SECTION 3 A: INTRODUCTION

A hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. The majority of hydrocarbons found naturally occur in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen.

The classification for hydrocarbons as follows: Saturated hydrocarbons (alkanes) are the simplest of the hydrocarbon species and are composed entirely of single bonds. The general formula for saturated hydrocarbons is CnH2n+2. Saturated hydrocarbons are the basis of petroleum fuels.

Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with double bond are called alkenes. Those with one double bond have the formula CnH2n.

A homologous series is a series of organic compounds with a similar general formula, possessing similar chemical properties, and shows a gradation in physical properties as a result of increase in molecular size and mass. For example, ethane has a higher boiling point than methane. This is due to the increase in the number of atoms making up the molecule. Each organic compound in a homologous series vary by an extra CH2 from the previous compound.

Hydrocarbons with the same molecular formula but different structural formulae are called isomers. This means that isomers have the same number of carbon and hydrogen atoms, but the carbon atoms are joined to each other in different ways. Simply changing the shape of the molecule does not make it a different isomer. Different isomers are different molecular compounds. They will have different physical properties (for example boiling point).

The more carbon atoms a compound has, the greater the number of isomers there can be. Propane (3 carbon atoms) has 0 isomers. Butane (4 carbon atoms) has 2 isomers. Pentane (5 carbon atoms) has 3 isomers.

Butane (C4H10) has two structural isomers Butane (C4H10) has two structural isomers. Isomer 1 is the straight chain normal structure for butane.

Isomer 2 is a different structure with a branched chain which has a carbon* atom joined onto three other carbon atoms.

Changing the position of the double bond in an alkene makes a different isomer. Butene and pentene exist as different isomers.

SECTION 3 B: ALKANES

Alkanes are hydrocarbons in which each carbon atom is covalently bonded to four other atoms via four single covalent bonds. The other atoms can be either carbon or hydrogen.

methane, CH4 C H C H ethane, C2H6 C H propane, C3H8

butane, C4H10 C H pentane, C5H12 C H

Increasing melting and boiling point. The alkanes show a gradual change in melting and boiling points Increasing melting and boiling point.

Combustion of hydrocarbons Hydrocarbons are mainly used as fuels. The burning of a substance in air is called combustion. In chemistry, combustion means reacting with oxygen. Combustion is a form of oxidation. The main products of combustion from hydrocarbons are carbon dioxide, water and heat. The reaction is exothermic (gives out heat energy).

Incomplete combustion means burning in a lack of air (not enough oxygen). During incomplete combustion methane gas burns with a yellow flame (unlike the clear blue flame seen in complete combustion). methane + oxygen → carbon monoxide + water 2CH4 (g) + 3O2(g) → 2CO (g) + 4H2O (l) Carbon monoxide is a very poisonous gas. It cannot be seen or smelt. Faulty gas fires or boilers may produce carbon monoxide and poison the air in a room without anyone knowing.

Carbon monoxide acts as a poison by combining with haemoglobin in the blood. Haemoglobin normally reacts with oxygen from the air and transports the oxygen to the parts of the body which need it. Carbon monoxide is much more reactive with haemoglobin than oxygen is. It combines to form a stable compound with haemoglobin, preventing the transport of oxygen around the body.

CH4 + Br2 → CH3Br + HBr The organic product is bromomethane CH4 + Br2 → CH3Br + HBr The organic product is bromomethane. The reaction takes place in the presence of UV light.

SECTION 3 C: ALKENES

Alkenes form an homologous series with the general formula CnH2n Alkenes form an homologous series with the general formula CnH2n. Methene, CH2, does not exist Ethene, C2H4, has n=2 Propene, C3H6, has n=3 Butene, C4H8, has n=4

Ethene

Propene

Butene

An alkene may be distinguished from an alkane by shaking the hydrocarbon with bromine water. Bromine water is brown, and will lose its colour when it reacts with a double bond in an alkene. Bromine water will stay brown with an alkane because an alkane has no double bonds. Bromine adds across the double bond of an alkene to form a colourless alkane.

This is an example of an addition reaction This is an example of an addition reaction. An addition reaction occurs when two or more reactants join together to form a single product.