Be
careful not to draw "false" isomers which are just twisted versions of
the original molecule. For example, this structure is just the straight
chain version of butane rotated about the central carbon-carbon bond.
You could easily see this with a model.
This is the example we've already used at the top of this page.
Pentane, C5H12,
has three chain isomers. If you think you can find any others, they are
simply twisted versions of the ones below. If in doubt make some models.
Position isomerism
In position isomerism, the basic carbon
skeleton remains unchanged, but important groups are moved around on
that skeleton.
For example, there are two structural
isomers with the molecular formula C3H7Br. In one
of them the bromine atom is on the end of the chain, whereas in the
other it's attached in the middle.
If you made a
model, there is no way that you could twist one molecule to turn it
into the other one. You would have to break the bromine off the end and
re-attach it in the middle. At the same time, you would have to move a
hydrogen from the middle to the end.
Another similar example occurs in
alcohols such as C4H9OH
These are the
only two possibilities provided you keep to a four carbon chain, but
there is no reason why you should do that. You can easily have a
mixture of chain isomerism and position isomerism - you aren't
restricted to one or the other.
So two other isomers of butanol are: