Nomenclature, Structure of Double Bond — Core Principles

Core Principles

Alkenes are unsaturated hydrocarbons characterized by at least one carbon-carbon double bond ($C=C$). Their general formula is $C_nH_{2n}$. Each carbon in the double bond is $sp^2$ hybridized, resulting in a trigonal planar geometry with bond angles of approximately $120^circ$.

The double bond consists of one strong sigma ($sigma$) bond, formed by head-on overlap of $sp^2$ orbitals, and one weaker pi ($pi$) bond, formed by sideways overlap of unhybridized $p$ orbitals. The presence of the pi bond restricts rotation around the $C=C$ axis, which is crucial for geometrical isomerism.

IUPAC nomenclature for alkenes involves identifying the longest carbon chain containing the double bond, numbering it to give the double bond the lowest possible number, and replacing the '-ane' suffix with '-ene'.

Substituents are named and located alphabetically. Common examples include ethene and propene. This fundamental understanding of structure and naming is essential for comprehending alkene reactivity and stereochemistry.

Important Differences

vs Alkanes

Aspect	This Topic	Alkanes
Bond Type	Contain only carbon-carbon single bonds ($C-C$).	Contain at least one carbon-carbon double bond ($C=C$).
Hybridization of Carbon	All carbons are $sp^3$ hybridized.	Carbons involved in the double bond are $sp^2$ hybridized.
General Formula	$C_nH_{2n+2}$	$C_nH_{2n}$ (for monounsaturated alkenes)
Geometry around Carbon	Tetrahedral ($109.5^circ$ bond angles).	Trigonal planar ($120^circ$ bond angles) around $C=C$ carbons.
Rotation around C-C bond	Free rotation around single bonds.	Restricted rotation around double bonds.
Reactivity	Relatively unreactive (undergo substitution reactions).	More reactive due to the exposed pi bond (undergo addition reactions).

Alkanes are saturated hydrocarbons with only single bonds, $sp^3$ hybridized carbons, and free rotation, making them relatively unreactive. Alkenes, conversely, are unsaturated with at least one double bond, $sp^2$ hybridized carbons in the double bond, and restricted rotation. This structural difference, particularly the presence of the weaker pi bond, makes alkenes significantly more reactive, primarily undergoing addition reactions, and allows for the existence of geometrical isomers. Their general formulas ($C_nH_{2n+2}$ vs. $C_nH_{2n}$) also clearly distinguish them.