Chapter 9 — Hydrocarbons
Compounds of only carbon and hydrogen — the fuels in an LPG cylinder, the ethene that ripens fruit, the ethyne of a welding torch and the benzene ring at the heart of dyes and drugs. Alkanes, alkenes, alkynes and aromatic hydrocarbons: their structures, how they are made and how they react.
A kitchen LPG cylinder holds about $14\ \mathrm{kg}$ of a liquid that boils at roughly $0\ ^{\circ}\mathrm{C}$ — mostly butane and propane, squeezed into liquid form by a few bar of pressure. Open the valve and it boils, burns with a clean blue flame, and turns into nothing but carbon dioxide and water. In a fruit warehouse a few parts per million of another two-carbon gas, ethene, are piped in to ripen bananas evenly overnight. A welder's torch burns a third, ethyne, hot enough to melt steel. And the clear, sweet-smelling liquid benzene, whose six carbon atoms form a perfectly flat ring, is the starting point for nylon, polystyrene, aspirin and countless dyes. All four are hydrocarbons — compounds of carbon and hydrogen only — and the differences between them come down to one question: how are the carbon atoms bonded to each other?
This chapter answers it family by family. Alkanes have only single bonds; they are the least reactive hydrocarbons, but they burn, and in sunlight they swap hydrogen atoms for chlorine one at a time by a free-radical chain. The C–C single bond lets the two halves of a molecule turn, so even ethane has conformations. Alkenes have a C=C double bond whose π electrons are exposed above and below the plane: they add reagents across the bond, in a direction predicted by Markovnikov's rule (and reversed for HBr by peroxides), and the rigid double bond creates cis and trans isomers. Alkynes add twice across their triple bond, and the H on a terminal C≡C is weakly acidic. Benzene, finally, refuses to add at all: its six π electrons are spread around the ring, it is aromatic, and it reacts by electrophilic substitution, with groups already on the ring deciding where the next one goes. Mechanisms, isomer counts and product prediction from this chapter are tested in every Board, JEE and NEET paper.
- classify hydrocarbons as alkanes, alkenes, alkynes and aromatic compounds, write their general formulae and name and draw their isomers;
- prepare alkanes by hydrogenation, from alkyl halides (including the Wurtz reaction), by decarboxylation and by Kolbe's electrolysis, and predict the products;
- write the free-radical mechanism of halogenation, balance combustion equations, and describe controlled oxidation, isomerisation, aromatisation, the reaction with steam and pyrolysis;
- draw ethane in sawhorse and Newman projections and explain torsional strain and the energy difference between its staggered and eclipsed conformations;
- explain the σ + π structure of the double bond, decide when an alkene has geometrical (cis–trans) isomers, and compare their dipole moments and boiling points;
- prepare alkenes and alkynes by elimination and partial reduction, and predict the products of addition of $\mathrm{H_{2}}$, halogens, HX (Markovnikov and peroxide effect), sulphuric acid and water, with mechanisms;
- identify an alkene from its ozonolysis products and describe oxidation by Baeyer's reagent and acidified permanganate;
- explain the acidity of terminal alkynes, and the addition reactions and cyclic polymerisation of ethyne;
- explain the structure, resonance and stability of benzene, apply the Hückel $(4n+2)$ rule, write mechanisms for nitration, halogenation, sulphonation and Friedel–Crafts reactions, and predict the effect of substituents on rate and orientation.
Sections in this chapter
Alkanes
Classification of hydrocarbons; alkanes, their isomers and names; preparation (hydrogenation, alkyl halides, Wurtz, decarboxylation, Kolbe); free-radical halogenation, combustion and other reactions; conformations of ethane.
Alkenes
The σ + π double bond and geometrical isomerism; preparation by partial reduction and elimination; addition of $\mathrm{H_{2}}$, $\mathrm{X_{2}}$, HX, $\mathrm{H_{2}SO_{4}}$ and water; Markovnikov's rule and the peroxide effect; oxidation, ozonolysis and polymerisation.
Alkynes
The linear sp triple bond (1σ + 2π); preparation from calcium carbide and vicinal dihalides; acidity of terminal alkynes; addition of $\mathrm{H_{2}}$, halogens, HX and water; cyclic polymerisation to benzene.
Aromatic Hydrocarbons
Naming benzene derivatives; the Kekulé structures, resonance and stability of benzene; aromaticity and the Hückel rule; electrophilic substitution with mechanism; addition and combustion; directive influence of substituents; toxicity.