Alkenes and Alkynes
Double and triple bonds make hydrocarbons unsaturated.
About this topic
Alkenes contain at least one carbon-carbon double bond and have the formula CnH2n (ethene, propene); alkynes contain a triple bond and have the formula CnH2n−2 (ethyne). They are unsaturated and more reactive than alkanes because the pi bond is easier to break. Names end in -ene and -yne.
The characteristic reaction is addition: the double bond opens and atoms add across it. Hydrogenation with a nickel catalyst gives an alkane, halogen addition gives a dihaloalkane, and hydration with steam gives an alcohol. The bromine water test distinguishes alkenes from alkanes: orange bromine water is rapidly decolourised by an alkene because of addition. Double bonds are planar and rigid, which allows cis and trans isomers in some molecules.
Example: ethene, C2H4, is made from cracking larger hydrocarbons and polymerises into polythene. Fruits such as bananas ripen with ethene as a plant hormone. Ethyne burns in oxygen at about 3000 degrees Celsius in welding torches. Hydrogenation of vegetable oils produces margarine, but partial hydrogenation can make unhealthy trans fats.
A misconception is that the double bond is simply two identical single bonds; it is one sigma and one weaker pi bond. Also, do not forget that alkenes burn with a smokier flame than alkanes because of higher carbon content.
Key ideas
- Alkenes CₙH₂ₙ
- Alkynes CₙH₂ₙ₋₂
- Bromine water test
Quick check
Use cases: where Alkenes and Alkynes helps
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Frequently asked questions
What is Alkenes and Alkynes?
Double and triple bonds make hydrocarbons unsaturated.
What will I learn in Alkenes and Alkynes?
Alkenes CₙH₂ₙ. Alkynes CₙH₂ₙ₋₂. Bromine water test.
Is Alkenes and Alkynes part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Alkenes and Alkynes lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Alkenes and Alkynes?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
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