alkene
/ AL-keen /
An alkene is a hydrocarbon that contains at least one carbon-carbon double bond, written C=C. If you picture an alkane as a chain where every carbon holds hands with its neighbours using a single firm grip, an alkene is the chain where two neighbouring carbons grip each other twice — a stronger, shorter, and crucially more reactive connection. The simplest alkene is ethylene (ethene), H2C=CH2, the very gas that ripens bananas and is made in larger quantity than any other organic chemical on Earth.
Structurally, the two carbons of the double bond are sp2 hybridized: each is flat, with its three attached groups spread out in a plane at about 120 degrees. The double bond is made of two different bonds — one sigma bond (a strong head-on overlap, like the single bond in an alkane) plus one pi bond (a weaker, sideways overlap of p orbitals above and below the plane). Because the pi bond would have to break to let the carbons spin, rotation around a C=C is locked, which is why alkenes can exist as cis/trans (E/Z) isomers that an alkane never could.
Alkenes matter enormously because that pi bond is a region of exposed, loosely held electrons — a chemical handle. It makes alkenes electron-rich and eager to react with electron-seeking species, the entire family of electrophilic additions. The general formula is CnH2n for a single double bond (two hydrogens fewer than the matching alkane, which is its one degree of unsaturation). The name 'olefin' is an older synonym still used heavily in industry.
Ethylene (H2C=CH2) is the simplest alkene; propene (CH3-CH=CH2) and 2-butene (CH3-CH=CH-CH3) are slightly larger ones. 2-Butene exists as two distinct compounds, cis and trans, precisely because the C=C cannot rotate.
The locked C=C double bond gives alkenes both flat geometry and cis/trans isomerism.
A double bond is not 'twice as strong' as a single bond; the pi component is the weaker half, which is exactly why it is the part that breaks and reacts so readily.