Isomers & Optical Isomerism in Compounds having Stereocenters | Organic Chemistry PDF Download

Isomers – Compounds that have the same molecular formula but do not have identical structures.

Constitutional Isomers – Different connectivity of atoms.

Stereoisomers – Same connectivity of atoms.

Stereochemistry

“Branch of chemistry which deals with three dimensional structures of molecules and their effects on physical-chemical properties”

•  It plays an important role in determining the properties and reactions of organic compounds.

•  The properties of many drugs depend on their stereochemistry.

Stereoisomers that contain asymmetric centres

Everything has a mirror image. The fundamental question is whether the molecule and its mirror image are superimposable. Some molecules are like hands. Left and right hands are mirror images, but they are not identical, or superimposable.

Other molecules are like chairs. Two chairs are mirror images that are superimposable. A chair and its mirror image are identical and hence achiral.

•  A molecule or object that is not superimposable on its mirror image is said to be chiral.
•  A molecule or object that is superimposable on its mirror image is said to be achiral.

Achiral molecules usually contain a plane of symmetry (A plane of symmetry is a mirror plane that cuts the molecule in half, so that one half of the molecule is a reflection of the other half) but chiral molecules do not.

Below, the molecule labeled A and its mirror image labeled B are not superimposable. No matter how you rotate A and B, all the atoms never align. Thus, it is chiral and A & B are stereoisomers—collectively, they are called Enantiomers.

A carbon atom with four different groups is a tetrahedral stereogenic center.

•  With one stereogenic center, a molecule will always be chiral.
• With two or more stereogenic centers, a molecule may or may not be chiral.

[Note: The terms stereogenic center and chiral molecule are related but distinct. In general, a chiral molecule must have one or more stereogenic centers].

Compounds having Sterocentre: Enantiomers & Diasteromers

Enantiomers

Isomers which are non-superimposable mirror images of each other are called Enantiomers (optical antipodes or enantiomorphs). The co mpounds o f the type Cabcd exist in enant iomeric forms and are described as chiral and the carbo n atom with four different substituent is called a stereogenic centre. The phenomenon of enantiomerism is also known as optical iso merism. An important property of the co mpounds of the t ype Cabcd is that on interchanging any two groups at the stereocenter converts one enantiomer into another.

(i) (S) configurat ion (ii) (R) configuration

In Enantiomers the configuration at each stereocenter is changed i.e. R changes into S and S changes into R. in the above structure if we replace H with Cl and Br with H then we will get a new enantiomer.

Properties of Enantiomers:

1. Each enantiomer of a pair has the same physical and chemical properties in achiral environment.
2. When plane po larized light is passed through the so lut ion o f each enant io mer then the plane polarized light is rotated in opposite direction by the same amount.
3. Enantio mers react with achiral reagents at the same rate but with chiral reagents they react with different rate.

Diastereomers:

Stereoisomers which don’t have any mirror image relationship are called Diastereomers.

No mirror image relationship

Diastereomers are non-enantiomeric stereoisomers which have two or more stereocenter and differ only in configuration of at least one of them as shown in the above molecule.

(A) (2R, 3S)-tartaric acid (meso compound due to internal co mpensat ion i.e. plane of symmetry in the molecule)
(B) (2R, 3R)-tartaric acid (optically act ive – no element of symmetry is present).
(C) (2S, 3S)- tartaric acid ( optically act ive – no element of symmetry id present)

• (A) and (B) , (A) and (C) – Diastereomers (configuration at  least one stereocenter is same)
• (B) and (C) – Enant io mers.( configurat ion at both stereocenters are different).

Properties of Diastereomers 

1. Diastereomers have different physical properties like melt ing po int, boiling po int, solubilit y, retention times and R_f values.

2. Different rates of reaction even in achiral environments.

3. Diastereomers are not related by symmetry elements such as ϭ, I and S_n.

Epimers

When two Diastereomers differ in the stereochemistry at only one stereocenter then these are called Epimers. The term is quite general. However it is not used for mo lecules wit h only two stereocenters. Glucose and mannose are Epimers at C₂.

Anomers
α-D-Glucose and β-D-glucose are anomers. Anomers are two sugars that differ in configurat ion only at the carbon that was the carbonyl carbon in the openchain form. This carbon is called the anomeric carbon. The prefixes α - andβ - denote the configuration about the anomeric carbon. Because anomers, like epimers, differ in configuration at only one carbon, they too are a particular kind of Diastereomer. Notice that the anomeric carbon is the only carbon in the molecule that is bonded to two oxygens.

Trigonal planar stereocenters

A stereocenter is defined as an atom having groups of suitable nature so that interchange of any two groups will give a stereoiso mer. However all stereocenters are not tetrahedral the unsaturated carbon atoms of Cis and trans-2-butene are examples of trigonal planar stereocenters. Since an interchange of groups at these stereocenters gives a stereoisomer.

i. (Z)-2-butene ( groups of higher priority are on the same side)
ii. (E)-2-butene ( groups of higher priority are on the opp. side)

Invertomers 

Stereogenic nitrogen and phosphorus.

Acyclic amines of the type in which the three groups are different and the lone pair on the nitrogen is classed as a formal substituent meets all the requirements of a stereocenter. However no optical activity is observed in amines of this type, even though these are chiral. This is due to very rapid pyramidal inversion (energy barrier to inversion is small ~25 Kj moli^-1) which interconverts Enantiomers.

(i) (S)-ethylmethylpropylamine  (Sp³ hybridized state)
(ii) Transit ion state ( Sp² hybridized state)
(iii) (R)- ethylmethylpropylamine  (Sp³ hybridized state)

Homomers

A molecule may be written in two or more orientations which infact represent the same molecule but at a first sight look different. Such different orientations o f the same co mpound (which are superimposable) are called Homomers.