Beginner Topic 735 of 1102 Organic Chemistry
Alkenes
Hydrocarbons containing a carbon-carbon double bond with formula CnH2n.
In-depth explanation of Alkenes
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond, with the general formula CnH2n for acyclic members. The double bond consists of one sigma bond and one pi bond, and the carbons are sp2 hybridized with trigonal planar geometry. The pi electron cloud lies above and below the molecular plane and is a region of high electron density, so it attracts electrophiles and alkenes undergo addition reactions. Restricted rotation about the double bond gives rise to cis and trans geometric isomers when each carbon carries two different groups.
Going deeper: Alkenes
Key ideas: Alkenes have a C=C made of one sigma and one pi bond, so the pi electrons are nucleophilic and the characteristic reaction is electrophilic addition. Stability rises with substitution: tetra > tri > di > mono, and trans is usually more stable than cis. Prepared by dehydration of alcohols, dehydrohalogenation or partial hydrogenation of alkynes. Rotation about the bond is blocked, giving cis-trans isomerism.
Common mistakes and connections: A frequent error is thinking the double bond is two identical bonds; the pi bond is weaker (about 270 kJ/mol) and more reactive than the sigma bond. Students also forget carbocation rearrangement during hydration or HX addition. Connects to Markovnikov addition, ozonolysis, polymerization and E-Z notation.
Visualization of Alkenes
Ethene is planar with 120 degree angles; the C=C double bond is one sigma plus one pi bond and prevents rotation. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Ethene is a gas used by plants as a ripening hormone and is the monomer for polyethylene.
Frequently asked questions about Alkenes
What is Alkenes?
Hydrocarbons containing a carbon-carbon double bond with formula CnH2n.
Is Alkenes a beginner, intermediate or advanced topic?
Alkenes is a beginner topic in Organic Chemistry. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Alkenes?
Study these earlier topics in Organic Chemistry first: Functional Groups, Homologous Series, Alkanes.
What comes after Alkenes?
The next topics in Organic Chemistry are Alkynes and Cycloalkanes.
Can I practise Alkenes with quizzes?
Yes. The interactive lesson includes a self-check question on Alkenes, and the Organic Chemistry quiz draws on every topic in this area.
Where Alkenes fits in Organic Chemistry
Alkenes is topic 7 of 148 in Organic Chemistry.