NUCLEOPHILIC 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.
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.
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-
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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.