Isomerism

The compounds having the same molecular formula but different properties are called isomers. The phenomenon of formation of such compounds is called isomerism.

Types of isomerism

1. Structural isomerism
2. Stereoisomerism

Structural isomerism

The compounds having the same molecular formula but different structural formula is called structural isomers. The phenomenon of the formation of such compounds is called structural isomerism. Different types of structural isomerism are given below.

i. Chain isomerism

The compounds having the same molecular formula and same functional group but different chain lengths of carbon are called chain isomers. The phenomenon of the formation of such compounds is called chain isomerism.
examples:

ii. Functional isomerism

The compounds having the same molecular formula but different functional groups are called functional isomers. The phenomenon of the formation of such compounds is called functional isomerism.
Examples:

• Alcohols and ethers are functional isomers (C_nH_2n+2O)

\underset{Ethanol}{CH_{3}CH_{2}OH} \ \ \underset{Methoxy\ methane}{CH_{3}OCH_{3}}

• Aldehydes and ketones are functional isomers (C_nH_2nO)

\underset{Propanal}{CH_{3}CH_{2}CHO} \ \ \underset{Propanone}{CH_{3}COCH_{3}}

• Carboxylic acids and esters are functional isomers(C_nH_2nO₂)

\underset{Ethanoic\ acid}{CH_{3}COOH} \ \ \underset{Methyl\ methanoate}{HCOOCH_{3}}

• Primary secondary and tertiary amines are functional isomers.

(iii) Positional isomerism

The compounds having the same molecular formula, same functional group and same chain length but different positions of the functional group are called positional isomers. The phenomenon of the formation of such compounds is called positional isomerism.
Examples:

(iv) Metamerism

The compounds having the same molecular formula and same functional group but differ in the alkyl group present on either side of the same functional group are called metamers. The phenomenon of the formation of such compounds is called metamerism.
Examples:

v. Tautomerism

The isomers which are interconvertible to each other and establish an equilibrium state between them are called tautomers and the phenomenon of formation of such compounds is called tautomerism.
Example: keto-enol tautomerism

Acetone exists in keto-enol form

Similarly, acetaldehyde and amide also exist in keto-enol form.

Stereoisomerism

The compounds having the same molecular and structural formula but different spatial arrangements of atoms or groups of atoms in a molecule are called stereoisomers and the phenomenon of formation of such compounds is called stereoisomerism. They are also called configurational isomers.

Types of stereoisomerism

• Geometrical isomerism
• Optical isomerism

Geometrical isomerism

The isomerism caused by the different geometrical arrangement of two different groups about a carbon-carbon double bond is called a geometrical isomer. This isomerism is also called cis-trans isomerism.
The geometrical isomer in which similar groups are attached to the same side of the double bond is called a cis isomer. The geometrical isomer in which similar groups are attached to the different sides of the double bond is called a trans isomer.

Optical isomerism

The compounds which have the same molecular formula and structural formula but differ in configuration and activity towards plane-polarized light are called optical isomerism. The stereoisomerism which arises due to molecular chirality is called optical isomerism.
An optically active compound can exist in two isomeric forms:

a. (+) or d-isomer: Those isomers which rotate the plane-polarized light in the clockwise direction are called dextrorotatory (d) isomers.
b. (-) or l-isomer: Those isomers which rotate the plane-polarized light in an anticlockwise direction are called laevorotatory (l) isomers.

Preliminary idea of the reaction mechanism

Concept of cleavage of covalent bonds

During chemical reactions, chemical bonds in reactants are broken and new chemical bonds are formed in the product. Covalent bond undergoes cleavage in two different ways:

i. Homolytic cleavage (fission): If a covalent bond is cleaved by an equal distribution of bonded electrons, the process is called homolytic fission. It takes place when bonding atoms have equal electronegativity. It generally takes place in the presence of heat or sunlight. It leads to the formation of free radicals.

Cl-Cl\ \ \rightarrow \underset {Chlorine\ free\ radicals}{Cl^{\cdot} + Cl^{\cdot}}\\ CH_{3}-CH_{3}\ \ \rightarrow \underset {Methyl\ free\ radicals}{CH_{3}^{\cdot} + CH_{3}^{\cdot}}

ii. Heterolytic cleavage (fission): If a covalent bond is cleaved by unequal distribution of bonded electrons, the process is called heterolytic cleavage. It takes place when bonded atoms have different electronegativity. It generally occurs in polar solvents. It leads to the formation of cations and anions.

CH_{3}-Cl \rightarrow \underset {cation}{CH_{3}^{+}}\ + \ \underset {anion}{Cl^{-}}

Reaction intermediates

i. Carbocation: An organic species having carbon with a positive charge is called carbocation. eg. CH₃⁺(methyl carbocation), CH₃CH₂⁺(ethyl carbocation), etc.

ii. Carbanion: An organic species having carbon with a negative charge is called carbanion. eg. CH₃^– (methyl carboanion), CH₃CH₂^–(ethyl carboanion), etc.

iii. Free radical: An atom or group of atoms having odd or unpaired electrons is called a free radical. eg. CH₃^. (methyl free radical), Br^. (bromine free radical), etc.

Types of reagent

i. Nucleophile (nucleus loving): Atoms or groups of atoms that are rich in electrons and attack the electron-deficient centre are called nucleophiles. eg. OH^–, Cl^–, CN^–, NH₃, H₂O, CH₃^–, etc.

ii. Electrophile (electron loving): Atoms or groups of atoms that are deficient in electrons and attack the electron-rich centre are called electrophiles. eg. H⁺, CH3⁺, Cl⁺, NO₂⁺, SO₃, CO₂, etc.

Resonance

The phenomenon in which a molecule or ion cannot be represented by a single structure but represented by more than one structure to explain its properties is called resonance. More than one such structure of the same molecule or ion is called a resonating structure. The real structure of the molecule or ion is a hybrid of these structures called a resonance hybrid. It is represented by a double-headed arrow (⬌).
Atom or groups of atoms which donate electron shows a positive resonance effect (+R) and those which withdraw electron shows a negative resonance effect (-R).

Inductive effect

The permanent shifting of bonded electrons towards a more electronegative atom and forming a polarity is called the inductive effect. Formation of polarity means the formation of partial positive and partial negative charges.

i. Positive inductive effect (+I effect): Atom or group of atoms which loses an electron. eg. -CH₃, -CH₂CH₃, etc.

ii. Negative inductive effect (-I effect): Atom or group of atoms that gains an electron. eg. -NO₂, -CN, -COOH, -OH, -OR, -F, -Cl, -Br, -I, etc.

Some Important Questions

1. Write all possible isomers of a. C₃H₈O b. C₃H₆O c. C₃H₆O₂ with their IUPAC names.
2. Write the metamers of C₅H₁₀O with the IUPAC name.
3. Write an example of a cis-trans isomer.
4. What do you mean by optical isomerism?
5. Define homolytic and heterolytic cleavage.
6. Write any three examples of nucleophile and electrophile.