Main Difference – SN1 vs SN2 Reactions
SN1 and SN2 are two different types of nucleophilic substitution reactions in organic chemistry. But SN1 represents unimolecular reactions, where the reaction rate can be expressed by, rate = K [R-LG]. Unlike SN1, SN2 represents bimolecular reactions, and the rate of reaction can be expressed by, rate = K’ [R-LG] [Nu–]. In addition, SN1 pathway is a multi-step process, and SN2 pathway is a single step process. This is the main difference between SN1 and SN2 reactions.
What is SN1 Reaction
SN1 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 SN1 reaction can be expressed by rate = K [R-LG]. Furthermore, SN1 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 SN1 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 SN1 reaction given at the bottom expresses the variation of energy with reaction coordinates.
In addition, the rate of an SN1 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 SN1 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 SN1 reaction. Similarly, the nature of the leaving group affects the rate of SN1 reaction, because the better leaving, faster the SN1 reaction. But the nature of the nucleophile is unimportant in an SN1 reaction since the nucleophile is not involved in the rate determining step.
What is SN2 Reaction
SN2 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 SN2 reaction.
Here, nucleophile attacks the opposite direction of the leaving group. Thus, SN2 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 SN1 and SN2 reactions.
Difference Between SN1 and SN2 Reactions
SN1 Reaction: SN1 Reaction is unimolecular and a first order reaction. So substrate affects the reaction rate.
SN2 Reaction: SN2 Reaction is bimolecular or a second order reaction. So, both substrate and nucleophile affect the reaction rate.
SN1 Reaction: This is expressed as rate = K [R-LG]
SN2 Reaction: This is expressed as rate = K’ [R-LG] [Nu–]
No. of Steps in the Reaction
SN1 Reaction: SN1 Reaction has only 1 step.
SN2 Reaction: SN2 Reaction has 2 steps.
SN1 Reaction: A stable carbocation forms during the reaction.
SN2 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.
SN1 Reaction: This generally has two intermediate states.
SN2 Reaction: This generally has one intermediate state.
Key Factor of the Reaction/ Big Barrier
SN1 Reaction: Carbocation stability is the key factor of the reaction.
SN2 Reaction: Steric hindrance is the key factor of the reaction.
Reactivity Order based on –R group
SN1 Reaction: IIIry> IIry>> Iry
SN2 Reaction: Iry> IIry>> IIIry
Requirements of Nucleophile to proceed the reaction
SN1 Reaction: Weak or neutral nucleophile is required.
SN2 Reaction: Strong nucleophile is required.
Reaction Favorable Solvents
SN1 Reaction: Polar protic such as alcohol is a favorable solvent.
SN2 Reaction: Polar aprotic such as DMSO and acetone are favorable solvents.
SN1 Reaction: Product can be a racemic mixture because stereochemistry retention or inversion can happen.
SN2 Reaction: Inversion of stereochemistry happens all the time.
“Solvent effects on SN1 and SN2 reactions” by Chem540f09grp12 – Own work (Public Domain) via Commons Wikimedia