Stereoisomerism, types of stereoisomerism, optical activity, Geometrical isomerism in alkene, asymmetric and dissymmetric molecules, organic chemistry-3 with best notes

Stereoisomerism

Stereoisomerism is a form of isomerism in which molecules have the same molecular formula and same structural formula but different in arrangement of atoms in a space. It is also defined as the particular kind of isomers that are different from each other only in the way the atoms are oriented in a space. Ex– 1,2 dichloroethane.

Optical activity

The power of any substance to rotate the plane polarized light (PPL) from its axis to clockwise or anticlockwise direction is called “optical activity” and the substance is known as “optical active substance“.

Fastly ordinary light passes through Nicol prism, make it plane polarized light then pass this light from polarimeter tube in which we filled 1/solution, which we have to check optical activity.

If the plane polarized light goes unaffected or straight the solution or compound is optically inactive but if the plane polarized light rotate, then compound is optically active.

Rotation of PPL in two types

1. If the PPL rotates clockwise direction, the solution is dextro rotary denotes as “D”.
2. If PPL rotates anti clockwise direction, the solution is levo rotary denote as “L”.

Stereo isomerism has two types:

1. Optical isomerism

A pair of compounds which have identical physical properties such as melting point, stability and refractive index and also have identical structure but which have equal and opposite effect on plane polarized light, such compound are called optical isomer. It is a type of ester isomerism in which molecules have some molecular structure of formula and also have same physical properties melting point boiling point density etc but they differ in their towards light. They also have same specific rotation of life but with opposite sign, like lactic acid. It is a type of stereoisomerism.

2. Geometrical isomerism

It is also called as cis-trans isomerism. Geometrical isomerism results from a restriction in relation about double bonds or single bond in cyclic compound. It is a type of stereoisomerism.

Geometrical isomerism in alkene

The carbon atom is C≈C are SP² hybridized. The C≈C consist of pi bond and sigma bond. The sigma bond is formed by the overlap of sp² hybrid orbital & the π bond is formed by the overlap of p-orbitals. The present of π bond looks the molecule in one position. The two carbon atoms of C≈C & the four atoms, that are attached to then lie in the one plane of this position in space are fixed. Rotation around the C≈C is not possible because rotation will break the π bond. This restrictions of rotation about a C≈C is responsible for geometrical isomerism. It is a type of stereoisomerism.

These two compounds are referred as geometrical isomers, and these distinguished by the terms cis and trans from each other.

1. Cis-isomers
It is 1 in which two similar groups are on the same side of the double bond.

2. Trans-isomers
It is 1 in which two similar groups are on the opposite side of double bond.

Conversion of cis-isomers into trans-isomers

Geometrical isomers are stereoisomers because they have the same structure formula but different special arrangement of atoms (configuration).

The conversion of cis-isomers into trans-isomers or vice-versa is possible only if either isomer is heated on highly temperature or absorb light. The heat supplies the energy about (62 calories) to break the π bond so that rotation about sigma bond becomes possible upon cooling the reformation of π bond can take place in 3 ways giving the mixture of Trans to butane + cis 2 butene. It is a type of stereoisomerism.

asymmetric and dissymmetric molecules

Asymmetric and dissymmetric molecules are molecules that have different shapes or structures on each side. These molecules can be found in nature, and they have been used in the development of many products and materials. Asymmetric molecules are particularly useful for their ability to interact with other substances in a specific way, while dissymmetric molecules can be used to create materials with specific properties. In this article, we will discuss the structure of these molecules, their uses in industry, and how they can be synthesized. It is a type of stereoisomerism.

1. Chiral molecule, such molecules as have no element of symmetry these are symmetrical molecules are called Chiral molecules.
2. Achiral molecules, an asymmetrical carbon is a chiral center of symmetrical molecules are called achiral molecules.

Condition for optical activity

The following conditions should be fulfilled by a compound to show optical activity.

1. A compound should be asymmetric or chiral. Ex– lactic acid, malic acid

2. The compound should be such that it is known spray possible on its mirror image. Ex- Chloroiodomethane sulphonic acid

3. Optical activity due to crystalline structure- these are many substances which are optically active in that solid state. Ex- Quartz, sodium chlorate & benzyl.

Quartz exist in 2 crystalline form one of which is dextrorotatory and the other levorotatory. These two forms are mirror image and are not super impossible.

Enantiomers

A molecules and its mirror image when they are non-super imposable are known as Enantiomers. Chirality is the necessary and sufficient condition for the existence of enantiomers. A compound whose molecules are a chiral cannot exist as enantiomers. Example– lactic acid, 2-methyl-1-butanol.

Optical isomers main rotate the plane of polarization by equal and opposite amount this optical isomer are enantiomers. Enantiomers  have identical physical property impact for the direction of rotation of plane of polarized light. Example- 2-methyl-1-butanol have identical melting points, boiling point, densities, refractive index and any other physical constant except one rotates plane polarize light to the light and other rotate to left. It is a type of stereoisomerism.

Diastereoisomers

When the optical isomers rotate the plane of a polarization light by different amounts these are called Diastereoisomers. Stereoisomers which are non-superimposable & also not the mirror image of each other are called as Diastereoisomers.

Diastereoisomers contains the same functional group and hence show similar chemical properties but the chemical properties are not identical. Diastereoisomers have different chemical properties.

Meso compounds

A meso compounds is one whose molecules, which are superimposable on their mirror image even through the contain chiral center or asymmetric carbon atoms. A meso compounds is optically inactive due to same reason as any other compound whose molecules as achiral.

Optically active is due to the rotation causes by one molecule is cancelled by an equal and opposite rotation causes by another molecule that is the mirror image of First. In meso structures, 1 half of the molecule is the mirror image of the other half, so the molecules have the plane of symmetry but the molecules is a chiral. It is a type of stereoisomerism.

Representation of structure of molecules

1. Wedge formula

In the wedge formula, a solid or a heavy line indicates the bond above the plane of the paper (a bond pointing towards you) & a dashed or clotted line indicates a bone below the plane of a paper (a bond pointing away from you). Solid line or continuous line indicates the bond is in the plane the paper.

2. Fisher projection formula

It is the “2D representation” of molecular. In this method molecules are represented by cross line and the center of lines is assumed as carbon atom.

Convert wedge dash projection to fisher projection

This conversation is important for the (D&L) and (R&S) configuration of molecules. In this, 3D structure of molecules is convert into 2D structure.

Those atoms/group which are on plane, represent in vertical line in fisher projection. Then, watch wedge-dash structure from that side on which “above & below the plane” atoms/group attached. Then that atoms/group which is in your side, put it in left side and that atoms/group which is in your right side, put in right side in “Fischer projection”.