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.
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