Alkenes: Hydrocarbons with Double Bonds

What makes an alkene

Alkenes are hydrocarbons containing at least one carbon–carbon double bond. They follow the general formula CnH2n — two fewer hydrogens than the corresponding alkane — because the double bond replaces two C–H bonds.

They are called unsaturated: unlike alkanes, they can take up more hydrogen (or other atoms) by addition across the double bond. That capacity for addition defines nearly all alkene chemistry.

The simplest alkene is ethene (CH2=CH2), followed by propene (CH3CH=CH2). From butene upward, the position of the double bond matters and is numbered in the name.

The planar double bond

Each double-bond carbon is sp2 hybridized, with its three sigma bonds arranged in a flat triangle at 120° angles. The unhybridized p orbitals overlap sideways above and below this plane to form the pi bond.

The result is a rigid, planar unit: all six atoms joined to the two carbons lie in one plane. This flatness is not just a drawing convention — it is the real geometry, confirmed by experiment.

Most importantly, the pi bond locks rotation. Twisting around the double bond would break the sideways p-orbital overlap, which costs too much energy — so unlike single bonds, double bonds hold their substituents in fixed positions.

Naming alkenes

Alkenes take the suffix -ene, and the longest chain containing the double bond is the parent. The chain is numbered to give the double bond the lowest possible locant.

• CH2=CH2 → ethene
• CH3CH=CH2 → propene
• CH2=CHCH2CH3 → but-1-ene
• CH3CH=CHCH3 → but-2-ene

Substituents are named as with alkanes once the double bond is numbered. Note that but-1-ene and but-2-ene are genuinely different compounds — position isomers — with different properties.

Cis/trans isomerism

Because rotation is locked, 2-butene (CH3CH=CHCH3) exists as two distinct isomers. In cis-2-butene the two methyl groups lie on the same side of the double bond; in trans-2-butene they lie on opposite sides.

These are stereoisomers: same formula, same connectivity, different spatial arrangement. They cannot interconvert without breaking the pi bond, so each can be isolated as a pure substance.

Their properties differ measurably — trans-2-butene boils at 0.9°C while cis-2-butene boils at 3.7°C — a direct consequence of molecular shape. Shape-dependent properties appear throughout chemistry and biology.

Why alkenes are more reactive

The pi bond's electrons sit in exposed clouds above and below the bond axis, making them easy targets for electron-seeking reagents (electrophiles). This is the fundamental reason alkenes are far more reactive than alkanes.

Where alkanes shrug off bromine in the dark, alkenes decolorize it instantly at room temperature. Where alkanes need UV light to react at all, alkenes add hydrogen, halogens, and hydrogen halides under mild conditions.

Nearly every alkene reaction is an addition: the pi bond breaks and two new sigma bonds form to the attacking atoms. The addition-reactions article develops this pattern in full.

Addition reactions: an overview

Addition reactions convert the C=C double bond into single bonds while attaching two new atoms. Hydrogenation adds H2 (with a nickel catalyst) to give an alkane; bromination adds Br2 to give a dibromoalkane; addition of HBr gives a bromoalkane.

The pattern is always the same: two reactants become one product, the pi bond is consumed, and each carbon gains one new sigma bond. No atoms are lost as byproducts.

This reliability makes addition the workhorse of alkene chemistry — from laboratory tests for unsaturation to industrial processes consuming millions of tonnes of ethene and propene each year.

Polymerization to polyethene

Under the right conditions, thousands of ethene molecules add to each other in a chain reaction called addition polymerization:

n CH2=CH2 → –[CH2–CH2]n–

Each new monomer opens its double bond and links to the growing chain, producing polyethene — the plastic of shopping bags, bottles, and food wrap. No atoms are lost; the product is simply n monomers joined together.

Propene polymerizes similarly to polypropene, and substituted ethenes give PVC, polystyrene, and PTFE. A single reaction type thus underpins the entire commodity-plastics industry.

Common mistakes

Real-world applications

• Polyethene and polypropene — made by polymerizing alkenes — are among the most widely used plastics in packaging and containers.

• Ethene is a natural plant hormone that triggers fruit ripening; it is used commercially to ripen harvested fruit evenly.

• Alkenes are key feedstocks: ethanol, antifreeze components, and many solvents are manufactured from ethene and propene.

• Catalytic hydrogenation of unsaturated vegetable oils produces margarine and shortening.