Topic 4 · page 7 of 17
Alkanes and cycloalkanes
Alkanes and cycloalkanes are hydrocarbons with only single bonds. Their general formulae let you check any formula and tell a chain from a ring.
Read first: Homologous series and functional groups
Hydrocarbons and saturation 4.9
Alkanes are saturated hydrocarbons in which the carbon atoms are joined in chains. Cycloalkanes are saturated hydrocarbons in which the carbon atoms are joined in a ring. Both have only C–C and C–H single bonds.
General formulae 4.9
| General formula | Example | |
|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | propane, C₃H₈ |
| Cycloalkanes | CₙH₂ₙ | cyclopropane, C₃H₆ |
Where does 2n + 2 come from? A chain of n carbons has n − 1 C–C bonds. Each carbon makes four bonds, so there are 4n bond-ends in total. Of these, 2(n − 1) are used in the C–C bonds, leaving 4n − 2(n − 1) = 2n + 2 for hydrogen atoms.
Closing the chain into a ring joins the two end carbons, which each lose one hydrogen. Two hydrogens are lost, so CₙH₂ₙ₊₂ becomes CₙH₂ₙ.
Worked exampleUsing the general formulae
- The alkane with 8 carbons. H = 2 × 8 + 2 = 18, so C₈H₁₈.
- Cyclopentane. It is a cycloalkane with n = 5, so H = 2 × 5 = 10, and the formula is C₅H₁₀.
- Could C₇H₁₆ be a cycloalkane? For n = 7, a cycloalkane has 14 hydrogens, and an alkane has 16. C₇H₁₆ fits the alkane formula, so it is an alkane (heptane or one of its isomers).
Because they are saturated, alkanes have no functional group. The C–C and C–H bonds are strong, and they are almost non-polar, because carbon (2.5) and hydrogen (2.1) have very similar electronegativities. This is why alkanes are not very reactive. They burn, and they react with halogens in ultraviolet light, but they do not react with most common reagents such as acids, alkalis or oxidising agents.
Check yourself
- Define hydrocarbon and saturated.
- Give the molecular formula of the alkane with (a) 5 carbons, (b) 12 carbons.
- Give the molecular formula of (a) cyclobutane, (b) cyclohexane.
- A hydrocarbon has the formula C₆H₁₂. State two different types of compound it could be.
- Explain why a cycloalkane has two fewer hydrogen atoms than the alkane with the same number of carbons.
- Explain why alkanes are relatively unreactive.
Answers cover these until you have tried the questions
- A hydrocarbon is a compound of carbon and hydrogen only. A saturated compound contains single bonds only.
- (a) 2 × 5 + 2 = 12: C₅H₁₂. (b) 2 × 12 + 2 = 26: C₁₂H₂₆.
- (a) 2 × 4 = 8: C₄H₈. (b) 2 × 6 = 12: C₆H₁₂.
- It fits CₙH₂ₙ with n = 6, so it could be a cycloalkane (cyclohexane or an isomer) or an alkene (hexene or an isomer).
- To close the chain into a ring the two end carbons are joined by a new C–C bond. Each of them loses one hydrogen to make the bond, so two hydrogens are lost.
- Alkanes contain only C–C and C–H single bonds. These bonds are strong and almost non-polar, because carbon and hydrogen have similar electronegativities, so there is no region that attracts reagents.