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Chemistry 11 Notes for Chemistry Notes

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Substitution Reactions

Nucleophilic substitution reactions
Definition

A reaction in which a nucleophile displaces another nucleophile
and takes its position is called a nucleophilic substitution reaction.
Nucleophilic substitution reaction are represented by the symbol “SN“.

Explanation
In alkyl halides, the C-X bond is polar because C-atom is attached to a highly electro-negative halogen atom. Therefore, in alkyl halides carbon atom is electrophilic and halogen has a nucleophilic character.
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For example in iso propyl bromide, C-atom has partial positive charge and Br-atom has partial negative charge. The C-atom has capability to accept a pair of electrons to form a new bond. In doing so the C-Br bond breakes and Br– leaves the molecules. The net result is that one nucleophile displaces another nucleophile. Such displacement reactions are called nucleophilic substitution reactions.
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Following examples illustrate the nature of “SN” reactions:
OH– + CH3+-I– à OH – CH3 + I–
In this example hydoxyl ion is an attacking nucleophile while iodide-ion is called leaving group.
CH3-CH2-I + OH– àCH3-CH2-OH +I–
CH3-I + –OCH3 àCH3-O-CH3 +I–
CH3-Br + CH3COO– àCH3COOCH3 + Br–

General expression
A nucleophilic substitution reaction (SN).
Nu– + Rd+-Xd- à Nu-R + X–
In general it can be written as:
Alkyl halide + nucleophile à new compound + halide ion

Mechanism of SN reactions
There are two classes of SN-reactions:

  • SN1-reactions
  • SN2-reactions

SN1-reactions
Nucleophilic substitution reactions which take place in two steps are referred to as “SN1-reactions.
Mechanism
SN1 mechanism occurs in two steps. In the first step, the substrate ionizes slowly and reversibly to produce carbonium ion.
R-X àR+ + X–
In the second step, nucleophile reacts with carbonium ion to produce a new compound.
R+ + Nu– à R-Nu
Rate of SN1-Reactions
The rate of an SN1 reaction depends upon the concentration of substrate (R-X) only and not on the nucleophile.i.e.
Rate = K[R-X]
This means that it is a unimolecular reaction. Therefore, SN1 is a unimolecular nucleophilic substitution reaction.

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