10. HALOGEN DERIVATIVES - part 04 - Chiral atom and molecular chirality

Optical isomerism in halogen derivatives :

Isomers having the same bond connectivities, that is, structural formula are called stereoisomers.
Knowledge of optical isomerism, which is a kind of stereoisomerism will be useful to understand nucleophilic substitution reactions of alkyl halides

1. Chiral atom and molecular chirality

Let us, now, jot down the atoms/groups attached to each carbon in 2 - chlorobutane.
Four groups bonded to C-2 are all different from each other.
Carbon atom in a molecule which carries four different groups/atoms is called chiral carbon atom. Thus, the C-2 in 2-chlorobutane is a chiral carbon.
Chiral atom in a molecule is marked with asterisk (*). For example, CH₃-*CHCl-CH₂-CH₃
When a molecule contains one chiral atom,can not superimpose perfectly on its mirror image. It is called chiral molecule.
A chiral molecule and its mirror image are not identical

A chiral molecule and its mirror image both have the same structural and molecular formula.
Spatial arrangement of the four different groups around the chiral atom, however, is different.
Chiral molecule and its mirror image are stereoisomers of each other, i.e. similar to the relationship between left and right hands. Therefore it is called handedness or chirality.
Stereoisomerism in which the isomers have different spatial arrangements of groups/ atoms around a chiral atom is called optical isomerism.
The optical isomers differ from each other in terms of a measurable property called optical activity.

Remember...

The phenomenon of optical isomerism in organic compounds was observed first and its origin in molecular chirality was recognized later

2 Plane polarized light :

An ordinary lightconsists ofelectromagnetic waves having oscillations of electric and magnetic field in all possible planes perpendicular to direction of propagation of light.
When ordinary light is passed through Nicol's prism, oscillations only in one plane emerge out.
Such a light having oscillations only in one plane perpendicular to direction of propagation of light is known as plane polarized light.

Do you know ?

Nicol prism is a special type of prism made from pieces of calcite, a crystalline form of CaCO3,arranged in a specific manner.
Nicol prism is also called polarizer.

3 Optical activity :

When an aqueous solution of certain organic compounds like sugar, lactic acid is placed in the path of plane polarized light, the transmitted light has oscillations in a different plane than the original.
Incident light undergoes rotation of its plane of polarization.
The plane of polarization rotates either to theright (clockwise) or to the left (anticlockwise).
This property of a substance by which it rotates plane of polarization of incident plane polarized light is known as optical activity.
The compounds which rotate the plane of plane polarized light are called optically active compounds.
Those which do not rotate it are optically inactive compounds.
Optical activity of a substance is expressed numerically in terms of optical rotation.
The angle through which a substance rotates the plane of plane polarized light on passing through it is called optical rotation.
In accordance with the direction of optical rotation an optically active substance is either dextrorotatory or laevorotatory.

Dextrorotatory :

A compound which rotates the plane of plane polarized light towards right is called dextrorotatory.
Designated by symbol d- or by (+) sign.

Laevorotatory :

A compound which rotates plane of plane polarized light towards left is called laevorotatory.
Designated by symbol l or by (-) sign.

Optical isomerism :

Isomerism in which isomeric compounds have different optical activity is known as optical isomerism.
French scientist Louis Pasteur first recognized that optical activity is associated with certain type of 3-dimensional structure of molecules.
Pasteur introduced the term enantiomers for the optical isomers having equal and opposite optical rotation.
Few objects in our day to day life which exhibit superimposable and non-superimposablemirror image relationship.

Remember...

Optical activity is an experimentally observable property.
Chirality is a description of molecular structure.
Optical activity is the consequence of chirality.
Molecules which contain one chiral atom are chiral, that is, they are nonsuperimposable on their mirror image.
The two non-superimposable mirror image structures are called pair of enantiomers.
Enantiomers have equal and opposite optical rotation. Thus, enantiomers are a kind of optical isomers.

4. Enantiomers :

Optical isomers which are non-superimposable mirror image of each other are called enantiomers or enantiomorphs or optical antipodes.
For example, 2 - chlorobutane exists as a pair of enantiomers.
Have identical physical properties except the sign of optical rotation.
Magnitude of their optical rotation is equal but the sign of optical rotation is opposite.
Identical chemical properties except towards optically active reagent.
An equimolar mixture of enantiomers (dextrorotatory and laevorotatory) is called racemic modification or racemic mixture.
Racemic modification is optically inactivebecause optical rotation due to molecules of one enatiomer is cancelled by equal and opposite optical rotation due to molecules of the other enantiomer.
Racemic modification is designated as (dl) or by (±) sign

a. Fischer projection formula (cross formula) :

Two representations are used to represent configuration of chiral carbon and the 3-dimensional structure of optical isomers on plane paper.
These are -

a. Wedge formula :

When a tetrahedral carbon is imagined to be present in the plane of paper all the four bonds at this carbon cannot lie in the same plane.
The bonds in the plane of paper are represented bynormal lines,the bonds projecting above the plane of paper are represented by solid wedges (or simply by bold lines) while bonds going below the plane of paper are represented by broken wedges (or simply by broken lines).

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