Organic Chemistry

Module 7. Carbohydrates

Lesson 22
STEREOCHEMISTRY AND STABILITY OF ANOMERS

22.1 Introduction

• Branch of chemistry which deals with the structures of molecules

22.2 Isomerism

Distinctive feature of organic compounds

• Isomers: compounds which have same molecular formula but different structural formula
• Isomerism : phenomenon that deals with isomers
• Types: two: structural isomerism and stereoisomerism

22.3 Structural Isomerism

• Due to the difference in the arrangement of atoms within a molecule- structural isomers
• Divided into four kinds
• Chain isomerism, position isomerism, functional group isomerism and metamerism

22.4 Sterioisomerism (Space isomerism)

• Due to the difference in spatial position of atoms in a molecule
• Isomers differ from each other only in the way in which atoms in a molecule are oriented in space
• Types- three- optical isomerism, geometric isomerism and conformational isomerism

1. Optical isomerism (enantiomerism)

• Light - ordinary, monochromatic, plane polarized

• Ordinary light - rays of different wavelength, vibrating in all directions (unpolarized light)
• Monochromatic light - rays of single wavelength, vibrating in all directions
• Plane polarized light - vibrating only in one direction

• Substance which rotates plane of polarized light- optically active

• This property -phenomenon- optical activity –Dextro (+) or Laevo (-)

• For the compound to be optically active

• Must have at least one asymmetric (chiral) carbon atom

• Asymmetric carbon atom

• The carbon atom which has four different atoms or groups or atoms and groups

• Enantiomer - pair of compounds whose structures differ only in being mirror image of each other

• Phenomenon- enantiomerism

• Enantiomeric pairs have identical gross structure, but differ from each other in having different spatial arrangement of atoms and/or groups constituting them

• Enantiomers differ from each other in direction of rotating the plane of polarized light

• One - dextrorotatory - rotates clockwise
• The other - laevorotatory- rotates anti-clockwise

• Racemic modification (mixture) - denoted by (±)
• A mixture of equal parts of enantiomers (+ and – forms)
• Optically in active - because rotation caused by one isomer of the mixture is cancelled out by equal and opposite rotation caused by its enantiomer.
• Compounds with two asymmetric carbon atoms:
• These two asymmetric carbon atoms may be

• Dissimilar - atoms/ groups attached to one of the asymmetric carbon atom are different from those attached to the other
• Similar - atoms/groups attached to both the asymmetric carbon atoms are similar (identical)

• Stereochemistry of compound containing two dissimilar asymmetric carbon atoms - e.g. 3-chlorobutanol-2
• An optically inactive compound whose molecules are superimposed on their mirror images despite the presence of asymmetric (chiral) carbon atoms is known as mesocompound (having plane of symmetry).

2. Geometric isomerism

• The double bonded carbon atoms and the four atoms attached to them should be flat
• e.g. ethylene molecule of the type abc=cab
• The flatness is due to geometric arrangement of the bonding orbitals and their overlapping that leads to formation of π bond so hindered rotation
• In cis- arrangement similar groups (atoms) lie on the same side of the ethylenic carbons e.g.
• In trans- arrangement similar groups (atoms) lie on opposite sides of the ethylenic carbons e.g.
• Interconversion of these two structures (cis- and trans-) requires breaking of the π bond
• The breaking of π bond requires very high amount of energy - 70 kcal/mol
• Therefore, the two structures (cis- and trans-) have independent existence
• Such cis- and trans- structures are isomeric

• Referred to as geometric ( or cis-trans) isomers
• The phenomenon is referred as geometric isomerism or cis-trans isomerism

• The cis- and trans- isomers differ from each other only in the manner of the substituent atoms/group of atoms oriented in space.

• Therefore cis- and trans- isomers are a kind of stereoisomers

• Geometric isomers have different physical properties

• Melting points, boiling points, solubilites, densities, refractive index, dipole movements, heat of hydrogenation, selectivity etc
• Therefore can be separated easily

3. Conformational isomerism

• The electron distribution of the sigma molecular orbital is symmetrical around the inter nuclear axis of a sigma bond

• Therefore sigma bond (single covalent bond) permits free rotation about its axis e.g. H₃C-CH₃.
• As a result, it is possible for such a compound to have different relative arrangements of theirs atoms in space which can change into one another usually denoted as Eclipsed and Staggered in case of C₂H₆

• Such arrangements of atoms which can be converted into one another by rotation around single bonds are called conformations or rotational isomers.