Introduction

Objectives

After studying this unit, you should be able to:

• name hydrocarbons according to the IUPAC system of nomenclature;
• recognise and write structures of isomers of alkanes, alkenes, alkynes and aromatic hydrocarbons;
• learn the various methods of preparation of hydrocarbons;
• distinguish between alkanes, alkenes, alkynes and aromatic hydrocarbons on the basis of their physical and chemical properties;
• draw and differentiate between the various conformations of ethane;
• appreciate the role of hydrocarbons as sources of energy and in other industrial applications;
• predict the products of addition to unsymmetrical alkenes and alkynes on the basis of the electronic mechanism;
• comprehend the structure of benzene, explain aromaticity, and understand the mechanism of the electrophilic substitution reactions of benzene;
• predict the directive influence of substituents in a monosubstituted benzene ring; and
• learn about carcinogenicity and toxicity.

Hydrocarbons are the important sources of energy — that single line, which opens the

unit, is the reason this chapter matters far beyond the exam. A hydrocarbon is exactly

what the name says: a compound of carbon and hydrogen only. You already live surrounded

by them. LPG (liquified petroleum gas) burning in a kitchen stove, CNG (compressed

natural gas) powering a bus, and the newer LNG (liquified natural gas) are all

hydrocarbon fuels. Petrol, diesel and kerosene come from the fractional distillation of

petroleum found under the earth's crust; coal gas comes from the destructive distillation

of coal; natural gas is found in the upper strata during the drilling of oil wells, and

compressing it gives CNG. Among domestic fuels LPG burns cleanest, while kerosene causes

some pollution; among automobile fuels, petrol- and CNG-run vehicles pollute less than the

alternatives.

Fuels are only half the story. Hydrocarbons are also the raw materials from which polymers

such as polythene, polypropene and polystyrene are manufactured, higher hydrocarbons serve

as solvents for paints, and hydrocarbons are the starting materials for the manufacture of

many dyes and drugs.

This unit studies the hydrocarbon families in turn — how they are classified (§9.1), then

the alkanes (§9.2), alkenes (§9.3), alkynes (§9.4) and aromatic hydrocarbons (§9.5): for each, their nomenclature and isomerism, methods of preparation,

and physical and chemical properties — and closes with why some polynuclear hydrocarbons

are carcinogenic and toxic (§9.6).