Question

Select the mechanisms where the concentration of the nucleophile/base has no effect on the reaction rate

Answer 1

Reactions in which the concentration of Nucleophile or Base has no effect on the rate of reaction depends only on the concentration of substrate. Such reactions are called Unimolecular Substitution or Elimination Reactions. The Kinetics of such reaction is First order with respect to substrate.

Examples:

SN¹ Reactions:
                        These are Unimolecular Substitution reactions. These are two step reactions in which the first step is the rate determining step. In RDS the substrate is ionized to generate a carbocation and is the slowest step. In second step the Nu attacks carbocation and is the fastest step.

E¹ Reactions:
                     These are unimolecular Elimination reactions. These are also two step reaction. In first step carbocation is generated (RDS step) and in second step the base abstracts proton to yield alkene. These reactions only depend on the concentration of substrate.

Answer 2

Further explanation

SN1 stands for nucleophilic substitution and "1" means that the rate determining step is a single molecular reaction. This reaction involves a carboxylation agent and generally occurs in the reaction of secondary or tertiary alkyl halides, or in the presence of strong acids, secondary and tertiary alcohols. With primary alkyl halides, alternative SN2 reactions occur. In inorganic chemistry, SN1 is referred to as a dissociative mechanism. This reaction mechanism was first proposed by Christopher Ingold, et al. in 1940

The SN2 reaction is a type of nucleophilic substitution reaction mechanism in organic chemistry. In this mechanism, one of the bonds is broken and the other one is formed simultaneously, in other words, in one reaction stage. Because the two reacting species are involved in a slow step (the rate determining step), this leads to the name nucleophilic (bi-molecular) or SN2 substitution, the other major type of mechanism being SN1.

This type of reaction is very common and therefore has various names, such as "bimolecular nucleophilic substitution", or, among organic chemists, "associative substitution" or "exchange mechanism"

Unlike the SN2 reaction, the SN1 reaction goes through two stages (it does not include protonation or deprotonation). The determining stage of the reaction rate is in the first stage, therefore the reaction rate of the whole reaction is generally the same as the rate of carbocation formation and does not involve the concentration of nucleophiles. Therefore, nucleophilicity is not a factor in the rate of reaction and the overall rate of reaction depends only on the concentration of the reagent.

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Details

Class: college

Subject: biology

Keywords : SN1 reaction, SN2 reaction