Organic Chemistry - Alkenes

Alkenes are unsaturated hydrocarbons containing at least one carbon-to-carbon double bond ($C=C$).

The general molecular formula for alkenes is $C_nH_{2n}$.

The $C=C$ double bond consists of one $\sigma$ (sigma) bond and one $\pi$ (pi) bond. The $\pi$ bond is an area of high electron density, making alkenes more reactive than alkanes.

Alkenes undergo addition reactions, where the $\pi$ bond breaks and atoms are added to the carbon atoms previously involved in the double bond.

Bromine water test: When an alkene is added to orange-brown bromine water ($Br_2(aq)$), the solution becomes colorless. This is a standard test for unsaturation.

Hydrogenation involves the addition of $H_2$ to an alkene in the presence of a Nickel ($Ni$) catalyst at approximately $150^\circ C$ to form an alkane.

Hydration is the reaction of an alkene with steam ($H_2O(g)$) in the presence of a phosphoric acid ($H_3PO_4$) catalyst at $300^\circ C$ and $60\text{ atm}$ pressure to produce an alcohol.

Addition polymerization occurs when many small alkene molecules (monomers) join together to form a long-chain molecule (polymer) by breaking their double bonds.

Stereoisomerism ($E/Z$ or $cis/trans$) occurs in alkenes because the double bond restricts rotation, leading to different spatial arrangements of substituent groups.