Alkanes: Formula, Names and Shape – Alkanes (Senior 3 Chemistry) | Ganzaa

Alkanes

Lessons

Senior 3 · Chemistry

Alkanes

4 lessons

Lesson 1 of 4

Alkanes: Formula, Names and Shape

25 min

Bubbles that burn

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, $\mathrm{CH_4}$ . 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.

A family of saturated hydrocarbons

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 $\mathrm{C_nH_{2n+2}}$ , where n is the number of carbon atoms. Example: propane, $\mathrm{C_3H_8}$ , with n = 3 and $\mathrm{2(3)+2 = 8}$ 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.

Naming the first ten

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 $\mathrm{CH_2}$ than the one before it, so its relative molecular mass rises by 14 every time. In the condensed formula, $\mathrm{(CH_2)_4}$ is a short way of writing four $\mathrm{CH_2}$ groups in a row.

Table 1.1: The first ten alkanes (C = 12, H = 1)

Carbons, n	Name	Molecular formula	Condensed formula	$M_{r}$	State at 25 °C
1	methane	$\mathrm{CH_4}$	$\mathrm{CH_4}$	16	gas
2	ethane	$\mathrm{C_{2}H_{6}}$	$\mathrm{CH_3CH_3}$	30	gas
3	propane	$\mathrm{C_{3}H_{8}}$	$\mathrm{CH_3CH_2CH_3}$	44	gas
4	butane	$\mathrm{C_{4}H_{10}}$	$\mathrm{CH_3(CH_2)_{2}CH_3}$	58	gas
5	pentane	$\mathrm{C_{5}H_{12}}$	$\mathrm{CH_3(CH_2)_{3}CH_3}$	72	liquid
6	hexane	$\mathrm{C_{6}H_{14}}$	$\mathrm{CH_3(CH_2)_{4}CH_3}$	86	liquid
7	heptane	$\mathrm{C_{7}H_{16}}$	$\mathrm{CH_3(CH_2)_{5}CH_3}$	100	liquid
8	octane	$\mathrm{C_{8}H_{18}}$	$\mathrm{CH_3(CH_2)_{6}CH_3}$	114	liquid
9	nonane	$\mathrm{C_{9}H_{20}}$	$\mathrm{CH_3(CH_2)_{7}CH_3}$	128	liquid
10	decane	$\mathrm{C_{10}H_{22}}$	$\mathrm{CH_3(CH_2)_{8}CH_3}$	142	liquid

Definition

Molecular formula: shows the number of atoms of each element in one molecule. Example: butane, $\mathrm{C_4H_{10}}$ .

Condensed structural formula: shows the atoms joined to each carbon, written in order along the chain. Example: butane, $\mathrm{CH_3CH_2CH_2CH_3}$ .

Displayed formula (full structural formula): shows every atom and every bond as a line. Example: Figure 1.1.

Drawing alkanes

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 real shape is three-dimensional

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 $M_{r}$ . (b) An alkane has $M_{r} = 100$ . Find its molecular formula and name it. (C = 12, H = 1)

Solution. (a) $n = 9$ : hydrogens $= 2 (9) + 2 = 20$ , so $\mathrm{C_9H_{20}}$ , nonane. $M_r = 9 \times 12 + 20 \times 1 = 128$ .

(b) For any alkane $M_{r} = 12 n + (2 n + 2) = 14 n + 2$ . So $14 n + 2 = 100$ , $14 n = 98$ , $n = 7$ . The alkane is $\mathrm{C_7H_{16}}$ , heptane.

Worked example

Problem. (a) Which of these are alkanes: $\mathrm{C_6H_{14}}$ , $\mathrm{C_6H_{12}}$ , $\mathrm{C_3H_8}$ , $\mathrm{C_5H_{10}}$ , $\mathrm{C_{10}H_{22}}$ ? (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 $2 n + 2$ . $\mathrm{C_6H_{14}}$ : $2 (6) + 2 = 14$ , an alkane. $\mathrm{C_6H_{12}}$ : 12 is not 14, not an alkane. $\mathrm{C_3H_8}$ : $2 (3) + 2 = 8$ , an alkane. $\mathrm{C_5H_{10}}$ : 10 is not 12, not an alkane. $\mathrm{C_{10}H_{22}}$ : $2 (10) + 2 = 22$ , 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 $\mathrm{C_nH_{2n+2}}$ has $n - 1$ C–C bonds and $2 n + 2$ 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: $M_{r} = 14 n + 2$ . If $M_{r} - 2$ 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

State the general formula of the alkanes and explain why they are called saturated hydrocarbons.
Name the alkanes $\mathrm{C_3H_8}$ , $\mathrm{C_6H_{14}}$ and $\mathrm{C_8H_{18}}$ . Write the molecular formulas of nonane and decane.
Draw the displayed formula of pentane. How many C–C bonds and how many C–H bonds does it have?
Which of these are alkanes: $\mathrm{C_4H_8}$ , $\mathrm{C_7H_{16}}$ , $\mathrm{C_2H_6}$ , $\mathrm{C_5H_8}$ , $\mathrm{C_{12}H_{26}}$ ?
An alkane has a relative molecular mass of 86. Find its molecular formula and name it. (C = 12, H = 1)
One molecule of an alkane contains 18 hydrogen atoms. Name the alkane.
Calculate the percentage by mass of carbon in propane. (C = 12, H = 1)
A student draws methane as a flat square, with H–C–H angles of 90°. Describe the real shape and explain why the bonds take up that shape.

Show answersHide answers

$\mathrm{C_nH_{2n+2}}$ . All the bonds between carbon atoms are single bonds, so the molecule holds the most hydrogen it can; no more atoms can be added.
propane, hexane, octane; $\mathrm{C_9H_{20}}$ , $\mathrm{C_{10}H_{22}}$ .
$\mathrm{CH_3CH_2CH_2CH_2CH_3}$ drawn with every bond; 4 C–C bonds and 12 C–H bonds.
$\mathrm{C_7H_{16}}$ , $\mathrm{C_2H_6}$ and $\mathrm{C_{12}H_{26}}$ .
$14 n + 2 = 86$ , $n = 6$ : $\mathrm{C_6H_{14}}$ , hexane.
$2 n + 2 = 18$ , $n = 8$ : octane, $\mathrm{C_8H_{18}}$ .
$M_r = 3 \times 12 + 8 = 44$ ; $\frac{36}{44} \times 100 = 81.8\%$ .
Tetrahedral, with all H–C–H angles 109.5°. The four bonding pairs of electrons repel one another and spread as far apart as possible.

Sources and credits

Photo: Frozen Methane Bubbles, U.S. Geological Survey, via Wikimedia Commons, public domain (a work of the United States federal government).
Video: Alkanes & Alkenes, FuseSchool, embedded from YouTube.
Written by Ganzaa. The diagrams, table, worked examples and Practice items were drawn and written by Ganzaa.

1Bubbles that burn

2A family of saturated hydrocarbons

3Naming the first ten

4Drawing alkanes

5The real shape is three-dimensional

Bubbles that burn

A family of saturated hydrocarbons

Naming the first ten

Drawing alkanes

The real shape is three-dimensional