Naming and Formula of Alkanes
Alkanes are the simplest family of saturated hydrocarbons, forming a homologous series with a consistent general formula. Mastering their names and formula is a fundamental building block for all organic chemistry topics in the ESAT.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Alkanes are 'saturated' hydrocarbons, meaning their carbon atoms are joined only by single covalent bonds.
- They form a homologous series where each successive member differs by a -CH2- group.
- The general formula for any alkane is C(n)H(2n+2), where 'n' is the number of carbon atoms.
- The names of the first six straight-chain alkanes must be memorized: Methane, Ethane, Propane, Butane, Pentane, Hexane.
- As a homologous series, alkanes share similar chemical properties, though their physical properties (like boiling point) change predictably with chain length.
Formulae
C(n)H(2n+2) To determine the molecular formula of any straight-chain or branched alkane when you know the number of carbon atoms (n).
Definitions
- Hydrocarbon
- An organic compound that consists entirely of hydrogen and carbon atoms.
- Saturated
- A molecule containing the maximum number of hydrogen atoms for its carbon skeleton, with only single C-C bonds.
- Homologous Series
- A sequence of compounds with the same functional group and similar chemical properties, in which members differ by a constant unit, typically -CH2-.
Worked example
An alkane is found to have 10 hydrogen atoms. What is its name and molecular formula?
- 1
Start with the general formula for alkanes:
C(n)H(2n+2).
- 2
Set the hydrogen part of the formula equal to the known number of hydrogens:
2n + 2 = 10 - 3
Solve this simple equation for n (the number of carbon atoms):
2n = 8, so n = 4 - 4
The molecular formula is therefore C4H10.
- 5
Recall the prefix for a 4-carbon chain, which is 'but-'.
The name of the alkane is Butane.
Answer: Butane, C4H10
Common mistakes
- ×Confusing the alkane general formula (C(n)H(2n+2)) with the alkene formula (C(n)H(2n)). Remember alkanes have the maximum possible hydrogens, so you need the '+2'.
- ×Mixing up the prefixes for the first four alkanes. Ensure you know that Methane=1C, Ethane=2C, Propane=3C, and Butane=4C.
- ×Forgetting that 'saturated' specifically means only single C-C bonds are present.
No-calculator tips
- ✓To quickly find the number of hydrogens, just double the number of carbons and add two. For example, for pentane (5C), it's (2*5)+2 = 12 hydrogens.
- ✓To work backwards from hydrogens to carbons, subtract 2 then halve the result. For C(x)H(14), it's (14-2)/2 = 6 carbons (Hexane).