Chirality in chemistry - without 4-coordinate carbon.

Aisha Ali

Chirality in chemistry - without 4-coordinate carbon

Chirality is a term which may be applied to any asymmetric object or molecule. It is the property of non-identity of an object with its mirror image. A chiral compound is one which is not superimposable on its mirror image. This property of molecules has a great importance in the chemistry feild as it provides us with an understanding of the shapes of molecules which then in turn, gives us an insight on the way they react in a particular reaction. Chirality in chemistry is described in many ways but the universal defination of chirality was given by Lord Kelvin in 1904 who defined chirality in his ‘Baltimore Lectures’ on ‘Molecular Dynamics and the Wave Theory of Light’ as "I call any geometrical figure, or group of points, chiral, and say it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself."

Say if a left hand was transplanted in the place of the right hand. Now if we try and shake hands with someone else, it would not be possible. Whichever way the left hand was transplanted, the fingers would seem to be inverted. The moral is that the left hand and the right hand are different. On the other hand, the left hand and the right foot are also different. However, we may feel there exists a relationship between the left hand and the right hand that doesn't exist between the left hand and the rig

ht foot. If we put the left hand up to a mirror. The reflection in the mirror is of a right hand. So this proves that the left and right hands are mirror images.

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ht foot. If we put the left hand up to a mirror. The reflection in the mirror is of a right hand. So this proves that the left and right hands are mirror images.

Not all objects behave like a pair of hands. If we take a water glass, for example, it is identical to its image in the mirror. The underlying issue is the identity of an object and its reflection. Any object that is different from its reflection is said to be chiral. Otherwise, it is called achiral. The left hand and the right hand are chiral, whereas the water glass is achiral. This geometric phenomenon is called chirality. Basically, chirality is "handedness," that is the existence of left/right opposition. The phenomenon distinguish left hand from right.

Altough chirality plays a tremendous role in organic chemistry, chirality does not depend on carbon atom properties. Many non-carbon atoms also show chiral properties. Compounds like Nitrogen and Phosphorus and silicon can also be chiral centers if they have different substituents attached to them. If nitrogen had three different substituents attatched to it say Et, methyl and a hydrogen, then it would have a tetrahedral structure with the three substiuents as well as the lone pair of electrons. The nitrogen atom is known as the chiral center as it is the atom to which the sustituents are attached. If we then took the mirror image of that structure it would be non-superimposable on the first structure just like the right hand and left hand no matter how hard we tried we would not be able to impose the two structures on to each other. But sometimes chirality may not be observed this is due to the ability of the molecule to flip in side out. If however the nitrogen is a part of a three membered ring and also be attached to an atom which has atleast one pair of unshared electrons then this situation may be prevented. The compound 1-chloro-2,2-dimethylaziridine has these properties and can be resolved into its enantiomers. Although this compound has a 4-coordinate carbon chirality is at nitrogen and not at the carbon.

Phosphorus is a five coordinate compound which when has different substituents attached to it will give a chiral compound. A five-coordinate trigonal bipyramid with phosphorus as central atom with different substituents attached to it would give a chiral compound. (TPY-3-S)-bromochlorophosphine is an example of a phosphorus centered chiral compound. It consists of phosphorus with the H, Cl and Br attathed to it to give a pyramidal structure.

As well as the molecule not being superimposable on its mirror image another way of determining chirality is by checking the symmetry operation of the compound. The absence of a mirror plane or centre of inversion in the molecule would also allow the molecule to posess chiral properties.This property is more common in determining weather metal complexes are chiral.

Chiral metal complexes may be tetrahedral, trigonal bypyramidal, square planar or octahedral in shape. An eaxample of an octahedral chiral compound is the complex ion [ Cr(C2O4)]-3 .

Cis-amminechlorobis (ethylenediamine) cobalt (2+), [Cr(edta)]- and [Ru(bipy)3]2+ are all examples of octahedral chiral compounds. The structure of these complexes would show that there is no centre of inversion or mirror plane symmetry. To be a centre of chirality a metal group must be surrounded by at least three different types of monodentate ligands arranged in an appropriate manner to give an octahedral complex.

Pt (II) metal complex containing one molecule of meso-stilbenediamine and one molecule of isobutyldiamine can posess both square planar and tetrahedral structures. The tetrahedral structure would have the carbon atoms join together this would lead to the molecule to have a plane of symmetry and thus lead to the molecule to be achiral. The square planar structure would howver be chiral as there would be no plane of symmetery present in the molecule. Pt(NH3)2(NO2)2(Cl)2 is also a chiral complex but with an octahedral structure.

For a square planar structure we need to use two different specially chosen bidentate ligands or a chiral ligand. These are the only way by which square planar complexes may become chiral.

Chirality is an important aspect of chemistry for inorganic chemists as well as organic chemists. It is a concept which enables us to understand the different behaviour of the same molecules. Chirality in chemistry is of many forms and is posessed by molecules and complexes of different structures.

Bibliography:

Inorganic chemistry A.G Sharpe

Inorganic nomenclature Principles and practise B. Peter Block

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