What are the similarities and differences between structural, geometric, and optical isomers?

Question

Emilia Alvarez · Accepted Answer

Structural isomers differ in their connectivity; geometric isomers differ in their geometry.

Take a simple organic formula, #C_4H_8#. This has one degree of unsaturation. What does this mean? A saturated molecule has a formula of #C_nH_(2n+2)#. Each degree of unsaturation, here a double bond, OR a ring, reduces the hydrogen count by 2. Butene has 2 structural isomers: #"1-butene"#; and #"2-butene"#. For these isomers the connectivity with respect to the double bond is different. Now consider #"2-butene"#; because of the position of the double bond, the pendant methyl groups can be on the same side of the double bond, to generate the #"cis"# isomer, versus opposite sides, to generate the #"trans"# isomer. For both isomers, the connectivity is the same: #C_1# connects to #C_2#......to #C_4#. Nevertheless the geometry is different. Consider optical isomerism, which is a subset of geometric isomerism. Here, two optical isomers have exactly the same connectivity, but they are handed, meaning they are not superposable mirror images. For example, even though an identical twin's right hand is the mirror image of their left hand, they cannot shake hands with each other. (Interestingly, I am told that identical twins do not possess identical sets of fingerprints). Here is a tip with regard to chiral centres. If you have correctly represented a chiral centre, #CR_1R_2R_3R_4#, on the printed page, or by constructing a model (which you certainly should do if time permits), the interchange of any 2 substituents (i.e. #R_1# for #R_2# or #R_1# for #R_3#) results in depiction of the enantiomer. Interchange again (and it could be original pair OR the other two) you get back to the original stereoisomer. Capisce?

Addison Armstrong · Answer

undefined Structural isomers have different arrangements of atoms but the same molecular formula. Geometric isomers have the same connectivity of atoms but differ in the spatial arrangement of atoms due to restricted rotation around a double bond. Optical isomers are nonsuperimposable mirror images of each other and occur when a carbon atom is bonded to four different groups, creating chiral centers.