Alkenes

4CH1-4.234CH1-4.244CH1-4.254CH1-4.26

2 guided lessons · 18 checks · 4. Organic Chemistry → Alkenes

Guided lessons for this note

Which concept don't you understand yet?

We'll explain it simply and check your understanding. Pick any lesson — any order works.

0 of 18 checks correct so far

Lesson 1 of 2

The C=C functional group family

Alkenes are the unsaturated counterpart to alkanes, defined by one structural feature. This lesson identifies the C=C functional group, generates the family formula, and explains what unsaturated really means.

What you know already

From Halogens & Alkanes you already know how the saturated family swaps atoms under UV light. This note introduces its double-bonded cousins: the C=C functional group, the alkane-versus-alkene formula difference, and why that open double bond makes alkenes so much more reactive.

Key points of understanding

Alkenes are unsaturated hydrocarbons whose functional group is the C=C double bond.
The general formula of an alkene is CnH2n, two fewer hydrogens than the alkane recipe.
Unsaturated means the C=C bond can open, letting each carbon form one more single bond.
Because the double bond can open, alkenes are much more reactive than alkanes.
Each carbon of the double bond ends up with four single bonds once the incoming atoms attach.

Tell us your confidence level against the key points

Choose a confidence level to continue.

Prefer to read it all at once? Show the full lesson as a page

1.The C=C functional group family

4CH1-4.234CH1-4.244CH1-4.25

Key points

Alkenes are unsaturated hydrocarbons whose functional group is the C=C double bond.
The general formula of an alkene is CnH2n, two fewer hydrogens than the alkane recipe.

Guided content from the SME authoring corpus — note rn_H2zGfgH76qYV5cmr, pinned at 52acc7ec1650 · promoted SyllabAI/syllabai-resources e967e75f1339

Back to the revision note All revision notes

2.Drawing and naming the first four

4CH1-4.26

The first four alkenes carry position numbers in their names because the double bond can sit in different places. This lesson lines up ethene to pent-1-ene and reads the numbering system.

Double Bond Opening, IGCSE & GCSE Chemistry revision notes — A carbon-carbon double can break and form a single bond, allowing more atoms to attach to the carbon atoms

Key points

The first members are ethene C2H4, propene C3H6, but-1-ene C4H8 and pent-1-ene C5H10.
Names of longer alkenes carry a number showing where the C=C bond begins in the chain.
In pent-2-ene the double bond sits between the second and third carbon atoms.
Drawing a displayed alkene means every bond shown, with the double bond drawn as two lines.
The cis/trans and E/Z naming systems stay outside the requirement, which covers unbranched-chain isomers only.

First pass

Learn ethene, propene, but-1-ene and pent-1-ene with formulae C2H4 to C5H10. The number in the name says which carbon the double bond starts at, counted from one end.

Analogy — The name works like a street address for the double bond: ethene and propene live in tiny houses with one possible door, but from butene onward the number tells you which door is open.

Exam pointer — Practise drawing each displayed formula with the double bond as two clear lines; both carbons still show four bonds in total.

Going deeper

The table runs ethene C2H4, propene C3H6, but-1-ene C4H8 and pent-1-ene C5H10, each obeying CnH2n with one C=C. Position numbers count from the nearer end, so pent-2-ene places the double bond between carbons two and three, and only unbranched-chain isomers are named at this level.

Analogy — Longer alkene names are like hotel room numbers: the stem names the building size and the number tells you the floor where the double-bond event happens.

Exam pointer — Count from the end that gives the lower number, and remember cis/trans and E/Z labels are outside the requirement.

Checks

0 of 9 attempted · 0 correct

1.Which molecular formula belongs to propene?
2.Where is the C=C bond in pent-2-ene?
3.Why does ethene have the formula C2H4 rather than C2H6?
4.What does a displayed formula of but-1-ene show that its molecular formula C4H8 does not?
5.Which notation is explicitly not required for alkenes in this specification?
6.Give the molecular formula of but-1-ene.
Written response · recall
Your answer
7.Between which two carbon atoms does the double bond sit in pent-2-ene?
Written response · recall
Your answer
8.Explain why every alkene contains two fewer hydrogen atoms than the alkane with the same number of carbons.
Written response · explain why
Your answer
9.A student draws but-1-ene with the C=C shown as a single line between the first two carbons. State which family the drawing now represents and what the correct drawing must show.
Written response · application
Your answer