# How do structural isomers differ from other isomers?

Sep 1, 2016

Structural isomers differ in their connectivity. Geometric isomers have the same connectivity, but differ in their geometry.

#### Explanation:

What do I mean by $\text{connectivity}$? Well, let's turn to organic chemistry, the which offers the richest source of isomerism.

Now given a simple formula of ${C}_{4} {H}_{10}$, I can write 2 structural isomers: $\text{n-butane, } {H}_{3} C - C {H}_{2} C {H}_{2} C {H}_{3}$, and $\text{isobutane } {H}_{3} C - C H \left(C {H}_{3}\right) C {H}_{3}$. Clearly these are isomeric, because they have the same chemical formula. These are structural isomers because their connectivity is manifestly different: we have a straight chain isomer, and a branched one. Of course, as the chain grows, the opportunity for structural isomerism is magnified.

On the other hand, geometric isomerism may be illustrated with $\text{2-butylene, } {H}_{3} C - C H = C H C {H}_{3}$, which can generate 2 geometric isomers as shown![https://useruploads.socratic.org/5X7Gi0MESgaZUTYCojcq_butene_1.png) :

Looking at the isomers, clearly they have the same connectivity: $C 1$ connects to $C 2$............$C 4$. Nevertheless, because of the disposition of alkyl groups about the olefin, the isomers have distinct physical and chemical properties. These, we would classify as geometric isomers, given the disposition of alkyl groups about the olefinic bond.