Isomerism. Introduction

Transcription

1 Isomerism Introduction The existence of two or more compounds with same molecular formula but different properties (physical, chemical or both) is known as isomerism; and the compounds themselves are called isomers. The term was given by Berzelius. The difference in properties of two isomers is due to the difference in the arrangement of atoms within their molecules. Isomerism may be of two types: Structural isomerism When the isomers differ only in the arrangement of atoms or groups within the molecule, without any reference to space, these are known as structural isomers and the phenomenon as structural isomerism. Thus the structural isomers have the same molecular formula, but possess different structural formulae. Structural isomerism may again be of several types. (i) Chain, nuclear or skeleton isomerism This type of isomerism is due to the difference in the nature of the carbon chain (i.e. straight or branched) which forms the nucleus of the molecule, e.g., (ii) Position isomerism It is due to the difference in the position of the substituent atom or group or an unsaturated linkage in the same carbon chain. Examples are (iii) Functional isomerism 1

2 This type of isomerism is due to difference in the nature of functional group present in the isomers, e.g., (iv) Metamerism It is due to the difference in nature of alkyl groups attached to the same functional group. This type of isomerism is shown by compounds of the same homologous series. For example, 2

3 (v) Tautomerism Tautomerism may be defined as the phenomenon in which a single compound exists in two readily interconvertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen. The two different structures are known as tautomers of each other. Sometimes the term tautomerism is also called as desmotropism (Greek desmos-bond; tropos-turn), since the interconversion of the two forms involves a change of bonds or dynamic isomerism as the two forms are in dynamic equilibrium with each other. Other names for tautomerism are kryptomerism, allelotropism or merotropy; however, tautomerism is the most widely accepted term. There are several types of tautomerism of which keto-enol tautomerism is the most important. In this type, one form (tautomer) exists as a ketone while the other exists as an enol. The two simplest examples are of acetone and phenol. However, the most widely studied example of keto-enol tautomerism is that of acetoacetic ester (ethyl acetoacetate). The two forms are readily interconvertible by acid or base catalysts, and under ordinary conditions surface of the glass is sufficient to catalyse the interconversion. The exact composition of the equilibrium depends upon the nature of the compound, solvent, temperature, etc. The conversion of a keto form into enol from is known as enolisation. The two forms of acetoacetic ester have been isolated under suitable conditions. Keto-enol tautomerism in acetoacetic ester is proved by the fact that under ordinary conditions the compound gives the properties of the ketonic group as well as that of the enolic group. Note that in all the examples of keto-enol tautomerism the two isomeric forms are interconvertible by 3

4 the migration of a proton from one atom (carbon) to the other with the simultaneous shifting of bonds Distinction of tautomerism from resonance : The tautomeric forms are quite chemically distinct entities and can be separated (in suitable cases e.g. acetoacetic ester) and characterised. On the other hand, resonating forms differ only in the distribution of electrons and can never be separated from one another since neither of them has any real existence. The important differences between resonance and tautomerism can be summarised as below. 1. Tautomerism involves a change in the position of atom (generally hydrogen), while resonance involves a change in the position of the unshared or only. 2. Tautomers are definite compounds and may be separated and isolated. Resonating structures are only imaginary and can t be isolated. 3. The two tautomeric forms have different structures (i.e. functional groups). The various resonating structures have the same functional group. 4. Tautomers are in dynamic equilibrium with each other, resonating structures are not in dynamic equilibrium. 5. Tautomerism has no effect on bond length, while resonance affects the bond length (single bond is shortened while the double bond becomes longer). 6. Tautomerism does not lower the energy of the molecule and hence does not play any role in stabilising the molecule, while resonance decreases the energy and hence increases the stability of the molecule. 8. Tautomerism can occur in planar as well as non-planar molecules, while resonance occurs only in planar molecules. Stereo isomerism When isomers have the same structural formula but differ in relative arrangement of atoms or groups in space within the molecule, these are known as stereoisomers and the phenomenon as stereoisomerism. The spatial arrangement of atoms or groups is also referred to as configuration of the molecule and thus we can say that the stereoisomers have the same structural formula but different configuration. Stereoisomerism is of two types. (i) Geometrical isomerism The isomers which possess the same structural formula but differ in the spatial arrangement of the 4

5 groupsaround the double bond are known as geometrical isomers and the phenomenon is known asgeometrical isomerism. This isomerism is shown by alkenes or their derivatives. When similar groups lie on the same side, it is the cis-isomer; while when the similar groups lie on opposite sides, the isomer istrans. For example, Remember that geometrical isomerism is possible only when each of the doubly bonded carbon atom has two different groups (see examples above). Thus compounds of the following type will not show geometrical isomerism. Distinction between cis -and trans- isomers. (a) Generally, the cis-isomer (e.g. maleic acid) cyclises on heating to form the corresponding anhydride while the trans-isomer does not form its anhydride at all. (b) The cis-isomer of a symmetrical alkene (alkenes in which both the carbon atoms have similar groups) has a definite dipole moment, while the trans-isomer has zero dipole moment. For example, 1, 2-dichloroethylene and butene-2. In trans-isomer of the symmetrical alkenes, the effect produced in one half of the molecule is cancelled by that in the other half of the molecule. 5

6 In case of unsymmetrical alkenes, the cis-isomer has higher dipole moment than the corresponding trans-isomer. For example, 6

7 (ii) Optical isomerism This type of isomerism arises from different arrangements of atoms or groups in three dimensional space resulting in two isomers which are mirror image of each other. Optical isomers contain an asymmetric (chiral) carbon atom ( a carbon atom attached to four different atoms or groups) in their molecules. For example, lactic acid having four different groups on the central carbon atom is optically active; while succinic acid having two similar atoms on the central carbon atom is optically inactive. Optical isomers have similar chemical and physical properties and differ only in their behaviour towards plane polarised light. The isomer which rotates the plane polarised light to left is known as laevo (l) while that which rotates the plane polarised light to the right is known as dextro (d). For example, Note that thed andl forms of a compound are non superimposible mirror image of each other and such pairs are known as enantiomorphs or enantiomers. A compound can exist in enantiomeric forms if it has an asymmetric carbon atom and is devoid of the elements of symmetry, viz. (i) a plane of symmetry, (ii) a centre of symmetry and (iii) an alternating axis of symmetry. If a molecule possesses any of the above elements of symmetry, it is symmetrical; on the other hand, if it does not possess either of these elements of symmetry, it is asymmetric and hence is optically active and can exist in d and l forms. The number of optical isomers in a molecule containingn 7

8 number of different asymmetric carbon atoms is given by the relation 2 n. Furtermore, there will be 2 n 1 pairs of enantiomer and the same number of racemic modifications. Racemic modification is an equimolecular mixture of d and l forms of the same compound. The process of converting d or l form of an optically active compound into dl (racemic) form is known as racemisation. Since the rotation of d is cancelled by equal but opposite rotation of l, racemic mixture (r) is always optically inactive. This type of optical inactivity is known as optical inactivity due to external compensation. Now since dl mixture(r form) can be separated into d and l form (resolution), optical activity can be restored in the r form. The number of optical isomers in a compound containing n number of similar asymmetric carbon atoms is always less than 2 n. The classical and most important example is tartaric acid, CH(OH)COOH.CH(OH)COOH which can exist in the following isomeric forms. (i) d Tartaric acid. It rotates the plane polarised light to the right. The rotation due to the upper half is strengthened by rotation due to the lower half. It has no plane of symmetry. (ii) l Tartaric acid. It rotates the plane polarised light to left. Here again rotation due to upper half is strengthened by rotation due to lower half. It also has no plane of symmetry. The d and l tartaric acids are mirror image of each other (enantiomers). (iii)r Tartaric acid. It is equimolecular mixture of the d and l forms and hence optically inactive (eg.r lactic acid) due to external compensation. (iv) m Tartaric acid. It possesses a plane of symmetry 8

9 (denoted by dotted line) and hence superimposes on its mirror image (i.e., they are identical) and hence it is optically inactive. The optical inactivity is said to be due to internal compensation as the rotation due to the upper half of the molecule is balanced by the equal but opposite rotation due to the lower half. The meso isomer cannot be resolved into active (d andl ) isomers (difference from racemic tartaric acid). Remember that stereoisomers which are not mirror image (enantiomers) are known as diastereomers ordiastereoisomers. Thus m tartaric acid constitutes the diastereomer of d as well as of l tartaric acid. Prediction of number of optical isomers (i) When the molecule is unsymmetrical Number of d and l isomers (a) = 2 n (active) Number of meso forms (m) = 0 Where n is the number of chiral carbon atom (s). Common example is CH 3.CHBr.COOH 2 1 = 2 (ii) When the molecule is symmetrical and has even number of chiral carbon atoms Number of d and l isomers (a) = 2 (n 1) Number of meso forms (m) = 2 (n/2 1) 9

10 Common example is tartaric acid, HOOC. CHOH. CHOH.COOH (iii) When the molecule is symmetrical and has an odd number of chiral carbon atoms. Number of d and l forms (a) = 2 (n 1) 2(n/2 ½) Number of meso forms (m) = 2 (n/2 ½) Racemic Mixture or Racemic Modification As described earlier, a racemic modification is an equimolecular mixture of a pair of enantiomers, i.e., (+) and ( ) forms and is denoted by Racemic mixture is generally obtained in the following two ways. (i) By mixing equal amounts of the two enantiomers. (ii) By synthesis. The synthesis of a chiral compound from achiral compound in the absence of optically active agent or circularly polarised light always produces a racemic modification. For example, the formation of lactonitrile from acetaldehyde always results in a racemic modification in the following manner:- Resolution Separation of dl mixture of a compound into d and l isomers is known as resolution. This can be done 10

11 by several methods, viz. mechanical, biochemical and chemical method. Chemical method involves the formation of diastereomers and is found to be the best method for resolution. 11