Organic chemistry/Stereochemistry

Organic chemistry involves infinitely varied structures arising form a series of limited elements. Because of this, just giving the empirical formula of a compound is insufficient. For example, the empirical formula C2H6O, a molecule of only 9 atoms, can refer to the ether dimethyl ether or ethanol, depending on whether the oxygen is in the middle of or at the end of the carbon chain. The term for different compounds arising from the same empirical formula is isomers.

There are multiple types of isomers, and dimethyl ether and ethanol are examples of structural isomers. They have the same atoms, but their bond structure is different, leading to different physical and chemical properties. For example, ethanol is liquid at room temperature, whereas diethyl ether is gaseous. Another term for structural isomerism is constitutional isomerism.

If the compound has the same bonds, but their arrangement in space differs, they are considered stereoisomers. This notably occurs in cyclic hexane compounds. If two bromines are substituted for two hydrogen in cyclcohexane then it has either a cis or trans configuration. If both bromines lie in the same plane, either both above cyclohexane or both below, they are considered the cis isomer. If one is up and the other down they are trans isomer. cis-1,3 dibromocyclohexane trans-1,3 dibromocyclohexane

This can have an effect on the stability and chemical and physical properties of the element. These difference often occur because of steric strain, when two large groups are forced close together. The different properties are much less pronounced than in constitutional isomers.

Another form of isomerism is enantiomerism, or chirality. Two molecules are enantiomers have the same bonds to the same atoms, but do not have superimposable mirror images. A good example of chirality is your hands. Your left hand is exactly the same as your right hand, but you cannot align them on top of each other when they are both facing the same direction. This often arises when a carbon has four different groups bound to it. Chiral molecules have the same physical properties and exactly the same chemical properties, except when reacting with other chiral molecules. Thus, chiral molecules have potentially drastic differences in physiology. Enantiomers also have the notable difference of rotating plane polarized light the same number of degrees but in opposite directions. This leads to the designation of enantiomers based on the direction they rotate the light as either positive (+) or negative (-), However, since this is relative to which side is arbitrarily chosen to be positive, the absolute configuration is determined as either sinister (S) or rectus (R) based on the Cahn-Ingold-Prelog priority system.