Question

How many isomers of [insert straight-chained alkane here] exist?

This is a reference for those who need it! Saves lots of time in case you want to know how many for a large alkane.

Answer 1

You can find how many here:

http://www-jmg.ch.cam.ac.uk/tools/isomercount/

Since there is no set formula, it must be done computationally! A list of examples (including optical isomers):

methane: 1
ethane: 1
propane: 1
butane: 2
pentane: 3
hexane: 5
heptane: 11
octane: 24
nonane: 55
decane: 136
dodecane: 3396667

Answer 2

Actually, all you need is the number of carbon atoms in the alkane.

Explanation:

There is a formula, but it is complicated and requires a computer to do the calculations.

However, you can find the number of constitutional isomers of alkanes with up to 100 carbon atoms here.

Let's hope that your instructor doesn't ask you to draw all the isomers of `"C"_100"H"_202`!

With the software (from the link above), you can determine the number of isomers of `C_nH_(2n+2)` with more than 1000 carbon atoms.

You can even construct them and display them on the screen.