IUPAC & GOC of Class 11
The isomers which differ only in the orientation of atoms in space are known as stereoisomerism. It’s of two types.
(a) Geometrical isomerism: Isomers which posses the same molecular and structural formula but differ in arrangement of atoms or groups in space around the double bonds, are known as geometrical isomers and the phenomenon is known as geometrical isomerism. Geometrical isomerism are show by the compounds having the structure.
(i) Cis – trans isomerism: When similar groups are on the same side it is cis and if same groups are on the opposite side it is trans isomerism.
(ii) Syn - anti
Syn anti isomerism is not possible in ketoxime since only one form is possible two —CH3 groups are at one C.
(iii) In cyclic compounds:
Differentiating properties of cis-trans isomerism
(i) Dipolemoment: Usually dipole moment of cis is larger than the trans-isomer.
(ii) Melting point: The steric repulsion of the group (same) makes the cis isomer less stable than the trans isomers hence trans form has higher melting point than cis.
(iii) Different chemical properties: Syn-addition makes cis forms into meso and trans into d and l, anti addition makes cis into d – l and trans into meso.
E and Z nomenclature of Geometrical Isomerism
If all the four groups/ atoms attached to C = C double bond are different, then Cis and trans nomenclature fails in such cases and a new nomenclature called E and Z system of nomenclature replace it.
The group / atom attached to carbon - carbon double bond is given to higher rank, whose atomic weight is higher. If the two higher ranked group are across, it is called E form (E stands for the German word entgegen meaning thereby opposite) and the two higher ranked groups are on the same side, they are called Z-form (Z stands for German word Zusammen meaning thereby on the same side).
In general trans - isomer is more stable then cis isomer because in cis from, there will be more interaction in between groups.
Besides suitably substituted alkene and cycloalkane, suitably substituted oximes and azo compounds also exhibit geometrical isomerism.
(b) Optical isomerism: Any substance which rotates the plane polarised light (PPL) is said to be optically active. If a substance is optically active, it is non - superimposable on its mirror image. If a molecule of substance is superimposable on its mirror image, it cannot rotate PPL and hence optically inactive. The property of non-superimposibility on mirror image is called chirality. The ultimate criterion for optical activity is chirality, i.e., non-superimposibility on its mirror image.
If a molecule of organic compound contains 'one' chiral carbon, it must be chiral and hence optically active.
Chiral carbon: If all the four bonds of carbon are satisfied by four different atoms / groups, it is chiral. Here it should be noted that isotopes are regarded as different atoms / groups) chiral carbon is designated by an asterisk (*).
Optical isomerism in bromochloroiodomethane:
The structural formula of bromochloroiodomethane is
The molecule has one chiral carbon as designated by star. So molecule is chiral. It is non - superimposable on its mirror image.
According to Van't Hoff rule,
Total number of optical isomers should be = 2n; when n is number of chiral centre
The Fischer projections of the two isomers are
Stereoisomers which are mirror - image of each other are called enantiomers or enantiomorphs. (i) and (ii) are enantiomers. All the physical and chemical properties of enantiomers are same except two:
(i) They rotate PPL to the same extent but in opposite direction. One which rotates PPL in clockwise direction is called dextro-rotatory (dextro is Latin word meaning thereby right) and is designated by d or (+). One which rotates PPL in anti-clockwise direction is called laevo rotatory (means towards left) and designated by (–).
(ii) They react with optically active compounds with different rates.
Optical isomerism in compounds having more than one chiral carbons
If an organic molecule contains more than one chiral carbons then the molecule may be chiral or achiral depending whether it has element of symmetry or not.
Elements of symmetry: If a molecule have either
(a) a plane of symmetry, and / or
(b) centre of symmetry, and / or
(c) n-fold alternating axis of symmetry
If an object is superimposable on its mirror image; it cannot rotate PPL and hence optically inactive.
If an object can be cut exactly into two equal halves so that half of its become mirror image of other half, it has plane of symmetry.
Centre of symmetry: It is a point inside a molecule from which on travelling equal distance in opposite directions one takes equal time.
Thus, if an organic molecule contains more than one chiral carbons but also have any elements of symmetry, it is superimposable on its mirror - image, cannot rotate PPL and optically inactive. If the molecule have more than one chiral centres but not have any element of symmetry, it must be chiral.
Stereoisomerism in 2, 3-dibromopentane
The structural formula of 2, 3-dibromopentane is
The molecule contains two chiral carbons and hence according to Van't Hoff rule the total number of optical isomers should be 2n = 22 = 4 and it is. The four optical isomers are.
∙ Nonsuperimposable ∙ Non -superimposable
∙ Rotate PPL ∙ Rotate PPL
∙ Optically active ∙ Optically active
I, II, III and IV are four stereoisomers of 2, 3-dibromopentane.
I and II are enantiomers.
III and IV are also enantiomers
What is relation between I and III; or I and IV; or II and III; or II and IV?
All these pairs are diastereomers, stereoisomers which are not mirror - image of each other are called diastereomers.
I and III are diastereomers
I and IV are diastereomers
II and III are diastereomers
II and IV are diastereomers
Stereoisomerism in Tartaric Acid
The IUPAC name of tartaric acid is 2, 3 - dihydroxy butandioic acid. The structural formula is
The molecule contains two chiral carbon and the number of optical isomers should be 2n=22=4; but number of optical isomers reduces to 3 because one molecule has plane of symmetry.
Stereoisomers of Tartaric Acid
- Non - superimposable
- Can rotate PPL III and IV are same
- Optically active Rotation of IV by 180° yield III.
Superimposability means same in identity
I and II are enantiomers
III is meso-form of tartaric acid
A meso compound is one which is optically inactive although have more than one chiral carbons.
Number of Optical Isomers
Number of possible optical isomers in compounds containing different no. of asymmetric atoms.
(1) The molecule has no symmetry
The no. of d and l – forms a = 2n
n = no. of asymmetric atoms
The no. of meso l- forms m = 0
Total no. of optical isomers = a + m = 2n
(2) The molecule has symmetry and n is even
The no. of d and l forms a = 2n–1
Meso forms m =2n/2 - 1
Total = a + m
= 2n–1 +2n/2 - 1
a = 2n–1 = 22–1
m = 2n/2 - 1 = 21–1 = 2° = 1
Total = 2 +1 = 3
One of asymmetric centre is rotating plane polarised light towards right other towards left so total optical rotation is zero.
I & III or II & III which are not mirror image to each other are called diastereomers.
(3) The molecule has symmetry & n is odd
The no. of d and l forms a = 2n–1 - 2n-1/ 2
Meso forms m =2n-1/ 2
Total = a + m
e.g. Lactic acid
n = 1 no symmetry
d & l forms = 2n = 2
meso form = 0
total = 2
Difference between racemic mixture and meso compound
A racemic mixture contains equimolar amounts of enantiomers. It is optically inactive due to external compensation. It can be resolved into optically active forms. A meso compound is optically inactive due to internal compensation.
Optically active compounds having no chiral carbon
The presence of chiral carbon is neither a necessary nor a sufficient condition for optical activity, since optical activity may be present in molecules with no chiral atom and since molecules with two or more chiral carbon atoms are superimposable on their mirror images and hence inactive.
(i) Any molecule containing an atom that has four bonds pointing to the corners of a tetrahedron will be optically active if the four groups are different.
(ii) Atoms with pyramidal bonding might be expected to give rise to optical activity if the atom is connected to three different groups, since the unshared pair of electron is analogous to a fourth group.
Many attempts have been made to resolve such compounds, but until recently all failed because of umbrella effect, also called pyramidal inversion. The umbrella effect is rapid oscillation of the unshared pair from one side of XYZ plane to the other.
(iii) Biphenyls, containing four large groups in the ortho position so that there is restricted rotation, are optically active if the rings are asymmetrical.
If either or both rings are symmetrical, the molecule has plane of symmetry and optically inactive.
Ring B is symmetrical (having plane of symmetry) superimposable optically inactive.
Allenes, with even number of cumulative double bonds are optically active if both sides are dissymmetric.
The specific rotation is an inherent physical property of an enantiomer, which varies with the solvent used, temp (in °C) and wavelength of the light used. It is calculated from the observed rotation α as follows.
Where l = length of tube in decimeters (dm)
C = Concentration in gram cm–3, for a solution density in g cm–3, for a pure liquid.
Nomenclature of Optical Isomers
(1) Absolute and Relative Configuration
While discussing optical isomerism, we must distinguish between relative and absolute configuration (arrangement of atoms or groups) about the asymmetric carbon atom. Let us consider a pair of enantiomers, say (+)- and (–)- lactic acid.
We know that they differ from one another in the direction in which they rotate the plane of polarized light. In other words, we know their relative configuration in the sense that one is of opposite configuration to the other. But we have no knowledge of the absolute configuration of the either isomer. That is, we cannot tell as to which of the two possible configuration corresponds to (+) - acid and which to the (–) - acid.
(2) D and L system
The sign of rotation of plane-polarized light by an enantiomer cannot be easily related to either its absolute or relative configuration. Compounds with similar configuration at the asymmetric carbon atom may have opposite sign of rotations and compounds with different configuration may have same sign of rotation. Thus d-lactic acid with a specific rotation + 3.82o gives l-methyl lactate with a specific rotation -8.25°, although the configuration (or arrangement) about the asymmetric carbon atom remains the same during the change.
+ 3.82 - 8.25o
Obviously there appears to be no relation between configuration and sign of rotation. Thus D-L-system has been used to specify the configuration at the asymmetric carbon atom. In this system, the configuration of an enantiomer is related to a standard, glyceraldehyde. The two forms of glyceraldehyde were arbitrarily assigned the absolute configurations as shown below.
D configuration L configuration
If the configuration at the asymmetric carbon atom of a compound can be related to D (+)-glyceraldehyde, it belongs to D-series; and if it can be related to L(–)-glyceraldehyde, the compound belongs to L-series. Thus many of the naturally occurring α-amino acids have been correlated with glyceraldehyde by chemical transformations. For example, natural alanine (2-aminopropanoic acid) has been related to L(+)-lactic acid which is related to L(–)-glyceraldehyde. Alanine, therefore, belongs to the L-series. In general, the absolute configuration of a substituent (X) at the asymmetric centre is specified by writing the projection formula with the carbon chain vertical and the lowest number carbon at the top. The D configuration is then the one that has the substituent 'X' on the bond extending to the 'right' of the asymmetric carbon, whereas the L configuration has the substituent 'X' on the 'left'. Thus,
R2 ––C––X X––C––R2
D configuration L configuration
When there are several asymmetric carbon atoms in a molecule, the configuration at one centre is usually related directly or indirectly to glyceraldehyde, and the configurations at the natural (+)-glucose there are four asymmetric centres (marked by asterisk). By convention for sugars, the configuration of the highest numbered asymmetric carbon is referred to glyceraldehyde to determine the overall configuration of the molecule. For glucose, this atom is C–5 and, therefore, OH on it is to the right. Hence the naturally occurring glucose belongs to the D-series and is named as D-glucose
However, the above system of nomenclature based on Fischer projection formulae, has certain disadvantages. Firstly before a name can be assigned to a compound, we must specify how its projection formula is oriented.
Secondly, sometimes the two asymmetric carbon atoms having the same kind of arrangements of substituents are assigned opposite configurational symbols. Thus for (–)-2, 3-butanediol we have