Concepts: classification, alkane structure and reactions

2.1 How hydrocarbons are classified

A hydrocarbon is a compound made only of carbon and hydrogen. The chapter groups them by the kind of carbon-carbon bond and by the shape of the skeleton. Alkanes are saturated: every carbon is joined to four other atoms by single bonds, the molecules are open chain or cyclic, and the open chain alkanes have the formula CnH2n+2. Alkenes contain at least one double bond and have the formula CnH2n when the molecule is open chain and has one double bond. Alkynes contain at least one triple bond and have the formula CnH2n-2. Aromatic hydrocarbons, of which benzene is the parent, contain a planar ring with delocalised electrons, and they behave differently from the ordinary unsaturated compounds.

Hydrocarbons are important as fuels and as raw materials. Liquefied petroleum gas and compressed natural gas are hydrocarbon mixtures used as domestic and transport fuels, and petroleum and coal supply the starting materials for a very large number of organic products. The same simplicity of composition is the reason the chapter can compare the three families through a small set of reactions.

2.2 Structure and conformations of alkanes

In methane the carbon atom is sp3 hybridised, so the four carbon-hydrogen bonds point towards the corners of a regular tetrahedron. The H-C-H angle is about 109.5 degrees and all four bonds have the same length. In larger alkanes the tetrahedral units are joined to one another, and the carbon skeleton can be straight or branched.

Ethane has a carbon-carbon single bond, and rotation about this bond is possible. The rotation produces different arrangements of the hydrogen atoms, called conformations, without breaking any bond. In the eclipsed conformation the hydrogen atoms on the two carbons are as close together as possible, while in the staggered conformation they are as far apart as possible. The staggered arrangement is more stable because the electron clouds of the carbon-hydrogen bonds are separated, and the energy difference between the two forms is small, which is why rotation is fast at room temperature but not completely free. Sawhorse and Newman projections are the two ways of drawing these conformations, and the printed figures show both styles for ethane.

2.3 Reactions of alkanes

Alkanes are almost non-polar and are not attacked by acids, bases or oxidising agents under ordinary conditions. Their characteristic reaction is substitution. Halogenation happens in the presence of ultraviolet light or at a high temperature, and it proceeds through free radicals. In the initiation step the halogen molecule breaks into two radicals under light. In the propagation steps a halogen radical takes a hydrogen atom from the alkane to give hydrogen halide and an alkyl radical, and the alkyl radical then reacts with a halogen molecule to give the haloalkane and a new halogen radical, so the chain continues. In the termination steps radicals combine with one another and the chain stops. Because several hydrogen atoms may be replaced in turn, a mixture of products is obtained, and the relative amounts depend on the number and type of hydrogen atoms available. Lower alkanes do not undergo nitration or sulphonation under ordinary conditions.

Combustion is the other important reaction. With a good supply of air the alkane burns completely to carbon dioxide and water, while with a limited supply the products include carbon monoxide or carbon black. The heat released makes alkanes valuable fuels. Alkanes also crack when heated strongly, breaking into smaller molecules, and this process is used industrially to obtain alkenes from petroleum fractions.

2.3.1 Worked example

The combustion of one mole of an alkane needs eight moles of oxygen. For an alkane the oxygen requirement is three n plus one by two, so three n plus one equals sixteen and n equals five. The alkane is C5H12, and the sum of its carbon and hydrogen atoms is seventeen. The same calculation also shows that one mole of this alkane produces five moles of carbon dioxide and six moles of water.

2.3.2 Naming and physical behaviour

The names of alkanes follow the IUPAC rules: the longest continuous chain gives the parent name, the carbon atoms of that chain are numbered from the end nearest the first branch, and each branch is named as an alkyl group with its locant. Two different branches are listed alphabetically, and the same branch repeated is indicated by di, tri and so on. Because the chains are flexible, the same compound can be drawn in several ways, and the naming rules are the way the different drawings are recognised as one substance. The physical properties follow the size and the shape of the molecule: the boiling point rises with the number of carbon atoms because the van der Waals forces grow with the surface area, and among isomers the straight chain has the higher boiling point because a branched molecule is more compact and has less surface contact.