Question

In nucleophilic substitution reaction, order of halogens as incoming (attacking) nucleophile is: ${{I}^{-}}>B{{r}^{-}}>C{{l}^{-}}.$
The order of halogens as departing nucleophile should be:
A.$B{{r}^{-}}>{{I}^{-}}>C{{l}^{-}}.$
B.${{I}^{-}}>B{{r}^{-}}>C{{l}^{-}}.$
C.$C{{l}^{-}}>B{{r}^{-}}>{{I}^{-}}.$
D.$C{{l}^{-}}>{{I}^{-}}>B{{r}^{-}}.$

Answer 1

We know that the substitution of the existing nucleophile by the incoming and more reactive nucleophile is known as the nucleophilic substitution reaction. The acetate ion is a fairly good nucleophile. It pushes out the chloride from the compound and acquires its position. The nucleophilic substitution may proceed through the bimolecular or unimolecular pathways.

Complete answer:
We know that alkyl halides are the chain of hydrocarbons in which hydrogen is replaced by halogens. The reactivity of the alkyl halide is decided by the ease with which the halide leaves the substrate. The nucleophiles are the electron-rich species, and have an affinity towards the positive charge. Nucleophilic substitution is a fundamental class of reactions in which a leaving group (nucleophile) is replaced by an electron rich compound (nucleophile). The substitution of one nucleophile by the more reactive nucleophile is the nucleophilic substitution reaction. This process is said to be a one step reaction.
In this transition state, the negative charge is shared by both the incoming acetate nucleophile as well as outgoing chloride ions. The acetate ion has diminished negative charge because it has partly removed a pair of electrons from the carbon. The rate of the reaction depends only on the concentration of reactant (substrate) thus it is first-order kinetics. The reactivity order also depends on solvent. The above mentioned order is in polar protic solvent and if the solvent is polar aprotic then the order gets reversed. The extent of the reaction of the replacement of one nucleophile by the other depends on the leaving group's ability to knock itself out and the reactivity of the incoming nucleophile. The trend of the order of leaving group ability of halogen is: ${{I}^{-}}>B{{r}^{-}}>C{{l}^{-}}>{{F}^{-}}.$
Therefore, the correct answer is option B.

Note:
Remember that, in this reaction bond breaking and making occurs at the same time thus this is also known as the concerted mechanism. The nucleophilic reaction depends on the strength of a nucleophile. The ease at which the nucleophile is displaced or it’s leaving group ability depends on the capacity to accommodate a negative charge. Thus, here bromide leaves the molecule and inverts the configuration.