Alkanes
Senior 3 · Chemistry
Alkanes
Lesson 1 of 4
25 min
Push a stick into the black mud at the edge of a marsh or a fish pond and bubbles rise to the surface. Trap them in an upturned bottle full of water, bring a lit splint to the mouth, and the gas burns with a pale blue flame. The gas is methane, . Bacteria make it as they break down dead plants where there is no air, which is why it was long called marsh gas. Methane is the simplest member of the alkanes, the family this unit is about.
In the last unit you met hydrocarbons, compounds of carbon and hydrogen only, and the idea of a homologous series. Alkanes are the simplest series: every bond between carbon atoms is a single bond, so each carbon uses its four bonds on as many other atoms as it can hold. Nothing more can be added to the molecule, and that is what saturated means.
Definition
Alkane: a saturated hydrocarbon, in which all the bonds are single covalent bonds. Alkanes have the general formula , where n is the number of carbon atoms. Example: propane, , with n = 3 and hydrogen atoms.
Paraffins: an older name for the alkanes, from the Latin parum affinis, "little affinity", because they react with very few substances. Example: candle wax is sometimes called paraffin wax.
Each name is the stem for the number of carbon atoms, which you learnt in Unit 8, followed by the ending -ane. Each member has one more than the one before it, so its relative molecular mass rises by 14 every time. In the condensed formula, is a short way of writing four groups in a row.
Table 1.1: The first ten alkanes (C = 12, H = 1)
| Carbons, n | Name | Molecular formula | Condensed formula | State at 25 °C | |
|---|---|---|---|---|---|
| 1 | methane | 16 | gas | ||
| 2 | ethane | 30 | gas | ||
| 3 | propane | 44 | gas | ||
| 4 | butane | 58 | gas | ||
| 5 | pentane | 72 | liquid | ||
| 6 | hexane | 86 | liquid | ||
| 7 | heptane | 100 | liquid | ||
| 8 | octane | 114 | liquid | ||
| 9 | nonane | 128 | liquid | ||
| 10 | decane | 142 | liquid |
Definition
Molecular formula: shows the number of atoms of each element in one molecule. Example: butane, .
Condensed structural formula: shows the atoms joined to each carbon, written in order along the chain. Example: butane, .
Displayed formula (full structural formula): shows every atom and every bond as a line. Example: Figure 1.1.
A displayed formula is the safest way to see a molecule. Check every drawing with two rules: each carbon has exactly four bonds and each hydrogen has exactly one.
The four bonding pairs of electrons around a carbon atom repel one another and spread as far apart as they can. They point to the four corners of a tetrahedron, with an angle of 109.5° between any two bonds. A displayed formula is a flat map: it shows which atoms are joined, but its 90° angles are not real. It also means that a so-called straight chain is really a zig-zag.
Worked example
Problem. (a) Write the molecular formula of the alkane with 9 carbon atoms and find its . (b) An alkane has . Find its molecular formula and name it. (C = 12, H = 1)
Solution. (a) : hydrogens , so , nonane. .
(b) For any alkane . So , , . The alkane is , heptane.
Worked example
Problem. (a) Which of these are alkanes: , , , , ? (b) How many C–C bonds and C–H bonds are there in one molecule of butane?
Solution. (a) Test whether the number of hydrogens equals . : , an alkane. : 12 is not 14, not an alkane. : , an alkane. : 10 is not 12, not an alkane. : , an alkane.
(b) Four carbons in a chain are joined by 3 C–C bonds, and each of the 10 hydrogens makes one C–H bond. In general an alkane has C–C bonds and C–H bonds.
Warning
Common mistakes. (1) A question that asks for a structural or displayed formula is not answered by a molecular formula such as C₄H₁₀: the molecular formula does not show how the atoms are joined. (2) Count the bonds on every atom of your drawing: a carbon with three or five bonds, or a hydrogen with two, is a wrong structure. (3) The ending -ane does not mean "one carbon": it means "single bonds only". Decane has ten carbons.
Tip
Mass check: . If is not a multiple of 14, the compound is not an alkane.
FuseSchool: Alkanes and Alkenes. Watch the first half, on the alkanes, for how the general formula and the names follow from the number of carbon atoms.
Practice
Sources and credits
