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Chirality

The science of organic chemistry is based on the relationship between molecular structure and properties. That part of science which deals with structure in three dimensions is called stereo chemistry.

Optical activity is a part of stereo chemistry, and study of enantiomers and diastereomers.

 

Chirality Definition

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Chirality is defined as the property by which objects differ only as an image in a mirror differs from the object that produces it.In general we use the term chiral to molecules and their two-or three-dimensional representations. However chirality does not apply to stationary objects such as these but also to time dependent physical entities represented by vectors and vector fields, either static or changing , some of them originating in translational or rotational movement.

Pyramidal Inversion

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Conversion of one form of enantiomer into another is called as pyramidal inversion. The chiral centers in most molecules are carbon. But, sometimes, atoms other than carbon can also be chiral centers. Silicon, like carbon has a tetrahedral arrangement of bonds when it bears four substituents. A large number of organo silicon compounds in which silicon bears four different groups have been resolved into enantiomers.

The trigonal pyramidal molecules are considered as chiral if the central atom contains three different groups. To understand the details of chirality of substances of this nature, however, the pyramidal inversion that inter converts enantiomers should resolve slowly at room temperature.

At nitrogen the pyramidal inversions are usually very fast and that results in the failure of resolving chiral amines due to rapid racemization.

Pramidal Inversion
Phosphorus is in the same group of the periodic table as nitrogen and tri coordinate phosphorus compounds, like phosphines, amines, etc are trigonal pyramidal. Phosphines undergo pyramidal inversion much slower than amines and a number of optically active phosphines have been prepared.

The tri coordinate sulfur compounds are found to be chiral when the sulfur atom bears three different substituents. The pyramidal inversion rate at sulfur is also found to be rather slow. The sulfoxides are the most common compounds in which sulfur has a chirality center.

Sulfoxides with Chirality Center

Chiral Amines

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Everything has a mirror image, but not all things are super imposable on their mirror images. Mirror-image super imposability characterizes many objects we use everyday.

In 1894, William Thompson coined a word for this property. H defined an object as CHIRAL if it is not super imposable on its mirror image. "A molecule is considered as chiral if at the least two of its mirror-image forms are not super imposable in three dimensions." Opposite of chiral is achiral. A molecule that is super imposable on its mirror image is achiral.

Chirality occurs in molecules that contain a carbon that is attached to four different groups. Sometimes, Nitrogen, sulfur or phosphorus can act as a chiral center too.


Chirality in amines


Amines are simply ammonia in which one or more hydrogen atoms have been replaced by organic groups. Nitrogen uses sp3 orbitals, which are directed to the corners of a tetrahedran. Three of these orbitals overlap s orbitals of hydrogen or carbon; the fourth contains an unshared pair of electrons. Amines, then, are like ammonia, pyramidal and with very nearly the same bond angles, 108o in trimethylamine, for example.

From an examination of models, we can see that a molecule in which nitrogen carries three different groups is not super imposable on its mirror image; it is chiral and should exist in two enantiomeric forms (I and II) each of which - separated from the other- might be expected to show optical activity.

Chirality in Amines
But, such enantiomers have not yet been isolated- for simple amines- and spectroscopic studies have shown why; the energy barrier between the two pyramidal arrangements about nitrogen is ordinarily so low that they are rapidly inter converted.

Just as rapid rotation about carbon - carbon single bonds prevents isolation of conformational enantiomers, so rapid inversion about nitrogen prevents isolation of enantiomers like I and II. Evidently, an unshared pair of electrons of nitrogen cannot ordinarily serve as a fourth group to maintain configuration.

Optical Activity

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The phenomenon of optical activity was discovered by the French Physicist Jean-Baptiste Biot in 1815.
  1. Optical activity is nothing but any chiral substance's ability to rotate the plane polarized light and this is measured by using an instrument called a polarimeter.
  2. The light which is utilized for measuring the optical activity has couple of properties. It consists of a single wavelength and it is plane polarized.
  3. The wavelength used is most often 589 nm which corresponds to the yellow light produced by a sodium lamp.
  4. Light from the ordinary electric lamp is composed of waves vibrating in many different planes. When it is passed through Nicole prism, made of Calcite, CaCO3, or Polaroid lens, light is found to vibrate in only one plane and is said to be plane polarized or simply polarized.

When a solution of a known concentration of an optically active material is placed in the polarimeter , the beam of polarized light is rotated through a certain degrees to the right (clockwise) or to the left (anticlockwise).

The compound which rotates the light to the right or in clockwise direction is said to be Dextrorotatory. It is usually indicated by a positive sign in the brackets (+).

The organic compound which rotates the plane polarized light to the left is said to be Leavorotatory. It is indicated by a negative sign in between the brackets. (-)

The optical rotatory powers of two isomers are equal in magnitude, but opposite in sign.

An equi molar mixture of the two isomers, dextro and leavo- rotatory, therefore, will not rotate the plane polarized light at all and is said to be a Racemic mixture. Organic compound containing a Chiral center is only optically active. A carbon atom bonded to four different groups is said to b a chiral center. Such a molecule is also called as Asymmetric, that is, without symmetry.

Example


Optical Isomerism of Lactic acid

Lactic acid (2 - hydroxide propanoic acid) is an example of a compound which shows optical isomerism. It contains one asymmetric carbon atom.

Lactic Acid

Two three dimensional structures are possible for Lactic acid.

  Optical Isomerism of Lactic Acid

These structures are not identical because they cannot be superimposed on each other.                       

Quaternary Ammonium Salt

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When all the four hydrogen of the ammonium ion (NH4 +) has been replaced by alkyl groups, the compound is called Quaternary Ammonium salt.

The amine salts are usually named as substituted ammonium salts.

Example

Quaternary Ammonium Salt

Tetraalkylammonium halides are obtained by treating ammonia with excess of an alkyl halide or by heating a tertiary amine with an appropriate alkyl halide.

For example


(CH3)3N + CH3I $\to$ (CH3)4N+I- 

Tetraalkylammonium halides have properties very similar to the ionic inorganic salts. They are crystalline solids. Their aqueous solutions conduct an electric current.

Optical activity in Quaternary ammonium salts


In quaternary ammonium salts, compounds in which four alkyl groups are attached to nitrogen, all four sp3 orbitals are used to form bonds and quaternary nitrogen is tetrahedral.

Quaternary ammonium salts in which nitrogen holds four different groups have been found to exist as configurational enantiomers, capable of showing optical activity.

Chirality Examples

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This compound has no chirality center and is not chiral. Carbon 1, 2, 4 ans 5 are all bonded to at least two hydrogen. Carbon 3 has one hydrogen, one hydroxy, and two ethyl groups attached to it.

Chirality Examples
Carbon-1,4,5 and 6 are all bonded to two or more hydrogens, so they are not chirality centers. However carbon 3 is bonded to four different groups (hydrogen, methyl, propyl, isopropyl) and is a chirality center. Because the molecules has only chirality center, it is chiral. Chirality centers are sometimes marked with an asterisk(*).

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