Isomers chem question?

Question

I have a chem question which says :
Which of the following structures are isomers?
There are 5 structures which I cannot do on here but how do I know if it's an isomer?
eg. of one is CH3-CH2-CH-CH2-CH3 with one going upwards from the CH in the middle -CH3

Answer 1

Count the longest chain of connected carbon atoms - take care, they may not be drawn in a straight line in the question. In your case it is five. Then look for branches off that chain. In your case there is a branch of one methyl group at the third position in the chain. In your case it doesn't matter which end you start from but if it makes a difference start from the end which gives you the lowest position. So, this molecule is 3-methylpentane (3 for the position of the substituent and pentane for the length of the longest chain - 5). Now do the same for the other structures. If you come up with a different name (and hence structural formula) but the same molecular formula (i.e. no. of each type of atom) you have a different structural isomer. E.g.

...........CH3...CH3
............|..........|
CH3--CH----CH--CH3

is 2,3-dimethylbutane and is an isomer of the first example.

Answer 2

There are different types of isomers but in this case it sounds like it is a molecule with the same molecular formula but different molecular structure.

Answer 3

Add up the total number of C atoms and the total number of H atoms. If the molecules look different, but they have the same no of C atoms and H atoms as each other, then they are isomers.