Alkanes

The hydrocarbons that emerge from the fractional distillation of crude oil (u4:c23) are mainly alkanes — and this chapter is devoted to them, because they are the simplest family of hydrocarbons in chemistry. Every molecule in the family is built from just two kinds of bond: carbon–carbon single bonds and carbon–hydrogen single bonds. Nothing else. That one structural fact explains almost everything you are about to read: why the family has a neat formula, why its members behave in similar ways, why the family is described as unreactive, and why — despite that — every alkane will burn. The structures and reactions of these compounds build directly on the bonding and formula work from the introduction to organic chemistry (u4:c22).

Start with the family portrait. The small alkanes — methane, ethane, propane, butane — are gases at room temperature, and their most important job is the one happening around you right now: they burn as fuels. Methane is the major component of natural gas, the fuel that likely heats your home; ethane and propane are present in small quantities in natural gas too, and both are important constituents of liquefied petroleum gas (LPG) from crude oil distillation. The first five members of the family are laid out in the table below — learn to read it, because it is the map of everything else in this chapter.

Two columns of that table deserve a second look. The molecular formula counts atoms: C₃H₈ tells you a propane molecule contains 3 carbon atoms and 8 hydrogen atoms, and nothing about how they are joined. The structural formula shows the order of the chain: CH₃CH₂CH₃ says carbon–carbon–carbon, with the hydrogen atoms attached along it. A third representation exists too — the displayed formula — in which every single bond is drawn as an explicit line and every atom is shown. For methane it is a central C with four H atoms each joined to it by one line (H–C–H on top and bottom, H on each side); for ethane it is H–C–H above and below each of the two carbon atoms joined by a central C–C line; and each further member lengthens the carbon backbone by one carbon, with hydrogen atoms filling every remaining bond. Drawing displayed formulae for these first five alkanes, and naming the straight-chain members, is a skill this chapter trains — specification statement 4.21 asks for exactly that, up to five carbon atoms.

Now the word that will follow the family for the rest of the chapter: saturated. The alkanes are saturated-hydrocarbons because they contain only C–C single bonds and have no double (C=C) or triple (C≡C) bonds anywhere in the molecule. Every carbon atom is using all four of its bonds on four neighbours, and there is no double bond anywhere waiting to react. Keep the definition in exactly that shape — single bonds only — because it is the version the mark scheme rewards.