How can you work out/estimate the number of isomers a given hydrocarbon can have?

I have noticed in a lot of Chemistry A OCR papers that they often ask 'Estimate how many isomers ........ can give.' and I'm never sure how to go about these questions!

1 Answer
Apr 16, 2018

There really is no easy way of doing that


The number of isomers for hydrocarbons can be calculated using Recursive Combinatorial Function, but this math is not easy... AT ALL!!

There are also some programs/apps that will calculate the number of isomers when you enter in the number of carbons (this is for the longer hydrocarbons)

I believe the easiest way is to memorize the number of hydrocarbon isomers for structures that have up to 10 carbons.

Here's a list for number of isomers for Methane (1 C) - Decane (10 C):

Methane - 1
Ethane - 1
Propane - 1
Butane - 2
Pentane - 3
Hexane - 5
Heptane - 9
Octane - 18
Nonane - 35
Decane - 75

It gets quite crazy after this point. For example, Octadecane (18 C) has 60,523 isomers.

I wish I could give you the answer you were looking for, but I don't know of any easy method(s) of estimating the number of isomers for hydrocarbons.

I will ask for a double check in case some chemistry wizard on here knows an easy way to estimate the number of isomers.

Hope this helps (c: