What are alkanes?
Hydrocarbons are made up of only carbon and hydrogen. If their atoms have only single bonds, they are called alkanes, or saturated hydrocarbons.
Carbon forms 4 bonds, and hydrogen forms 1. In a straight chain, each of the two end carbons has 3 hydrogens, and each middle carbon has 2. So an alkane with n carbons has 2n + 2 hydrogens: the general formula is CₙH₂ₙ₊₂.
| C atoms | Name | Formula | Boiling point, °C | State at 25°C |
|---|---|---|---|---|
| 1 | methane | CH4 | −161.5 | gas |
| 2 | ethane | C2H6 | −88.6 | gas |
| 3 | propane | C3H8 | −42.1 | gas |
| 4 | butane | C4H10 | −0.5 | gas |
| 5 | pentane | C5H12 | 36.1 | liquid |
| 6 | hexane | C6H14 | 68.7 | liquid |
The first part of the name shows the number of carbons: meth-, eth-, prop-, but-, pent-, hex-. The ending -ane means saturated. Neighboring members differ by one CH2 group, so together they are called a homologous series. The longer the chain, the higher the boiling point.
Lengthen the chain in the Nalukh chem app
This simulator is part of full access. The app is free to download, and 2 chapters and 5 simulators are open without sign-up.
How do you use the simulator?
When the simulator opens, you see methane.
- Press the plus button below the picture to add one carbon and lengthen the chain. The minus button removes one.
- The number of carbons appears between the buttons, and the name and formula of the alkane appear in the top line.
- In the picture, the dark balls are carbon, the light balls are hydrogen, and the lines are bonds.
- The longest chain is Hexane, with 6 carbons. The plus button does not work past that.
Tasks to try in the simulator
- Add carbons up to Butane and count the hydrogens in the picture. Answer: 10. There are 3 + 3 at the two ends and 2 + 2 in the middle.
- Compare the formulas of Propane and Butane. What is added when the chain grows by one carbon? Answer: 1 carbon and 2 hydrogens.
- The simulator stops at 6, so work out the formula of the alkane with 7 carbons in your notebook. Answer: C7H16, heptane.
- Count the bonds of each carbon in the picture. Answer: Always 4.
Straight chains and isomers
The simulator shows only unbranched chains. Starting with butane, one formula can fit several structures: C4H10 has 2 isomers, C5H12 has 3, and C6H14 has 5. The simulator draws the chain as a zigzag. In real molecules too, the bonds of carbon form angles of about 109.5° with each other, so the chain is not a straight line.
Methane is the main component of natural gas, and butane is the gas in lighters. When alkanes burn, they form carbon dioxide and water. Practice combustion reactions in the balancing equations practice article.
