Classification of Isomers

 

Isomers: Compounds that have identical chemical and molecular formulas but differ in the nature of sequence of bonding of their atoms or in their arrangement of atoms in space. According to their topology they are classified as either structural or stereoisomers. Isomers are broadly classified into two broad categories:

 

1.1

Structural Isomers: Compounds that have the same atoms present but differ in their order of connectivity. They are also called as constitutional isomers. They have the same molecular formula but different structures. It can be distinguished by planar diagrams such as fischer projections.

 

1.1.1

Skeletal Isomers: Compounds that have the same functional groups, but differ in the length of the side chains. They are also called as chain isomers. For example: pentane, 2-methyl butane and 2,2 dimethyl propane.

 

1.1.2

Positional Isomers: Compounds that have the same functional groups, but are present on different positions of the chain. For example: butan-1-ol and butan-2-ol.

 

1.1.3

Functional Isomers: Compounds that have different functional groups. For example: ethanol and methoxy methane.

 

1.1.4

Tautomers: Compounds whose structures differ in arrangement of atoms but which are in dynamic equilibrium with each other.

 

1.1.4.1

Keto-Enol tautomerism:

 

1.1.4.2

Ring-Chain isomerism: seen in case of glucose

 

1.2

Stereoisomer: Compounds that have the same chemical formula, same atoms, same connectivity and differ only in the arrangement of their atoms in space.

 

1.2.1

Anomers: Stereoisomers where the molecule is cyclized and the difference in configuration is about the anomeric carbon only. In case of aldoses the anomeric carbon is C1 and for ketoses it is C2. e.g. sugar hemiacetal. Glucose in open chain form is not chiral at C1 but in ring form has two optically active stereoisomers: alpha & beta glucose.

 

1.2.2

Rotamers and Conformers: On the basis of spatial arrangement of atoms in the molecule that can be achieved by rotation(or torsion) around one or more single bonds, they are classified as rotamers and conformers. Conformers assume the chair/boat and equitorial/axial forms. Rotamers assume the different newmann projections (staggered/eclipsed/gauche).

 

1.2.3

Configurational Isomers have a chiral (stereogenic center). Chiral center refers to a carbon atom attached to four different groups. The molecule is said to possess chirality and to have a stereogenic

center.

 

1.2.3.1

Enantiomer: Stereoisomers that are non-identical, mirror-symmetric for all atoms, nonsuperimposable, optically active (e.g. levo/dextro-rotatory), inverted only by breaking bonds and remaking them in the reverse sense. e.g.: D-glucose and L-glucose.

 

 

 

1.2.3.2

Diastereomer: Stereoisomers that are not mirror images, but have identical configuration for at least one asymmetric center and at least one different configuration for the remaining asymmetric centers. e.g. Threonine has 2 chiral centers and therefore 4 diastereomers.

 

1.2.3.2.1

Epimers: They are a special case of diastereoisomerism where there is a difference for one and only one asymmetric center. e.g. D-glucose and D-mannose; D-glucose and D-galactose are epimers.

 

1.2.3.3

Meso-isomer (Achiral molecules): super-imposable mirror images which have more than one stereogenic center. Meso-isomers have two planes of symmetry; the usual mirror plane of reflection and a second plane perpendicular to it through the molecule (in the ``middle'' of the molecule).  The asymmetric centers are distributed around this second place so that they are mirror inverses of each other.  Hence optical rotations from the two ``halves'' of the molecule cancel out.

 

1.2.3.4

Geometric Isomers: Stereoisomers which are isomeric about double bonds.  If the bond is C=C, the terms are cis/trans; if the bond is C=N, the terms are syn (cis-like) and anti (trans-like). For example: cis-2-butene and trans-2-butene.

 

Chiral Center Naming Classification:

• +/- Indicates the direction in which plane of polarized light is rotated(clockwise/anticlockwise).

• D/L (Dextrorotatory/Levorotatory): Plane of polarized light is rotated to the right or left.

• alpha/beta stereochemistry of the anomeric carbon. It is used for sugars.

• R-S convention: The four groups surrounding the stereocenter are given a priority from a -> d (from highest to lowest). The molecule is then observed from the side with the lowest priority group. If the remaining three groups form a clockwise array (a->b->c), then it is R convention. If it is in anti-clockwise then S convention. Priority is assigned according to atomic number, with the higher the atomic number the higher the priority. This is used for chiral molecules (enantiomers/diastereomers/epimers).

 

• E-Z convention: The two groups attached to the carbon around the double bond are given the priority. If the two highest priority groups are on the same side of the double bond, then the molecule is given the Z convention(similar to cis); and if on the opposite side then E convention(similar to trans). e.g.: 1-chloro-1-bromo-2-iodoethene is differentiated on basis of E-Z convention. Priority is assigned according to the atomic number; with the higher the atomic number, the higher the priority. Hyrdogen always has the lowest priority. If there are two identical atoms attached to the stereocenter (say the carbon of the methyl group and carbon of the ethyl group) then work along the chain of the attached group until a difference occurs.