Question #e7063

1 Answer
May 7, 2017

To draw isomers from a molecule, we have to know which atoms it contains and how many of them. To do this, the best thing is to draw the structure from the name you provided: 1-ethyl-2-methylcyclobutane.

We follow the steps below to draw carbohydrate structures:

  1. Draw the longest chain of carbon atom.
  2. Give every carbon atom a number.
  3. Draw the substituents at the specific carbon atom.
  4. Make sure every atom has sufficient amount of covalent bonds.

So for your structure, this gives us:

  1. The longest chain is at the end of the name, in this case: cyclobutane, which is a chain of 4 carbon atoms in a circle.
  2. We give every carbon atom a number.
  3. We draw a ethyl group (2 carbon atoms) at position 1 and a methyl group (1 carbon atom) as substituent at position 2
  4. We quickly control our structure and everything seems fine.

The definition of isomers is that it has the same molecular formula but a different structure. Therefore we have do find out the molecular formula of the drawn structure. To do this, just count the amount of carbon and hydrogen atoms of your structure. For clarification, I have drawn the structure below with every hydrogen atom on it, but it is also possible to leave those out.
We count:
Therefore the molecular formula is #C_7H_14#.

Now we need to draw isomers that contain 7 carbon atom and 14 hydrogen atoms.

The easiest procedure to draw all the isomers is to start with the longest carbon chain possible. We can for example here make a circular chain with 7 carbon atoms and 14 hydrogen atoms.
Then we make the chain 1 shorter and find all the isomers with the longest circular chain of 6.
We keep on following this procedure, now for 5 carbon atom in a circular chain.

As you can see, there are a lot of isomers with this molecular structure. In fact, I did a quick search on this website and found out that there are at least 55 isomers. I am not going to draw all of them, but with the procedure written above, you should be able to draw a lot of them. I have here some more, to visualise that with shorter cycles, a number of isomers go up.

Just work always from longest chain to shorter chain, so you do not miss any structures.