Alkanes
Senior 5 · Chemistry
Alkanes
Lesson 1 of 4
22 min
The cooking-gas cylinder in many Kigali kitchens holds liquefied petroleum gas (LPG), a mixture of propane, , and butane, , kept liquid under pressure. A candle is made of paraffin wax, alkanes with roughly 20 to 40 carbon atoms. The petroleum jelly rubbed on dry skin is a soft mixture of still longer chains. A gas, a soft solid and a hard solid: the only real difference between them is the length of the carbon chain. This unit explains why.
Definition
Alkane: a saturated hydrocarbon, containing only carbon and hydrogen joined by single bonds. Open-chain alkanes have the general formula . Example: hexane, .
Cycloalkane: a saturated hydrocarbon whose carbon atoms form a ring; general formula . Example: cyclohexane, .
Alkyl group: an alkane minus one hydrogen atom, , written R–. Example: ethyl, .
Table 1.1: Main fractions of crude oil (approximate ranges; refineries vary)
| Fraction | Carbon atoms per molecule | Boiling range / °C | Main use |
|---|---|---|---|
| refinery gas | 1–4 | below 30 | LPG for cooking, fuel for the refinery |
| petrol (gasoline) and naphtha | 5–10 | 30–180 | car fuel; feedstock for cracking |
| kerosene (paraffin) | 10–16 | 160–250 | jet fuel, lamps and stoves |
| diesel (gas oil) | 14–20 | 220–350 | lorries, buses, generators |
| lubricating oil, fuel oil, wax | 20–40 | above 350 | engine oil, ships, candles |
| bitumen | more than 40 | residue | road surfaces, roofing |
Every carbon in an alkane is hybridised (Unit 1), with its four bonds at 109.5°. A "straight-chain" alkane is therefore really a zigzag, which is why skeletal formulae are drawn that way. The groups can rotate freely about each C–C σ bond, so a chain is constantly flexing into different shapes called conformations; these are not isomers, because they interconvert millions of times a second.
The rules of Unit 1 apply. For alkanes, the decisions are about the chain and the numbers:
Figure 1.1: The longest chain is the one with the most carbon atoms, however it is drawn.
For cycloalkanes, add "cyclo" to the stem. With one substituent no number is needed (methylcyclohexane); with two or more, number round the ring to give the lowest locants (1,2-dimethylcyclopentane).
Figure 1.2: Cycloalkanes, general formula CₙH₂ₙ. Each corner is a CH₂ group.
Worked example
Problem. Name .
Solution. Draw it out. The longest chains have five carbons (through the ethyl branch or along the end ; both are five long and both carry two branches). The branches are a methyl and an ethyl group. Numbering from the left gives locants {2, 3}; from the right, {3, 4}. Choose {2, 3}, and write the branches alphabetically: 3-ethyl-2-methylpentane, .
Worked example
Problem. Write the structural formula and molecular formula of 4-ethyl-2,2-dimethylhexane.
Solution. "Hexane": a chain of six carbons. "2,2-dimethyl": two groups on C2. "4-ethyl": a group on C4. Fill each carbon up to four bonds with H:
Carbons: 6 + 2 + 2 = 10, so the formula is , which fits .
Warning
A name with a branch on C1, or an ethyl group on C2, is always wrong. "1-methylbutane" is pentane; "2-ethylbutane" is 3-methylpentane; "2-ethylpropane" is 2-methylbutane. In each case the branch was really part of a longer chain.
Tip
Check every alkane name with the general formula: count the carbons in the name, then . If your structure has a different number of hydrogens, you have drawn it wrongly.
The Organic Chemistry Tutor: IUPAC Nomenclature of Alkanes (11 min). Watch how he finds the longest chain when it is drawn with bends, and how he numbers the chain to give the branches the lowest locants.
Practice
Sources and credits
