Saturated hydrocarbons: alkanes and isomers · Chemistry Class 9–10 | StemPy Lab
Course 8.2·Lesson 1 of 3
Saturated hydrocarbons: alkanes and isomers
A hydrocarbon has only carbon and hydrogen. In a saturated hydrocarbon, the alkane, all the bonds are single bonds, and the formula is CnH2n+2. Compounds with the same formula and different structures are isomers.
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Hydrocarbons
A hydrocarbon is a compound of only carbon and hydrogen. They are the simplest carbon compounds, and they are the main parts of natural gas and petrol.
Saturated hydrocarbons: the alkanes
In a saturated hydrocarbon, every bond between carbon atoms is a single bond. Each carbon has the most hydrogens that it can take. These compounds are the alkanes. Their names end in -ane.
Number of C
Name
Formula
1
Methane
CH4
2
Ethane
C2H6
3
Propane
C3H8
4
Butane
C4H10
5
Pentane
C5H12
6
Hexane
C6H14
The pattern is the general formula
CnH2n+2
With n=4: C4H10. With n=12: C12H26.
The structural formulae of four alkanes. Methane, CH4: one carbon with four hydrogens. Ethane, C2H6: two carbons in a row, each with three hydrogens. Propane, C3H8: three carbons in a row. Butane, C4H10: four carbons in a row. Every carbon has four single bonds.
The alkanes are a series
Each alkane has one more CH2 than the one before, and its mass is 14u more. As the chain gets longer, the boiling point rises. Methane to butane are gases, and pentane and larger are liquids at room temperature.
A graph of the boiling point of the alkanes, in kelvin, against the number of carbon atoms, from 1 to 8. It rises from 112 kelvin for methane to 399 kelvin for octane. A dashed line marks room temperature, 298 kelvin. Methane to butane are below it and are gases, and pentane and above are liquids.
You should find that each step along the chain adds 14g/mol, the mass of one CH2.
Isomers
Butane, C4H10, can be drawn in two ways.
A straight chain of four carbon atoms: butane.
A branched chain, with three carbon atoms around one in the middle: 2-methylpropane (isobutane).
Both have the same molecular formula, C4H10, but different structures. Such compounds are called isomers, and they have different properties. For example, their boiling points are different.
Two carbon skeletons of C4H10. Butane is a chain of four carbon atoms. 2-Methylpropane has three carbon atoms around a central carbon atom. They have the same molecular formula and different structures, so they are isomers.
Pentane has three isomers, and the number grows very fast with the length of the chain.
Practise with new numbers
Try it in Python
This program lists the alkanes, their masses and their boiling points.
Remember these
A hydrocarbon has only carbon and hydrogen. An alkane has only single bonds. Formula: CnH2n+2.
Each member has one more CH2, which is 14u.
Isomers have the same formula and different structures. Butane has two.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What is the formula of the alkane with 9 carbon atoms?
Propane boils at −42∘C. Is it a gas or a liquid in a room at 25∘C?
Two compounds are both C5H12, yet they boil at different temperatures. How can that be?
Is C3H6 an alkane?
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