Formula for alkanes

Before the formula, get the two key words right — examiners give marks for them.

• Hydrocarbon — a compound made of only carbon and hydrogen atoms (nothing else).
• Saturated — the molecule contains only single bonds between carbon atoms (no C=C double bonds). Every carbon is bonded to as many hydrogen atoms as possible — the molecule is "full up" with hydrogen.

So an alkane is a saturated hydrocarbon: only C and H, joined by single bonds.

Because alkanes have no C=C double bond, they are fairly unreactive — apart from combustion (burning) and substitution with halogens, they do not react easily. (In particular, alkanes do not react with bromine water — that is the test that tells alkanes apart from the unsaturated alkenes.)

The bonding rule for every carbon and hydrogen — this is what makes your drawings correct:

• Each carbon atom forms 4 bonds.
• Each hydrogen atom forms 1 bond.

If you draw a carbon with only 3 lines coming off it, or a hydrogen with 2, the structure is wrong and you lose the mark.

Every alkane fits the same pattern. The general formula is:

$C_n H_{2n+2}$

Here n is the number of carbon atoms. To find the molecular formula, count the hydrogens with H = 2n + 2.

Substituting n = 1, 2, 3, 4 gives the first four alkanes (learn this order):

n	Name	Working (2n + 2)	Molecular formula
1	Methane	2(1) + 2 = 4	CH₄
2	Ethane	2(2) + 2 = 6	C₂H₆
3	Propane	2(3) + 2 = 8	C₃H₈
4	Butane	2(4) + 2 = 10	C₄H₁₀

A memory trick for the order of names: Monkeys Eat Peanut Butter → Methane, Ethane, Propane, Butane.

It works backwards too: if a hydrocarbon has the formula C₅H₁₂, check it against CₙH₂ₙ₊₂. With n = 5, 2n + 2 = 12 — it matches, so C₅H₁₂ (pentane) is an alkane. If a hydrocarbon does not fit 2n + 2 (for example C₃H₆, where 2n + 2 would be 8, not 6), it is not an alkane.

This family of molecules — all fitting the same general formula and differing by one −CH₂− each step — is called a homologous series. The alkanes are one homologous series.

You need to be able to draw alkanes two ways.

Displayed formula — shows every atom and every bond as a separate line. (Examiners call these "full structural formulae".)

Condensed structural formula — groups the hydrogens with each carbon, e.g. CH₃CH₂CH₃ for propane. Faster to write, but you must still be able to expand it to a displayed formula.

Alkane	Condensed formula	Hydrogens
Methane	CH₄	4
Ethane	CH₃CH₃	6
Propane	CH₃CH₂CH₃	8
Butane	CH₃CH₂CH₂CH₃	10

Here are the displayed formulae of the first four. Notice every carbon has 4 lines and every hydrogen has 1:

Methane CH₄        Ethane C₂H₆

    H              H   H
    |              |   |
H - C - H      H - C - C - H
    |              |   |
    H              H   H

Propane C₃H₈

  H   H   H
  |   |   |
H-C - C - C-H
  |   |   |
  H   H   H

Butane C₄H₁₀ (straight chain)

  H   H   H   H
  |   |   |   |
H-C - C - C - C-H
  |   |   |   |
  H   H   H   H

Tip for a perfect displayed formula:

1. Draw the carbon chain first (the carbons in a line, joined by single bonds).
2. Add hydrogens until every carbon has 4 bonds in total.
3. Count up: the hydrogens must equal 2n + 2.

Structural isomers are molecules with the same molecular formula but a different arrangement of atoms (a different structure). They are different compounds.

You must be able to draw the isomers of two alkanes:

C₄H₁₀ has 2 structural isomers:

1. Butane — a straight chain of 4 carbons (CH₃CH₂CH₂CH₃).
2. 2-Methylpropane — a chain of 3 carbons with a CH₃ branch on the middle carbon ((CH₃)₃CH).

Butane                2-Methylpropane

  H   H   H   H            H
  |   |   |   |            |
H-C - C - C - C-H      H - C - H
  |   |   |   |            |
  H   H   H   H        H - C - C - H   ← branch
                           |   |
                           H   H
                           C
                          /|\
                         H H H

C₅H₁₂ has 3 structural isomers:

1. Pentane — a straight chain of 5 carbons (CH₃CH₂CH₂CH₂CH₃).
2. 2-Methylbutane — a 4-carbon chain with a CH₃ branch on the second carbon.
3. 2,2-Dimethylpropane — a central carbon with four CH₃ groups around it (a 3-carbon chain with two branches on the middle carbon).

How to find isomers without missing any:

1. Start with the longest straight chain (the unbranched alkane).
2. Shorten the main chain by one carbon and attach that carbon as a CH₃ branch somewhere along the chain — each different position is a new isomer.
3. Keep checking the molecular formula stays the same (same number of C and H).
4. Watch for duplicates — a "branch on the end carbon" just gives you the straight chain again, so it does not count as a new isomer.

The number of isomers grows quickly: butane (C₄H₁₀) = 2, pentane (C₅H₁₂) = 3, hexane (C₆H₁₄) = 5.

Branched isomers usually have a slightly lower boiling point than the straight chain, because branching stops the molecules packing closely together — but for spec 4.20 you mainly need to draw them correctly.