What are Alkanes

Section 1 of 5The first five alkanes

Name	Molecular formula	Displayed formula (description)
Methane	CH4	One carbon atom with four single bonds to four hydrogen atoms
Ethane	C2H6	Two carbons joined by a single bond, each carbon also bonded to three hydrogens
Propane	C3H8	A chain of three carbons; the end carbons carry three hydrogens, the middle carbon carries two
Butane	C4H10	A chain of four carbons; the two end carbons carry three hydrogens each, the two middle carbons two each
Pentane	C5H12	A chain of five carbons with three hydrogens on each end carbon and two on each of the three middle ones

Section 2 of 5How to draw an alkane

1

Write the carbon atoms in a straight row, using the number from the name: meth- is 1, eth- is 2, prop- is 3, but- is 4 and pent- is 5.

2

Join neighbouring carbon atoms with single lines, each line representing one shared pair of electrons.

3

Add hydrogen atoms until every carbon atom has exactly four bonds in total.

4

Count the carbons and hydrogens and check the totals fit C_nH_2n+2.

Section 3 of 5Reactions of alkanes

Alkane chemistry

Generally unreactive

• Generally unreactive The strong, single C–C and C–H bonds mean alkanes do not react with acids, alkalis or most oxidising agents under normal conditions.

Combustion

• Combustion Alkanes burn in plenty of oxygen to give carbon dioxide and water, releasing a lot of energy, which is why they are useful fuels.

Cracking

• Cracking Long-chain alkanes can be broken into shorter, more useful molecules, including alkenes.

Reaction with halogens

• Reaction with halogens In the presence of ultraviolet light, alkanes react with halogens such as chlorine, replacing hydrogen atoms with halogen atoms.

Section 4 of 5Key terms

Term	Meaning
Hydrocarbon	A compound made of hydrogen and carbon atoms only.
Saturated	Containing only single bonds between carbon atoms.
Homologous series	A family of compounds with the same general formula, the same functional group and similar chemical properties.
General formula	An algebraic formula that describes every member of a homologous series, such as CnH2n+2.
Displayed formula	A diagram that shows every atom and every covalent bond in a molecule.

Section 5 of 5Exam technique

1

Giving alkanes the formula C_nH_2n. That is the general formula of alkenes; alkanes are C_nH_2n+2.

2

Drawing a carbon atom with five bonds or only three. Each carbon always forms exactly four bonds.

3

Using 'saturated' to mean 'full of water'. It means all carbon–carbon bonds are single.

4

Mixing up the prefixes. Remember meth-, eth-, prop-, but-, pent- as 1 to 5 carbons.

5

Writing C₃H₈ as C₃H₈ with full-size numbers. Subscripts matter: the 3 and the 8 are atom counts.