I suppose you would wonder what is the big deal as both will still show the same chemical reactions. Well if you passed a polarised light through a solution of (S)-2-ethylpentane it would rotate the light in a direction opposite, to a same amount, to that of similar concentration of solution (R)-2-ethylpentane. This phenomenon is known as optical isomerism. We said the isomers are optically active. Optically active isomers are known as enantiomers. If the light is rotated to in a clockwise direction we say the rotation is dextro or (+) if it is anti-clockwise it is levo or (-).

So it rotates light, what is such a big deal? If Organic Chemistry is the Chemistry of God then there must be some good reason why natural organic products are present in only one of the two possible configurations. Glucose is (+)-glucose, fructose is (-)-fructose, sucrose is (+)-sucrose, and all natural amino acids have the (S) structure?

Note: There is no correlation between (+) and (-) to (R) and (S). (+) and (-) describe the optical property of the molecule, (R) and (S) is the convention for the configuration.

Carbons with four different groups attached to it are known as chiral carbon, meaning this carbon centre can rotate light.

DIASTEREOMERS

What happens when a molecule has two chiral carbons?

You can try any twist and turn, but you will not be able to put any two of the molecules side by side and have all the groups correspond to each other one-to-one. So these four molecules are configurationally different. They are known as diastereomers.

In general when the molecule has n number of chiral carbons, you can have a possible maximum of 2ⁿ number of diastereomers.

Let us consider the molecule 2,3-dichlorobutane. Here we have a molecule with two chiral carbons. So it is possible to have four diastereoisomers.

(A)				(B)

Let us consider pair (A). Take the molecule on the right and flip it 180 degrees you will see that there is a one-to-one correspondence of all the groups with the molecule on the left. So these two molecules are configurationally identical and so are not optically active. Molecules that have chiral carbons and yet are not optically active are known as meso compounds. You can easily identify meso compounds by their plane of symmetry.

In chirality a chiral carbon is different from a chiral compound.

For the pair (B) you can twist and turn you will never get a one-to-one correspondence of all the groups for the pair. Also you can never get a one-to-one correspondence of all the groups of these pairs with the molecule in (A). So the maximum diastereoisomers possible for 2,3-dichlorobutane is three.

ALKENE - STEREOISOMERS

The π−bond in an alkene holds the two carbons rigidly in three-dimensional space and do not allow spinning about the CC σ−bond axis. This means that there are two possible structures.

trans-2-pentene

cis-2-pentene

Let us illustrate with 2-pentene. The structure with two similar groups on opposite side of the π−bond would be known as the trans-isomer (trans is Latin for "across"; like in trans continental). The other structure would be known as cis (cis is Latin for "on this side"). If the various groups attached to the carbons are bulky then needless to say they will have more space to themselves if they were in the trans configuration. The enthalpy of formation for trans-pentene is about 3.8 kJ mole‾� less than the cis-pentene. Meaning it is more stable.

As far as physical properties are concern there are not much difference between the two forms. The boiling point for trans-2-pentene is 36.4�C and for cis-2-pentene it is 36.9�C. This presents a problem in the separation of the trans- and cis-isomers when both are products of the same reaction.

CONFORMATION

Even when the molecule has the same configuration not all the molecules in the sample will have the same conformation. Conformation is the orientation of the molecule in 3-D space. It is possible for molecules to stretch out in a linear conformation but unless there are special considerations they prefer to go with the flow. For an alkane it will twist and turn at each carbon centre at 109� dictated by the sp�-hybridistion. Also with the spinning about the CC σ−bond the entire molecule is in constant motion.

So imagine all the groups attached to a carbon spinning like a propeller upon the CC σ−bond axis, and extend this to each and every carbon centre in the alkane. There is definitely a wild party going on in the molecule with the groups having to avoid knocking into each other.

SAWHORSE STRUCTURE     NEWMAN PROJECTION Staggered (Anti)   Eclipsed     Staggered (Gauche)

Just to get some idea of the situation, let us consider butane. The 3-D molecule is best represented by either the sawhorse structure or the Newman projection. (The methyl groups are represented by the black dot). It is obvious that the staggered (anti) conformation would offer the groups more individual space, while the eclipsed conformation must be the most cramp. The staggered (gauche) conformation is somewhere in between.

(A)

◄`►

◄`►

(B)

As for cycloalkanes they are not so free to party as their ends are tied. But they do their best to have a good time. They flip-flop, invert and twist inside out; all the while keeping the bond-lengths and the bond angles intact. It is obvious that the chair form (A and B) allows the groups to have more individual space.

Sideview

Axial Equatorial

Topview

If there were alkyl groups attached to the ring the situation would become even more complicated. The chair form with the groups in the equatorial positions would certainly encounter less steric repulsion (and so more stable), compared with being in the axial position.

In conclusion, molecules are in constant motion about their bonds. The conformation that allows the groups to stay away from each other would be the most preferred conformation. That is, if you visit the molecule the probability of finding it in that conformation is very good.