STEREOCHEMISTRY
Stereochemistry
Stereochemistry is defined as study of organic chemicals considering three dimensional structural properties. It is a sub discipline of chemistry, involves the study of the relative spatial arrangement of atoms within a molecules.
Isomers
Isomers are defined as compounds having same molecular formula but different structures. Isomers also have same molecular mass. However these compounds differ in their physical chemical and structural properties. Isomers may not belong to the same class of organic compounds e.g. the compounds whose structural formulae are shown below, are isomers. The one on the left is a carboxylic acid, while the one on the right is an ester and both have same molecular formula C5H10O2. These compound differ in physically, chemically, structurally.
Isomers Tree
Constitutional isomers
Constitutional isomers are also known as structural isomers At least one atom is bonded to a different atom in each molecule. The isomers molecules differ in their connectivity. A branch, functional group, a multiple bond etc are not connected to same atom.
Chain isomers
Chain isomers are those isomers in which the carbon chain arrangement is varied for the same molecular formula. e.g. pentane & 2-methylbutane differ in their structure
Positional isomers
Positional isomers are those isomers which have same carbon chain but differ in the position of a functional or substituted group e.g. In 1-bromopropane & 2bromoprpane location of functional group differ.
Functional isomers
Functional group isomers have different group present in the same chain of carbon but their molecular formula is same e.g. Ethnol & methoxymethane have same molecular formula but different function is attached to same carbon chain with two carbon.
Tautomerism
Tautomerism is phenomenon in which there is a dynamic equilibrium between the two isomers, i.e. they co-exist at the same time in an equilibrium situation
The equilibrium is very much on the left, but the small % of the 'enol' form is an important intermediate in certain reactions of ketones. The enol form is an important intermediate in the iodination of propanone).
Stereoisomer
Stereoisomers or Stereomerism, is difined as the phenomenon of different location of substituent or function group in space linked to the same atom and having same molecular formula as well as mass. The difference of location in space is the cause of varying physical and chemical properties.
Conformational isomer
Conformational isomers or conformers are related to a central atom about which bonded groups may vary in their arrangement so that the molecules may be super imposable. These isomers are a type of stereo isomers. The isomers are produced by rotation of the groups about their bonds. Different types of structure notation are used to represent conformers such as dash and wedge, saw horse, or Newman projection.
The energy of each of these projections (Anti, Eclipsed Gauche conformation) differs from each other. As the energy is different these can be isolated at low temperature Carbon no, 3 rotate about its sigma-bond with carbon no. 2. (see figure above) This rotation results in a mixture of conformers being present at any given moment.
Conformation in cyclic compound (cyclohexane)
Cyclohexane is close ring structure. And forms three type of conformers
All forms exist. Chair formation has lowest energy. The ring flip-flop between all forms The energy graph of each conformer is shown below
Ring strain
In a ring structure like cyclohexane a strain develop due to angle strain. This strain is defined as ring strain. Angle strain is caused by the deviation from normal bond angle. Cyclohexane has sp3 hybridization. Normal bond angle for sp3 hybridization is 109.5 The ring strain is minimum for cyclohexane. It increases with increase or decrease in size of the ring. However it reduces to zero after nine carbon rings. Low ring strain means low energy high stability and vice -versa
Equatorial & Axial hydrogen
Two hydrogen are attached to each carbon The hydrogen coming out of center of ring is called equatorial hydrogen and the hydrogen projected upward or downward is called axial hydrogen. When ring flip-flop between various shapes all the hydrogen bond interchange their location and no position is more favorable than other. However when ring has substituent groups attached crowding occur very often in axial position. The crowding causes higher strain higher energy lower stability. Hence most substituent groups are present in equatorial position.
Cis- trans isomerism are possible in ring structure even without double bond.
Geometric isomers
Geometric isomerism or cis-trans isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. Such isomers typically contain double bonds which cannot rotate, but they can also arise from ring structures where the rotation of bonds is greatly restricted.
The isomers in which the functional group or substituents are locked opposite side of the double bond is called trans isomer The figure shows two chlorine atoms or the two methyl groups are locked on opposite sides of the double bond.
The isomer in which the function group or the substituent are on the same side of the double bond is called cis-isomer. The figure above shows the two chlorine atoms or the two methyl groups are attached on the same side of the double bond.
Optical activity and Optical isomers
Rotation of plane polarized light is called optical activity. The molecules which posses this property are cal