Physical Properties and Preparation of Alkenes

Like the alkanes, alkenes are essentially non-polar molecules with only weak dispersion forces

between them, so their physical-property trends run parallel to the alkanes': boiling points rise

with chain length (ethene and propene are gases, higher alkenes are liquids), branching lowers the

boiling point, and alkenes are insoluble in water but soluble in non-polar organic solvents.

Preparation from alkyl halides (dehydrohalogenation). Heating an alkyl halide with

potassium hydroxide dissolved in ethanol (as opposed to aqueous KOH, which favours substitution

instead) eliminates $\text{HX}$ across two adjacent carbons to generate the double bond, e.g.

When the starting halide can eliminate in more than one direction (e.g. 2-bromobutane), the more

highly substituted, more stable alkene predominates -- Saytzeff's rule -- so 2-bromobutane

gives mainly but-2-ene rather than but-1-ene.

Preparation from alcohols (dehydration). Heating an alcohol with a concentrated dehydrating

acid, most often $\text{H}_2\text{SO}_4$ (or passing its vapour over hot alumina,

$\text{Al}_2\text{O}_3$, at $\approx 350\,°\text{C}$), removes a molecule of water across two

adjacent carbons:

As with dehydrohalogenation, an alcohol that can eliminate towards more than one neighbouring

carbon again follows Saytzeff's rule and favours the more substituted alkene.

Preparation from alkynes (partial hydrogenation). An alkyne can be partially reduced,

stopping cleanly at the alkene stage, using hydrogen over a specially "poisoned" catalyst called

Lindlar's catalyst (palladium on calcium carbonate, deactivated with lead acetate and quinoline);

because the two hydrogens add to the same face of the triple bond (syn addition), this route gives …