Question

$C{H_3}Br + N{u^ - }\xrightarrow{{}}C{H_3} - Nu + B{r^ - }$
The decreasing order of the rate of the above reaction with the following nucleophiles ( $N{u^ - }$ ) A to D is:
A) $Ph{O^ - }$ B) $Ac{O^ - }$ C) $H{O^ - }$ D) $C{H_3}C{H_2}{O^ - }$
(A) $D > C > A > B$
(B) $D > C > B > A$
(C) $A > B > C > D$
(D) $B > D > C > A$

Answer 1

Hint:A reagent that brings an electron pair is known as a nucleophile . It is a nucleus loving species. During a polar organic reaction, a nucleophile attacks an electrophilic centre of the reacting substrate which is that particular atom or part of the electrophile that is electron deficient or has low electron density. The tendency to attack electrophile determines the strength of the nucleophile.

Complete step-by-step answer:
The strength of a nucleophile depends on the electron density on the electron donating atom. In the given options, Oxygen is the atom which has lone pairs of electrons to donate. The nucleophile which has more electron density on oxygen atom will have more reactivity as nucleophlile.
The option (D), $C{H_3}C{H_2}{O^ - }$ has electron donating ethyl group attached to oxygen atom which will help in increasing the electron density on oxygen.

The option (C), $H{O^ - }$ don’t have electron donating group as $C{H_3}C{H_2}{O^ - }$, so it will have less electron density on O atom than $C{H_3}C{H_2}{O^ - }$.

The option (B) $Ac{O^ - }$ or $CH_3COO^-$, has resonance structure through which negative charge will be distributed on two O atoms. So, it will have lower electron density on the O atom than $C{H_3}C{H_2}{O^ - }$ and $H{O^ - }$.

The option (A) $Ph{O^ - }$, also has resonance structure due to phenyl ring and electron density is less on O atom due to this resonance. However, $Ph{O^ - }$ is more stabilised than $Ac{O^ - }$. So, it is a better nucleophile than $Ac{O^ - }$.

Therefore, the decreasing order of the rate of the given reaction with the given nucleophiles is $D > C > A > B$.

Option ‘A’ is correct

Note: In neutral molecules such as alkyl halides, due to the polarity of the C-X,
the bond becomes partially positively on the carbon atom and hence the carbon atom becomes an electrophilic centre at which a nucleophile can attack.