Structural and Geometric Isomers?

Question

How, when I am shown diagrams of isomers, can I determine which is a structural and which is a geometric isomer?

Answer 1

A structural isomer has the name number of atoms, but a very different structure.

One example would be C2H6O.

These atoms can form ethanol (ethyl acohol) or dimethyl ether.

These two compounds have nothing in common and you certainly wouldn't want to drink the ether. They are structural isomers.

Geometric isomers have the same genral chemical structure, but the physical geometry of the atoms are positioned differently. For exampe in 1,2 dichloroethane, the 2 chlorine atoms are attached one each to each carbon. But they can be geometrically positioned next to each other (cis isomer) or opposite each other (trans isomer). The geometry of an isomer affects the properties of the compound, especially as it may interact with proteins & enzymes in the body.

Another category is chiral isomers, in which there are "left handed" and "right handed" isomers of the same compound. This also has a very big influence on how these molecules react in your body.

Answer 2

The structural view will be a straight chain with the branch(es) off downward or upward.

The geometric isomer will be drawn 'isometrically' so that a branch can be shown at an angle to the main chain and whether it goes behind or in front of the main chain.