How to determine the nucleophilic substitution mechanism as SN1 SN2 reaction from the reaction condi

First, under alkaline conditions, the competition of SN2 and E2 reactions generally occurs, rather than SN2 and SN1. Because in the mechanism of producing carbocations (such as the SN1 reaction), carbocations act as Lewis acids.

It will react with the base in the reaction system. In the same way, under acidic conditions, SN1 and E1 reactions are more likely to occur. When writing the mechanism, it is important to consider the relationship between the reaction intermediate and the reaction environment, if the reaction is carried out in a strong alkaline system, then the mechanism will not contain acidic intermediates, and vice versa.

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This book has a detailed explanation of the mechanism of this type of reaction, and those who are interested can refer it to themselves. <>

SN1 is a nucleophilic substitution reaction in which only reactants (independent of nucleophile concentration) are involved in the reaction rate control step.

SN2 is a nucleophile and a nucleophile both participate in the nucleophilic substitution reaction of the reaction rate control step, the difference between SN1 and SN2 is whether the nucleophile is involved in E1 is that the reactant ionizes first, L breaks down, and at the same time generates a positive carbon ion, and then loses the hydrogen atom and generates bonds. The reaction is carried out in two steps, and only the reactant molecules participate in the ionization step that determines the rate. Therefore, the rate of E1 is directly proportional to the concentration of the reactant and has nothing to do with the concentration of the base.

E2 is a one-step reaction completed, the cleavage of L, the neutralization of hydrogen atoms and bases, and the formation of bonds are carried out in synergy, and the reactants and bases participate in the reaction at the same time. The rate of E2 is proportional to both the reactant concentration and the base concentration. The e1 <> can be distinguished by the change in the key

The reaction mechanism of SN1 and SN2 is as follows:

SN1: The reaction mechanism of SN1 is that the reactants first dissociate into carbocations and negatively charged departing groups, and after the molecules are dissociated, the carbocations immediately interact with nucleophiles.

Combine. SN2: The reaction mechanism of SN2 is the hybridization of aliphatic SP3.

The carbon atom is attached to the leaving group, and the nucleophile attacks the carbon atom directly behind the leaving group to form a new compound.

Quality. 1. SN1 leads to racemization of the product; This is because OH can attack from both sides of the carbocation and form two compounds with opposite configurations. However, most of the racemization products are not 1:

1 generation, which can be explained from the winstein ion-pairing mechanism.

2. SN1 will lead to the rearrangement of products. This is because carbocations have undergone a rearrangement towards more stable carbocations.

It should be noted that SN1 and SN2 are only two of the more extreme mechanisms of nucleophilic substitution reactions, which are ideal. Under the influence of factors such as ion pairings, the actual reactions will have more or less components of these two mechanisms, and some are even difficult to classify.

Due to the formation of carbocation intermediates in the SN1 reaction, rearrangement is an important feature of this reaction, and it is also an important experimental basis to support the SN1 mechanism. If there is a rearrangement in a nucleophilic substitution reaction, then the substitution is generally SN1 mechanism. In the SN2 reaction, no carbocation intermediates are formed, and no intermediates are formed, so no rearrangement occurs.

The Walden transformation is a stereochemical feature of the SN2 reaction, and the reason for this characteristic is the backside attack of the nucleophile. If the nucleophile is an isotope.

For example, with optical rotation.

2 iodo-methane.

In the case of pathway exchange reaction with radioactive iodine anion, the speed of racemization is twice that of isotope exchange during the reaction.

It is generally believed that SN1 and SN2 are the two limit mechanisms of nucleophilic substitution reactions, and there is a region of SN1 and SN2 mixing mechanism with different degrees between these two limit mechanisms. The ion-pairing mechanism can unify SN1 and SN2 and other conditions, and is a relatively complete affinity substitution mechanism.

SN1 is a single-molecule nucleophilic substitution reaction of Woodmine; SN2 is a bimolecular nucleophilic substitution reaction.

SN1 is divided into 2 steps, first the carbonaceous bond is broken to form a carbocation, and then the nucleophile.

Attack carbocation formation product (Because it can be attacked from 2 directions, it is possible to form the enantiomer of Zen and Ichiga orange punch.

product and the total reaction rate.

It is only proportional to the concentration of the reactant and not to the concentration of the reagent).

SN2 is a synergistic process that occurs simultaneously with the breaking of old bonds and the formation of new bonds. The reaction rate is proportional to both the reactant concentration and the reagent concentration. The product of configuration flipping is generally generated. See:

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The SN1 reaction mechanism is carried out in steps, the reactants are first dissociated into carbocations and negatively charged departing groups, this process requires energy, and it is a step to control the reaction rate, that is, a slow step. When the molecule is dissociated, the carbocation immediately binds to the nucleophile at an extremely fast rate, which is a fast step.

Therefore, the carbocation stability determines the activity of the SN1 reaction, and the reactivity a>c>d>b.

In the SN2 reaction, the nucleophile carries a pair of lone pairs of electrons to attack the electrophilic-deficient central atom, forming a transition state and leaving the group.

No carbocations are formed in the reaction, and the rate control step is the synergistic step mentioned above, and the reaction rate is proportional to the concentration of the two substances, so it is called a bimolecular nucleophilic substitution reaction. In inorganic chemistry, the antimorphic amulate mechanism of bimolecular nucleophilic substitution reaction is often called "exchange theory".

Therefore, the SN2 reactivity is determined by the size of the attacking group, i.e., steric hindrance.

By judging the carbocation intermediates, the nucleophilic substitution mechanism was judged to be SN1 and SN2 reactions. There is an over-elimination elevation of carbocation intermediates in the SN1 reaction, so the more stable the carbocation formed (how to judge the stability will not be repeated or bridged), the more conducive to the SN1 reaction.

Nucleophilic substitution: Nucleophilic substitution reaction is one of the basic reactions of organic chemical reactions, which is mainly divided into two processes: SN1 and SN2. Then the essence of SN2 is a bimolecular nucleophilic substitution reaction, but many students will confuse these two processes, because the SN1 reaction is also a reaction of two molecules.