How many isomers are there of butanal?

You have to remember that butanal is an aldehyde, a group that only exists on terminal atoms. Thus, the aldehyde group can only go on the terminal carbons. There are only two isomers, and they are 1-butanal and 2-methylpropanal.

• "3-methylpropanal" is identical to 1-butanal.

• "2-butanone" is not an aldehyde anymore, but a ketone.

The isomers should be:

I just moved the methyl group inwards as much as possible. You can see that the aldehyde group in 2-methylpropanal can then be moved around the outer carbon, but since there is rotational symmetry, it turns out to be the same isomer.

Thus, there are only two (straight-chained) isomers total that are also aldehydes.

If you are not restricting yourself to only aldehydes, i.e. if you are looking for isomers of #"C"_4"H"_8"O"# in general, then

2-butanone would work, and so would crotyl alcohol, cyclobutanol, 2-methoxypropene, and tetrahydrofuran.

That would total seven.

You can find their structures in the link above (except for cyclobutanol, which is just a four-membered ring with an #"OH"# off of it).

Including butanal, there are three carbonyl-containing isomers. These are

1. Butanal
2. Butanone
3. 2-Methylpropanal

In addition there are many unsaturated and cyclic alcohols as well as unsaturated and cyclic ethers (too many to draw here).