Summary

This chapter surveyed the four families of hydrocarbons -- compounds of carbon and hydrogen only

-- moving from the least to the most reactive. Alkanes ($\text{C}_n\text{H}_{2n+2}$,

$sp^3$ carbons, only sigma bonds) are named by the longest-chain/lowest-locant IUPAC rules, show

chain isomerism, rotate freely (but not with equal energy) about each $\text{C--C}$ bond between

staggered (more stable) and eclipsed (less stable, torsionally strained) conformations, and react

mainly by free-radical substitution (halogenation, via initiation/propagation/termination, with

$3° > 2° > 1°$ reactivity) and by combustion and pyrolysis/cracking.

Alkenes ($\text{C}_n\text{H}_{2n}$, $sp^2$ carbons at the double bond, planar, $120°$ bond

angles) can show cis-trans geometrical isomerism when each double-bond carbon carries two

different groups, are made chiefly by elimination (dehydrohalogenation, dehydration, or partial

alkyne reduction), and react by electrophilic addition -- following Markovnikov's rule under

normal ionic conditions but reversing to anti-Markovnikov for HBr specifically when peroxides are

present (the radical-chain Kharasch effect) -- as well as by ozonolysis (cleaving the double bond

to two carbonyl compounds, used to locate its position) and by oxidation with cold dilute or hot

concentrated $\text{KMnO}_4$.

Alkynes ($\text{C}_n\text{H}_{2n-2}$, $sp$ carbons at the triple bond, linear, $180°$ bond

angles, one sigma plus two pi bonds) are distinguished by the unusual acidity of a terminal

$\equiv\text{C--H}$ hydrogen -- a direct consequence of the high $s$-character of an $sp$ orbital

-- are prepared from calcium carbide (ethyne) or by double dehydrohalogenation of dihalides

(higher alkynes), and undergo electrophilic addition twice over, including the mercury-catalysed

Markovnikov hydration (Kucherov reaction) that gives a ketone (or, uniquely for ethyne, an

aldehyde) via a keto-enol tautomerisation.

Aromatic hydrocarbons, headed by benzene ($\text{C}_6\text{H}_6$), owe their unusual

stability to a fully delocalised, resonance-stabilised pi system that satisfies Huckel's

$(4n+2)$-pi-electron rule, giving all six ring bonds an identical, intermediate bond length and a

substantial resonance energy. This stability is why benzene reacts by electrophilic

substitution rather than addition -- generate the electrophile, attack to form the

resonance-stabilised arenium ion, lose a proton to rearomatise -- covering nitration, sulphonation, …