group, the presence of bulky substituents on or near the carbon atom have a dramatic inhibiting effect. Of the simple alkyl halides, methyl halides react most rapidly in S N reactions because there are only three small hydrogen atoms. Tertiary halides are the least reactive because bulky groups hinder the approaching Configuration Spacial arrangement of functional groups around carbon is called its configuration. See the structures (A) and (B) given below carefully.
These are the two structures of the same compound. They differ in spacial arrangement of functional groups attached to carbon. Structure (A) is mirror image of Structure (B). We say configuration of carbon in structure (A) is mirror image of the configuration of carbon in structure (B).
Haloalkanes and Haloarenes nucleophiles. Thus the order of reactivity followed is: Primary halide > Secondary halide > Tertiary halide. (b) Substitution nucleophilic unimolecular (S N ) S N reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.). The reaction between tert - butyl bromide and hydroxide ion yields tert -butyl alcohol and follows the first order kinetics, i.e., the rate of reaction depends upon the concentration of only one reactant, which is tert - butyl bromide.
It occurs in two steps. In step I, the polarised C—Br bond undergoes slow cleavage to produce a carbocation and a bromide ion. The carbocation thus formed is then attacked by nucleophile in step II to complete the substitution reaction. Fig.
. : Steric effects in S N reaction. The relative rate of S N reaction is given in parenthesis Step I is the slowest and reversible. It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent.
Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion. Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. In