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General Organic Chemistry
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Electronic Displacements in OrganicMolecules
Inductive effect:
Inductive effect may be defined as the permanent displacement of electrons forming
a covalent bond towards the more electronegative element or group.
The inductive effect is represented by the symbol, the arrow pointing towards the more electronegative
element or group of elements. Thus in case of n-butyl chloride inductive effect may be represented as
below.
Resonance and Resonance (Mesomeric) effect.
Resonance:
The phenomenon in which two or more structures can be written for the true structure of a
molecules, but none of them can be said to represent if uniquely, is referred to as resonacne or
mesomerism. The true structure of the molecule is said to be a resonance hybrid of the various possible
alternative structures which themselves are known as resonating structures or canonical structures. Every
two adjacent resonating structures are represented by inserting a double headed arrow between them.
Thus the actual structure of benzene may be represented in the following two ways.
Electromeric effect:
This type of temporary displacement of electrons take place in compounds
containing multiple covalent bonds (e.g. C=C, C=O, C°N, etc.) or an atom with a lone pair of electrons
adjacent to a covalent bond. The effect involves complete transference of a pair of electrons from a
multiple bond to an atom, or from a multiple bond to another bond, or from an atom with a free pair of
electrons to a bond. it is the p-electrons of a multiple bond, or the p-electrons of an atom, which are
transfered. Since the effect involes complete transference of electrons, it leads to the development of full
+ and - charges within the molecule. It is important to note that the electromeric effect is purely a
temporary effect and is brought into play only the requirement of attacking reagent, it vanishes out as
soon as the attacking reagent is removed from reaction mixture.
Breaking of a Covalent Bond
Homolytic fission or homolysis:
In homolytic bond fission one electron of the bonding pair goes with
each of the departing atom or group resulting in two electrically neutral fragments or atoms generally
known as free radicals. e.g.
Thus a free radical may be definedas the atom or group of atoms having a single, odd or unpaired
electron. Now since the homolytic fission always results in the formation of free radicals, the reactions
involving such (homolytic) fission are known as free radical reactions and are said to proceed via a free
radical mechanism.
Hetereolytic fission or heterolysis:
In this type of fission the electrons pair forming the covalent
bond goes to a single atom or group and thus electricity charged fragments (ions) are formed. Thus the
reaction involving heterolytic fision are known as ionic reaction sand are said to proceed via ionic (polar)
mechanism. The heterolytic fission of the covalent bond can occur in either of the following two ways.
(i) When the electrons pair between C and X leaves the organic group and remains with the departing
substituent X and thus the latter attains a negative charge (due to gain of electrons) while the former
attains a positive charge (due to loss of electrons)
Reaction Intermediates
Free radicals:
A free radical is a species which has as odd or unpaired electron. Free radicals
themselves are electricially neutral, however, due to the presence of odd electron, these are paramagnetic
in nature. Again because of the presence of odd electron, free radicals are in constant search for another
electron to pair up and hence these are highly reactive species. Carbon free radicals are named after the
parent alkyl group and adding the word free radical.
A free radical may have an sp2 hybridized carbon in which odd electron remains in the p orbital the shape
of this type of free radical will be planar. Alternatively, free radical may have sp3 hybridized carbon atom.