The Reactive Cousins
🛢️ The Reactive Cousins
Alkenes have a double bond — like having a spare hand ready to grab onto something! Alkanes use all their "hands" (bonds) holding hydrogens. Alkenes have two carbons sharing a double grip (C=C), so they can let go with one hand and grab something new. That's why they're so reactive — and perfect for making polymers!
The key difference is saturation. Alkanes are "saturated" — completely full of hydrogen atoms. Alkenes are "unsaturated" — they have a C=C double bond which can "open up" and add new atoms across it.
Without alkenes, we wouldn't have plastics, polymers, or many of the materials that make modern life possible. They're the building blocks that join together to make everything from plastic bottles to phone cases.
Chemistry glossary
- What are alkenes?
- Unsaturated hydrocarbons containing a C=C double bond
Earn the mark scheme marks
🧠 Memory Aid: AlkENE = doublE bond = Extremely reactive
The letters in alkENE remind you:
- ENE — contains a C=C doublE bond (the E in ENE)
- E — Extremely reactive compared to alkanes
- General formula: CₙH₂ₙ — just 2n hydrogens (count: 2 × carbons)
For the bromine water test: "AlkENE turns it clear, alkaNE leaves it there" — alkenes decolourise bromine water; alkanes do not.
For addition reactions: DOUBLE bond → becomes SINGLE bond when atoms ADD across it. Double → Single = Addition.
Now try it yourself
Quiz · Question 1 of 20
What is the general formula for alkenes?
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This topic in real past papers
Every real exam question we've found on alkenes, with a full worked answer.