# What is the difference between a structural isomer, a geometric isomer, and an enantiomer?

What do I mean by connectivity? Take a straight chain butane chain, n -butane; ${C}_{1}$ is connected to ${C}_{2}$ is connected to ${C}_{3}$ is connected to ${C}_{4}$, and a formula of ${C}_{4} {H}_{10}$. Now ${H}_{3} C - C H \left(C {H}_{3}\right) C {H}_{3}$, isobutane, is a structural isomer of butane, because ${C}_{2}$ is connected to ${C}_{3}$, which is NOT connected to ${C}_{4}$. The connectivity of $\text{n-butane}$ and $\text{isobutane}$ is different, hence these are structural isomers.
Let's stick with a four-carbon chain, and consider 2-butene, ${H}_{3} C - C H = C H C {H}_{3}$. This is a structural isomer of butene (what is the other?), but this will nevertheless support geometric isomerism. $\text{Trans-2-butene}$ has precisely the same connectivity as $\text{cis -2-butene}$, ${C}_{1}$ is connected to ${C}_{2}$ is connected to ${C}_{3}$ is connected to ${C}_{4}$, BUT in the cis isomer, the methyl groups (${C}_{1}$ and ${C}_{4}$) are on the same side of the double bond, but for the trans isomer, the methyl groups are on opposite sides. So for geometric isomers, spatial geometry is different while connectivity is the same.
When we consider enantiomers, we introduce another aspect of $\text{(geometric) isomerism}$, in the handedness of the molecule. The connectivity is the same, however, the geometry about the chiral centre is different. Even for a simple acid, say alanine, ${H}_{3} C - C H \left(N {H}_{2}\right) C \left(= O\right) O H$, because $C 2$ bears 4 different groups, it is capable of generating optical isomerism.