Nomenclature, Structure and Preparation of Alkanes, Alkenes and Alkynes

IUPAC nomenclature of alkanes. The name of an alkane is built in three steps: (1) identify the

longest continuous carbon chain -- this fixes the parent name (methane, ethane, propane, butane,

pentane, hexane, ...); (2) number the chain from whichever end gives the lowest set of locants to

the substituents; (3) name each substituent as a prefix (methyl, ethyl, ...), cite its locant, and

list multiple different substituents alphabetically, e.g. 2,2-dimethyl-4-ethylhexane is written

4-ethyl-2,2-dimethylhexane. Identical substituents are grouped with di-, tri-, tetra- and given

every locant, e.g. 2,4-dimethylpentane for $(\text{CH}_3)_2\text{CHCH}_2\text{CH}(\text{CH}_3)_2$.

Isomerism in alkanes beyond $\text{C}_3\text{H}_8$ is chain (skeletal) isomerism: the same

molecular formula can be arranged as a straight chain or as one or more branched chains, e.g.

$\text{C}_5\text{H}_{12}$ exists as n-pentane, 2-methylbutane (isopentane) and

2,2-dimethylpropane (neopentane) -- three structurally distinct compounds with different physical

properties (boiling points 36 °C, 28 °C and 9.5 °C respectively, falling as branching increases

and the molecule becomes more compact/spherical).

Alkenes are named by replacing the '-ane' ending of the parent chain (chosen to include both

double-bond carbons) with '-ene', and numbering to give the double bond the lowest possible

locant, cited before the suffix, e.g. but-1-ene vs but-2-ene. At the double bond each carbon is

$sp^2$ hybridised: three $sp^2$ orbitals form sigma bonds at $120^\circ$ in one plane, and the

remaining unhybridised $p$ orbital on each carbon overlaps sideways with its neighbour's to form

the pi bond, locking the six atoms of the $\text{C}{=}\text{C}$ unit coplanar and preventing free

rotation about the double bond.

Alkynes take the suffix '-yne' by the same numbering rule, e.g. but-1-yne vs but-2-yne. The

triple-bonded carbons are $sp$ hybridised: two $sp$ orbitals form sigma bonds at $180^\circ$

(perfectly linear), and the two remaining unhybridised $p$ orbitals on each carbon, mutually

perpendicular, overlap with the neighbouring carbon's $p$ orbitals to form two mutually

perpendicular pi bonds -- so the triple bond is one sigma plus two pi bonds, shorter

($\approx 120$ pm) and stronger than either a single or a double carbon-carbon bond, though the

two pi bonds are individually weaker than the sigma bond and are what reacts first.

Preparation. Alkenes are most commonly made by an elimination reaction that removes two

atoms/groups from adjacent carbons: dehydrohalogenation of an alkyl halide with hot alcoholic KOH,

or acid-catalysed dehydration of an alcohol with concentrated $\text{H}_2\text{SO}_4$ or hot …