This article is on the Organic Chemistry Some Basic Principles and Techniques Notes class 11 of Chemistry. The chapter has been divided into two articles. This unit (Part 2) is on Isomerism and Stereoisomerism. The second unit (Part 1) is on the Classification of organic compounds and Nomenclature of Organic Compounds.
Organic Chemistry – Some Basic Principles and Techniques Part 2
ISOMERISM
Molecules with the same molecular formula, but different arrangements of atoms are called Isomers and the phenomenon is thus called Isomerism. They have different physical and chemical properties.
STRUCTURAL ISOMERISM
Compounds which have the same molecular formula but different structural formulas are called isomers and the phenomenon is known as isomerism.
Type of Structural Isomerism:
(a) Chain Isomerism: Chain isomers possess the same molecular formula, but different number of carbon in parent–chains (straight or branched).
(b) Position Isomerism: Compounds having the same molecular formula but the position of the functional group, multiple bond or branches along the same chain length of carbon atoms varies.
(i) The same molecular formula
(ii) The same length of carbon chain
(iii) The same functional group
In this structure, three carbon atoms form a chain, and X is joined at the end in (I), while at the middle carbon in (II). To be specific.
Note: In disubstituted derivatives, position isomerism also exists because of the relative positions occupied by the substituents on the benzene ring. Thus, Chlorotoluene, C6H4(CH3)Cl exists in three isomeric forms–ortho, meta and para.
(c) Functional Group Isomerism: When isomers have the same molecular formula but different functional groups, these compounds are called functional group isomers. The following pairs of families show this isomerism.
E.g. (i) Molecular formula: C2H6O
CH3–CH2–OH and CH3–O–CH3
Ethyl alcohol Dimethyl ether
(Alcohol) (Ether)
(ii) Molecular formula: C3H6O
(iii) Molecular formula: C3H6O2
(iv) Molecular formula: CH3NO2
(d) Metamerism: This type of Isomerism is due to the different position of polyvalency functional group (like S, N, O, and CO,) in a molecule, with alkyl groups around it. Members belong to the same homologous series.
E.g. (i) Diethyl ether and methyl propyl ether
CH3CH2OCH2CH3 Diethyl ether
CH3OCH2CH2CH3 Methyl propyl ether
(ii) Diethylamine and methyl propylamine
CH3CH2–NH– CH2CH3 Diethylamine
CH3CH2 CH2–NH– CH3 Methyl propylamine
STEREOISOMERISM
Those compounds having same molecular and structural formula but a different arrangement of group or atom in space is called stereo and phenomenon is termed as stereoisomerism.
It is divided into two parts:
(i) Configuration isomerism (ii) Conformational isomerism
Configurational isomerism is further divided into two parts:
(i) Geometrical isomerism (ii) Optional isomerism
GEOMETRICAL ISOMERISM
The main criteria for geometrical isomerism are the restriction in rotation:
In alkenes, (C=C) bond is made of σ –and π –bonds. A π –bond is made by the sideways overlap of the unhybridised π–orbitals of two C atoms above and below the plane of two C atoms. If one of the atoms is rotated through 90o, orbitals will no longer overlap and π –bond would break, which requires 25kJ/mol of energy. Hence, the rotation around (C=C) bond is not free but restricted. Due to this restricted rotation, the relative position of atoms or groups attached to the C atoms of the double bond gets fixed, which results in the formation of two distinct forms called geometrical isomers.
Types of geometrical isomerism: There are 3 types of geometrical isomerism on the basis of groups attached to double bond or the site of restricted rotation. They are mentioned below:
Cis-Trans Isomers
This kind of isomerism is used when there is at least a common group on both sides of the double bond or the site of restricted rotation (especially cycloalkane).
The isomer (I) in which similar groups or atoms lie on the same side of the double bond is called cis–isomer, whereas the isomer (II) in which the similar atoms or groups lie on the opposite side of the double bond is called the trans–isomer.
Properties of cis-trans isomer
(i) Stability: Cis < Trans
Cis forms are less stable than trans because of the mutual repulsion between the same group e.g.
(ii) Dipole moment: Asymmetrical trans molecule has zero dipole moment, even if trans molecule is not symmetrical than its dipole moment is less than that of the cis isomer.
µ(Sym trans)=0
(unsym trans) µ < cis isomer
(iii) Polarity: Cis > Trans
(iv) Solubility: Soluble in a polar solvent. Cis > Trans
(v) Boiling point: Cis >Trans (due to high polarity)
(vi) Melting point: Cis < Trans. It is due to better packing in the crystal due to symmetry. But the heat of hydrogenation, heat of combination, density refractive index is higher in cis isomer than trans.
(vii) Heating effect: Maleic acid from anhydride at 100oC but fumaric acid forms anhydride at 250oC
E–Z Isomerism
A better system E–Z, is applicable for those type of compounds which cannot be express by cis-trans nomenclature.
E → Entegen (opposite) Z → Zusamann (same)
E–form: When two same priority groups present on the opposite side of the double bonded is known E–form and when the same priority group present on the same side of the double bond is called Z–form.
Priority rule: Chann, Ingold & Prelong proposed a sequence rule:
1: When an atom or group of atoms which are directly attached to the stereogenic centre have a higher atomic number, they will have higher priority. For example
2: When the atomic number will be the same, then higher atomic weight or group of an atom have higher priority
3: When both the atomic number and atomic weight are the same, then priority will be decided by the next joining atom.
4: If multiple bonded group attach to the double bonded carbon, then they are considered in the following manner.
Some E–Z configuration of the following compounds
Syn–Anti Isomerism
When the –OH group and H atom is on the same side of the double of C and N, then it is the syn form otherwise anti-form.
In unsymmetrical Ketoxime, if –OH and the alphabetically alkyl are present on the same side of the double bond, then it is syn form and another isomer is anti-form.
Geometrical Isomerism in Cyclic Compounds
There is a restriction in rotation about the bonds in geometrical isomerism which is also seen in cycloalkanes.
OPTICAL ISOMERISM
Compounds that can rotate plane-polarized light, are called optically active compounds and this phenomenon is called optical activity. Plane polarized light can be obtained by passing ordinary light through Nicol prism. A solution of an Optical active compound can rotate plane-polarized light either left or to the right through the angle α. This property of a substance of rotating plane-polarized light is called optical activity and the substance passing through it, is said to be optically active.
The observed rotation of the plane of polarized light produced by a solution depends upon–
(i) The amounts of the substance in a tube.
(ii) On the strength of the solution examined.
(iii) The temperature of the experiment and the wavelength of the light used.
Chirality: Optical Activity
All these substances are known to exist in three stages.
(i) One rotating the plane of polarised light to the left this form is named laevorotatory.
(ii) One rotating the plane of polarized light exactly to some extent but to the right. This form is named dextrorotatory.
(iii) An inactive form that does not rotate plane-polarized light at all this is a mixture of equal amounts of (+) and (–) forms and hence it is optically inactive. It is named ( ± )–mixture or Racemic mixture. (Latin, Racemic–a mixture of equal compounds).
Polarizer and analyser are parallel. No optically active substance is present. Polarized light can get through the analyser.
- Polarizer and analyser are crossed.
- No optically active substance is present.
- No polarized light can emerge from the analyser.
- The substance between polarizer and analyser is optically active.
- The analyser has been rotated to the left (from observer’s point of view) to permit rotated polarized light through (substance is levorotatory).
- Clockwise direction is also said to be dextrorotatory, and one that rotates plane–polarized light in a counterclockwise direction is said to be levorotatory (Latin: dexter, right, and leavus, left).
Check Part 1 of the chapter here: ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES (PART 1)