2.5.10Chemical Bonding, Application Of Chemical Reactions and Organic Chemistry
Naming Alkanes and Alkenes
5 min read 7 parts Lesson 50 of 54 1 quiz
Section 1 of 7Alkanes: saturated hydrocarbons
General formula
- General formula Alkanes have the general formula CnH2n+2. With 5 carbons the formula is C5H12.
Saturated
- Saturated Every carbon atom forms four bonds, and all the carbon-carbon bonds are single.
- A compound with only single bonds is saturated.
Properties
- Properties Alkanes are colourless and fairly unreactive.
- They burn in combustion reactions and can be cracked into smaller molecules.
- Physical properties change gradually as the chain gets longer.
Section 2 of 7Alkenes: unsaturated hydrocarbons
General formula
- General formula Alkenes have the general formula CnH2n. With 4 carbons the formula is C4H8.
Unsaturated
- Unsaturated Alkenes contain at least one carbon-carbon double bond, C=C, in addition to single bonds.
- This is their functional group.
Reactivity
- Reactivity The double bond makes alkenes more reactive.
- They burn, take part in addition reactions, and join together in addition polymerisation to make plastics.
- The smallest alkene has 2 carbons, since a double bond needs two carbon atoms.
Section 3 of 7The naming pattern
| Carbons | Prefix | Alkane (formula) | Alkene (formula) |
|---|---|---|---|
| 1 | meth- | methane, CH4 | none |
| 2 | eth- | ethane, C2H6 | ethene, C2H4 |
| 3 | prop- | propane, C3H8 | propene, C3H6 |
| 4 | but- | butane, C4H10 | but-1-ene, C4H8 |
| 5 | pent- | pentane, C5H12 | pent-1-ene, C5H10 |
| 6 | hex- | hexane, C6H14 | hex-1-ene, C6H12 |
Section 4 of 7Naming branched molecules (HT only)
1
Find the longest continuous carbon chain (for an alkene it must include the C=C). This chain gives the stem: meth, eth, prop, but, pent or hex.
2
Decide the ending: -ane if every bond is single, or -ene if there is a double bond, with the position of the double bond given as a number such as but-1-ene.
3
Identify each side-chain: a methyl group, −CH3, or an ethyl group, −C2H5.
4
Number the main chain from the end that gives the lowest numbers, and state which carbon the side-chain is on.
5
Write the name as side-chain position + side-chain + main chain, for example 3-methylpentane or 2-methylbut-1-ene.
Section 5 of 7Worked examples (HT only)
Branched names
2-methylpropane
- 2-methylpropane The longest chain is 3 carbons (propane) with a methyl group on carbon 2.
- The number 2 is not strictly needed because a methyl could only sit on the middle carbon.
2-methylbutane
- 2-methylbutane The longest chain is 4 carbons with a methyl on a middle carbon, which is numbered 2 to get the lowest number.
3-ethylpentane
- 3-ethylpentane A 5-carbon chain with an ethyl group on the middle carbon.
2-methylbut-1-ene
- 2-methylbut-1-ene A 4-carbon chain with the double bond starting at carbon 1 and a methyl on carbon 2.
- Beware that some molecules are drawn 'backwards' so carbon 1 may be on the right.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Alkane | A saturated hydrocarbon with only single bonds, general formula CnH2n+2. |
| Alkene | An unsaturated hydrocarbon containing a C=C double bond, general formula CnH2n. |
| Saturated | Containing only single carbon-carbon bonds. |
| Functional group | The atom or group of atoms that gives a family of compounds its characteristic reactions. |
| Side-chain | A group such as methyl or ethyl attached to the main carbon chain. |
Section 7 of 7Exam technique
1
Choosing a chain that is not the longest. A molecule drawn as a propane chain with an ethyl branch is really a butane with a methyl branch.
2
Saying that an alkene is 'unsaturated because it has no hydrogen'. It is unsaturated because it has a C=C bond.
3
Trying to name a one-carbon alkene. A double bond needs at least two carbon atoms, so ethene is the first.
4
Numbering from the wrong end of the chain; always give the side-chain or double bond the lowest possible number.