Thursday, January 17, 2008

Ch.20 Inductive and Resonance Effects - Core Points for Review


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;

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. Due to the inductive effect, which is a permanent effect in the molecule, in case of methyl chloride, methyl group has slight positive charge

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.

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.


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.

Bond fission.

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.

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.

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.

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.

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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