Chemistry · Grades 10–12
Alkenes, alkynes and unsaturation
Recognize bond types and the restrictions of simple formula patterns.
Learn the idea
Alkene
An alkene contains a C=C bond. An acyclic hydrocarbon with one double bond has formula CₙH₂ₙ.
Alkyne
An alkyne contains a C≡C bond. An acyclic hydrocarbon with one triple bond has formula CₙH₂ₙ₋₂.
Position
Position numbers distinguish where a multiple bond begins on the parent chain.
Not unique
A molecular formula may also fit rings or other unsaturation patterns, so connectivity is needed.
Worked examples
Which bond identifies an alkene group?
A carbon–carbon double bond is the relevant group.
C=C.
For an acyclic hydrocarbon with one C=C bond and four carbons, how many hydrogens are predicted?
The restricted formula CₙH₂ₙ gives eight hydrogens.
8
Check your understanding
For an acyclic hydrocarbon with one C≡C bond and three carbons, how many hydrogens are predicted?
Which simple compound contains a triple carbon–carbon bond?
- Ethyne.
- Ethanol.
- Ethane.
Why does C₄H₈ alone not prove a molecule has a C=C bond?
Answers and marking notes
Answers and marking notes
- 4
2n − 2 = 2 × 3 − 2 = 4.
- Ethyne.
Ethyne contains C≡C.
- A saturated monocyclic structure can have the same formula.
Composition alone does not establish connectivity.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.