Substitution Reactions
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DEFINITION | ||
| 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. | ||