Physical and Chemical Properties — Core Principles

Core Principles

Alkenes are unsaturated hydrocarbons featuring at least one carbon-carbon double bond. Their physical properties are largely dictated by molecular size and geometry. Smaller alkenes (C2-C4) are gases, C5-C17 are liquids, and larger ones are solids.

Melting and boiling points generally increase with molecular mass but decrease with branching. Cis-trans isomerism causes cis isomers to have slightly higher boiling points due to dipole moments, while trans isomers often have higher melting points due to better crystal packing.

Alkenes are nonpolar, making them insoluble in water but soluble in organic solvents, and they are less dense than water.

Chemically, the electron-rich $\pi$-bond makes alkenes highly reactive, primarily undergoing electrophilic addition reactions. Key reactions include hydrogenation (addition of $\text{H}_2$ to form alkanes), halogenation (addition of $\text{X}_2$ to form dihalides), hydrohalogenation (addition of $\text{HX}$ to form alkyl halides, following Markovnikov's rule, or anti-Markovnikov with HBr/peroxides), and hydration (addition of $\text{H}_2\text{O}$ to form alcohols, following Markovnikov's rule).

They also undergo oxidation reactions like Baeyer's test (cold, dilute $\text{KMnO}_4$ for diols) and oxidative cleavage (hot $\text{KMnO}_4$ or ozonolysis for aldehydes, ketones, or carboxylic acids).

Alkenes can also polymerize and undergo combustion.

Important Differences

vs Alkanes

Aspect	This Topic	Alkanes
Bonding	Contain C-C single bonds (saturated)	Contain C=C double bonds (unsaturated)
Reactivity	Less reactive, undergo substitution reactions (free radical)	More reactive, undergo electrophilic addition reactions
Hybridization	All carbons are $\text{sp}^3$ hybridized	Double bond carbons are $\text{sp}^2$ hybridized
General Formula	$\text{C}_n\text{H}_{2n+2}$	$\text{C}_n\text{H}_{2n}$
Test for Unsaturation	Do not decolorize bromine water or Baeyer's reagent	Decolorize bromine water and Baeyer's reagent
Molecular Geometry	Tetrahedral geometry around each carbon, flexible rotation	Planar geometry around double bond carbons, restricted rotation (leading to cis-trans isomerism)

The fundamental difference between alkenes and alkanes lies in their bonding and saturation level. Alkanes are saturated, containing only C-C single bonds, making them relatively unreactive and primarily undergoing free radical substitution. Alkenes, being unsaturated with a C=C double bond, are significantly more reactive due to the exposed $\pi$-electron cloud, readily undergoing electrophilic addition reactions. This difference in reactivity is exploited in qualitative tests like the bromine water test or Baeyer's test, which alkenes pass but alkanes do not. Furthermore, the $\text{sp}^2$ hybridization of double bond carbons in alkenes leads to planar geometry and restricted rotation, giving rise to geometric isomerism, which is absent in alkanes.