Difference Between SN1 and SN2 Reactions

Main Difference – S_N1 vs S_N2 Reactions

S_N1 and S_N2 are two different types of nucleophilic substitution reactions in organic chemistry. But S_N1 represents unimolecular reactions, where the reaction rate can be expressed by, rate = K [R-LG]. Unlike S_N1, S_N2 represents bimolecular reactions, and the rate of reaction can be expressed by, rate = K’ [R-LG] [Nu^–]. In addition, S_N1 pathway is a multi-step process, and S_N2 pathway is a single step process. This is the main difference between S_N1 and S_N2 reactions.

What is S_N1 Reaction

S_N1 indicates the unimolecular nucleophilic substitution reactions in organic chemistry. Their rate determining step of the mechanism depends on the decomposition of a single molecular species. So that, the rate of an S_N1 reaction can be expressed by rate = K [R-LG].  Furthermore, S_N1 is a multi-step reaction, which forms an intermediate and several transition states during the reaction. This intermediate is a more stable carbocation, and the reactivity of the molecule depends on the R- group. The following figure illustrates the mechanism of an S_N1 reaction.

At the first step, loss of the leaving group (LG) forms a more stable carbocation. This is the slowest step or the rate determining step of the mechanism. Subsequently, nucleophile attacks rapidly on the electrophilic carbon to form a new bond. The energy profile diagram of S_N1 reaction given at the bottom expresses the variation of energy with reaction coordinates.

In addition, the rate of an S_N1 reaction depends on the alkyl side chain attach with the leaving group. The reactivity of R- groups can be ordered as follows.

Reactivity order: (CH₃)₃C-  >  (CH₃)₂CH-   >  CH₃CH₂–  >  CH₃–

In an S_N1 reaction, the rate determining step is the loss of the leaving group to form the intermediate carbocation. Among primary, secondary and tertiary, the tertiary carbocation is very stable and easier to form. Therefore, compounds with a tertiary R group enhance the rate of S_N1 reaction. Similarly, the nature of the leaving group affects the rate of S_N1 reaction, because the better leaving, faster the S_N1 reaction. But the nature of the nucleophile is unimportant in an S_N1 reaction since the nucleophile is not involved in the rate determining step.

What is S_N2 Reaction

S_N2 indicates the bimolecular nucleophilic substitution reactions in organic chemistry. In this mechanism, separation of leaving group and formation of new bond happen synchronously. Therefore, two molecular species involve with the rate determining step, and this leads to the term bimolecular nucleophilic substitution reaction or SN2. The rate of the SN2 reaction can be expressed by rate = K [R-LG] [Nu^–]. In inorganic chemistry, this reaction also called “associative substitution” or “interchange mechanism.” The following figure illustrates the mechanism of S_N2 reaction.

Here, nucleophile attacks the opposite direction of the leaving group. Thus, S_N2 reaction always leads to an inversion of stereochemistry. This reaction works best with methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile.  In addition, the stability of the leaving group as an anion and the strength of its bond to the carbon atom both affect the rate of reaction.

The following figures illustrate the energy profile diagram of S_N1 and S_N2 reactions.

Difference Between S_N1 and S_N2 Reactions

Rate Law

S_N1 Reaction: S_N1 Reaction is unimolecular and a first order reaction. So substrate affects the reaction rate.

S_N2 Reaction: S_N2 Reaction is bimolecular or a second order reaction. So, both substrate and nucleophile affect the reaction rate.

Rate Expression

S_N1 Reaction: This is expressed as rate = K [R-LG]

S_N2 Reaction: This is expressed as rate = K’ [R-LG] [Nu^–]

No. of Steps in the Reaction

S_N1 Reaction: S_N1 Reaction has only 1 step.

S_N2 Reaction: S_N2 Reaction has 2 steps.

Carbocation Formation

S_N1 Reaction: A stable carbocation forms during the reaction.

S_N2 Reaction: A carbocation does not form during the reaction because the separation of leaving group and formation of new bond happen at the same time.

Intermediate States

S_N1 Reaction: This generally has two intermediate states.

S_N2 Reaction: This generally has one intermediate state.

Key Factor of the Reaction/ Big Barrier

S_N1 Reaction: Carbocation stability is the key factor of the reaction.

S_N2 Reaction: Steric hindrance is the key factor of the reaction.

Reactivity Order based on –R group

S_N1 Reaction: III^ry> II^ry>> I^ry

S_N2 Reaction: I^ry> II^ry>> III^ry

Requirements of Nucleophile to proceed the reaction

S_N1 Reaction: Weak or neutral nucleophile is required.

S_N2 Reaction: Strong nucleophile is required.

Reaction Favorable Solvents

S_N1 Reaction: Polar protic such as alcohol is a favorable solvent.

S_N2 Reaction: Polar aprotic such as DMSO and acetone are favorable solvents.

Stereochemistry

S_N1 Reaction: Product can be a racemic mixture because stereochemistry retention or inversion can happen.

S_N2 Reaction: Inversion of stereochemistry happens all the time.

Image Courtesy:

“Solvent effects on SN1 and SN2 reactions” by Chem540f09grp12 – Own work (Public Domain) via Commons Wikimedia