "uses up" the space two hydrogens would have taken.

• Ethane C₂H₆ (alkane) → ethene C₂H₄ (alkene).
• Propane C₃H₈ (alkane) → propene C₃H₆ (alkene).

You must be able to name and draw the first three alkenes. The names use the same stems as the alkanes (eth-, prop-, but-) but end in -ene instead of -ane.

n	Name	Molecular formula	Structural formula
2	Ethene	C₂H₄	CH₂=CH₂
3	Propene	C₃H₆	CH₂=CHCH₃
4	Butene	C₄H₈	CH₂=CHCH₂CH₃

A displayed formula shows every atom and every bond. The golden rule for alkenes: always draw the C=C as a double line.

Tips for drawing alkenes.

• Draw the carbon skeleton first, then add hydrogens so that every carbon has 4 bonds in total (count a double bond as 2 of them).
• The C=C must be two parallel lines — a common error is to draw only a single line.
• In propene and butene, only two carbons share the double bond; the rest are joined by single bonds.

Structural formula shorthand. Instead of drawing every bond, you can write a condensed structural formula:

• Ethene → CH₂=CH₂
• Propene → CH₂=CHCH₃
• Butene → CH₂=CHCH₂CH₃

Notice the = sign is kept in the shorthand — it tells the reader where the double bond is.

Exams often give you a formula or a structure and ask: is this an alkene? Use this two-part check.

1. Does the molecular formula fit CₙH₂ₙ? Count the carbons (n) and check the hydrogens equal 2n.

• C₄H₈ → 4 carbons, 8 hydrogens = 2 × 4 ✓ → fits CₙH₂ₙ → could be an alkene.
• C₄H₁₀ → 8 ≠ 10 → this is CₙH₂ₙ₊₂ → an alkane, not an alkene.

2. Is there a C=C double bond in the structure? If you are shown a displayed or structural formula, look for the double line (=) between two carbons.

• CH₂=CHCH₃ → has C=C ✓ → alkene (propene).
• CH₃CH₂CH₃ → all single bonds → alkane (propane).

Watch out: a formula like C₄H₈ fits CₙH₂ₙ, so it is unsaturated — but at IGCSE you should name it simply butene and draw it with one C=C double bond.