Saturday, October 20, 2007

Study Guide TMH JEE Ch.20 Inductive and Resonance Effects and Other Phenomena

JEE syllabus

Resonance and hyperconjugation;
Inductive and resonance effects on acidity and basicity of organic acids and bases; Polarity and inductive effects in alkyl halides;
Reactive intermediates produced during homolytic and heterolytic bond cleavage; Formation, structure and stability of carbocations, carbanions and free radicals.
Keto-enol tautomerism;
Determination of empirical and molecular formula of simple compounds (only combustion method);
Hydrogen bonds: definition and their effects on physical properties of alcohols and carboxylic acids;


Main Topics Covered in the TMH Book


These effects occur during bonding and in molecules

Inductive Effect

This effect arises when an electron withdrawing group (such as halogen) is attached to the end of a carbon atom.

i) Atoms or groups of atoms hvaing electron attracting capacity more than hydrogen are referred to as having -I (electron attracting) effect. The increasing tendency is


ii) Atoms or groups of atoms having smaller electron power than hydrogen are referred to as havng +I (electron repelling effect)


Due to the inductive effect, which is a permanent effect in the molecule, in case of methyl chloride, methyl group has slight positive charge and this provides a site for attack by a negatively charged species say (OH-)

2. Electromeric effect

It is a temporary effect which takes between two atoms joined by a multiple bond i.e., a double or triple bond. This occurs at the requirements of the attacking reagent and involves simulataneous transfer of a shared pair of electrons of the doube or triple bond to one of the linked atoms.

3. Resonance Effect or Mesomeric Effect;

There are many molecules whose behaviour cannot be explained by a single Lewis structure. To explain behavior of such molecules, two or more than two structures are proposed and the molecule is thought to be a resonance hybrid of those structures.

Examples of Molecules for which resonance structures were proposed:


The actual carbon-oxygen bond CO2 has been found to be 115 pm whereas the normal carbon-oxygen double bond is 122 pm and that for triple bond is 110 pm. This means carbon-oxygen bond in carbon dioxide is intermediate between a double and a triple bond. To account for this total three resonance structures were proposed

O=C=O <--> Oˉ-C≡O^+ <--> O^+≡C-Oˉ


When a H-C bond is attached to a double bond or triple bond, the sigma electrons of the H-C bond interact with the double bond or triple bond system.

the interactions between the electrons of pi systems of multiple bonds and the adjacent sigma bonds (Single H-C bonds) of the substituent groups in organic compound is called hyperconjugation. The concept was developed by nBaker and Nathan and is also known as Baker and Nathan effect.

Reactive intermediates produced during homolytic and heterolytic bond cleavage;

Organic reactins usually involve making and breaking of covalent bonds. The fission of bonds can take place in two ways.

Bond breaking is also known as bond fission.

1. Homolytic fission
2. Heterolytic fission
Heterolytic fission results in the formation of two different chemical species in the sense that one is a cation and the other an anion. Homolytic fission results in two electrically uncharged radicals.

Radicals have an unpaired electron...
Radicals are particles that have an unpaired electron. They may be single atoms (e.g. chlorine radical, Cl.) or groups of covalently bonded atoms (e.g. methyl radical,.CH3).

Some radicals, called biradicals, have two unpaired electrons, for example, .O. (1s22s22p4) and .O2..

Because of the unpaired electron, radicals can be very reactive. However, there are some that are relatively stable and behave somewhat like ordinary molecules. An example is nitrogen monoxide.

Heterolytic fission versus Homolytic fission...

The hydrogen chloride molecule (H-Cl) is polar owing to the greater electronegativity of the chlorine atom. Heterolytic fission is more common where a chemical bond is already polar. Hydrogen chloride is highly soluble in water and becomes fully ionised; it is a strong acid. Solvents with polar molecules favour heterolytic fission.

Homolyic fission is favoured by non-polar solvents, or by gaseous conditions, and the presence of visible or ultraviolet light.

Reaction Intermediates

The species produced during cleavage of bonds are called reaction intermediates. The important ones are:

1. Free radical: A free radical is an atom or group of atoms having an unpaired electron. Thee are produced during the homolytic fission of a covalent bond. These are very reactive. The free radicals are classified as primary, secondary or tertiary depending upon whether one, two or three carbon atoms are attached to the carbon atom carrying the odd electron.

The order of stability of alkyl free radicals is


2. carbocation: It is a group of atoms which contain positively charged carbon having only six electrons. It is obtained by heterolytic fission of covalent bond involving carbon atoms.

Relative stabilty: The methyl group has +I inductive effect. So the alkyl group (as a specific example methyl group)attached to +vely charged carbon (carbocation) tends to releaase electrons towards carbon. As a result, it decreases +charge on the carbocation. Due to this inductive effect, the positive charge on the carbocation gets reduced and dispersed (Dispersion is distribution of charge over other atoms in the molecule). The dispersal of charge results into stability. Therefore, more the number of alkyl groups, the greater will be the dispersal of charge and therefore, more stable will be the carbocation.

So the order of stabilty is


3. Carbanion: It is a species containing a carbon atom carrying a negative charge. They are generated during heterolytic fission of covalent bonds containing carbon, when an atom linked to carbon goes without the bonding electrons.

Carbanions are very reactive species.

They are classified as primary, secondary and tertiary.

The order of stability is reverse of that of carbocations and free radicals.

CH3‾ >1°>2°>3°

4. carbene: The carbenes are reactive neutral species in which the carbon atom has six electrons in the valence shell out of which two are shared. The simplest carbene is methylene (:CH2). It is formed wbehg diazomethan is decomposed by the action of light.

CH2N2 --> :CH2 + N2

Types of attacing reagents

1. Free radicals
2. Electrophiles
3. Nucleophiles

Typesof organic reactions

1. substitution reactions
2. Addition reactions
3. Elimination reactions
--i) α-Elimination
--ii) β-Elimination
4. Rearrangement reactions
5. Condensation reactions
6. Isomerism reactions

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