Question

How do you figure out which compounds will have optical isomers from condensed structural formula, also what are the differences between optical isomers (physical and biochemical)

Answer 1

Structural isomers have the same molecular formula but different connectivity. Optical isomers have the same molecular formula but rotate plane polarized light. Optical isomers occur when chiral centers exist in a molecule giving rise to enantiomers. Enantiomers are molecules with non-superimposable mirror images. One enantiomer will rotate plane polarized light in the clockwise direction and the other will rotate it in the counterclockwise direction.

source: http://wiki.answers.com/Q/What_is_the_difference_between_optical_and_structural_isomers

Answer 2

You really can't, unless you are very experienced. So the first thing to do is use your knowledge of chemical bonding to turn the condensed structure into a complete Lewis structure. Then you'll need to use your knowledge of structure, e.g. VSEPR, to turn the Lewis structure into a complete 3D model. It's best to do it with an actual model, if you possible can. At least you might consider, on an exam or something, seeing if you can carry around a little model of a tetrahedral (sp3 hybridized) carbon, so you can more easily imagine what the bonding around critical carbons looks like.

Once you've got the 3D model, look for chiral centers, which means a carbon with four different substituents. You can ignore any sp or sp2 hybridized carbons, because they can never be chiral centers, since they automatically have planes of symmetry. The most common mistake made by newbies is to forget that different lengths of hydrocarbon can be different substituents on a chiral carbon.

If you find any chiral centers, then your molecules has enantiomers (optical isomers). If you find no chiral centers, it will not.

There are very few physical differences between optical isomers except for the fact that one will rotate the plane of polarization of polarized light in the opposite way from the other. Indeed, they're called "optical" isomers largely because only through delicate optical experiments like this can you even tell the isomers apart. Generally, they will boil and melt at exactly the same temperature, look identical, smell and feel identical, and react identically with almost all inorganic or common organic reagents.

Biochemically, if the molecule participates in a reaction catalyzed by an enzyme, then almost always the enantiomers will have very different reactivities, because only one of them will "fit" into the active site pocket of the enzyme. So, for example, if a drug molecule contains chiral centers (and many do), only one of the optical isomers will be active. The most famous example of which I can think are the antidepressants Lexapro and Celexa. Celexa is a mixture of two optical isomers, and Lexapro is only one of them, the active one.