Question

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!

Answer 1

There really is no easy way of doing that

Explanation:

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: