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Lesson: Chapter 14 — Hydrocarbons and their derivatives

Alkanes 4 of 4

Branched chains

Activity 14.2 builds models of the alkanes, and shows that from four carbons on, the same formula can be built in more than one way.

Activity 14.2

With suitable materials and the help of your science teacher, build models of the structures of the alkanes with 1 to 5 carbon atoms.

For extra knowledge

For C4H10 and C5H12, the structures below are also correct, as well as the straight chains of table 14.3.

A branched C4H10: a chain of three carbons with a CH3 group on the middle carbon.
A branched C5H12: a chain of four carbons with a CH3 group on the second carbon.
Another C5H12: a chain of three carbons with two CH3 groups on the middle carbon.

To keep them readable, the branches are drawn as CH3 groups; each stands for a carbon with three hydrogens, drawn out in full in the textbook.

Why the formula does not fix the shape

With three carbons there is only one way to join them, so propane has one structure. With four there are two: all in a row, or three in a row with the fourth hanging from the middle. Count the atoms in both butane models and you get C4H10 each time, with every carbon still holding four bonds. Pentane has three such arrangements. Compounds with the same molecular formula but different structures are different substances, with different boiling points — which is why chemists draw structures and not just formulas.

A bent chain is not a branch

Drawing a straight chain with a bend in it does not make a new compound. It is a branch only if one carbon is joined to three or four other carbons.

One formula, several structures

C4H10 has two structures and C5H12 three: a straight chain and branched chains.