Difference Between SN1 and SN2

SN1 and SN2 are the two forms of nucleophilic substitution reaction. SN1 involves one molecule while Sn2 involves two molecules.

In order to understand SN1andSN2, it is important to know what a nucleophilic substitution reaction is? Only after one gets to understand all the terminologies pertaining to the nucleophilic substitution reaction, understanding the difference between SN1and SN2 becomes easier. Here in this article, we will discuss nucleophiles, substitution reactions, nucleophilic substitution reactions and the difference between Sn1 and Sn2. 

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What is Nucleophile?

A nucleophile is an atom or molecule that has an electron pair(s) to donate. In other words, it has extra unused electrons that are making it negatively charged. There are two types of nucleophile:

1. Neutral - Molecules that have a lone pair(s) of electrons but have an overall neutral charge are called neutral nucleophiles.

Example- We can take an example of NH3. The octet of this molecule is satisfied. It has an overall neutral charge. But the Nitrogen atom, in itself, is negatively charged as it has a lone pair of electrons. So, regardless of the molecule’s overall charge, the N atom will still get attracted towards the electron-deficient part of a molecule or an electron-deficient atom.

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2. Anions - Chemical species that have negative charges are known as anions. Example - hydroxide ion or OH- .

Substitution Reaction

Any chemical reaction where an atom or the functional group of a molecule is substitutedby another atom or functional group. Substitution reactions can be nucleophilic or electrophilic. In this article, we will focus on nucleophilic substitution reactions..

Nucleophilic Substitution Reaction

Any substitution reaction that involves replacing an atom or a functional group by a negatively charged ion or by an atom or functional group that has a lone pair of electrons. The negatively charged ion or the atoms/molecules having lone pairs of electrons will get attracted towards the positively charged area of an atom or compound and thus it will try to replace the functional group or atom already attached to the positive area.

Examples: Bromomethane. Its chemical formula is CH3Br. The alkyl CH3 is positive, while Bromine is negative. Now if it reacts with the Cyanide anion, the negatively charged Br will get replaced by CN-. The reaction is given below.

CH3Br + CN- → CH3CN + Br- 

Again, if you look at chloromethane orCH3Cl. Here, CH3 is positive and Cl is negative. If the CH3Cl reacts with the hydroxide ion the Cl gets replaced by the negatively charged hydroxide ion. The reaction would look like this -

CH3Cl + OH- → CH3OH + Cl-

It is happening in each of the two reactions that one negatively charged atom/molecule is getting replaced by another negatively charged atom/molecule.

Types of Nucleophilic Substitution Reaction

There are two types of nucleophilic substitution reaction:

• SN1 Reaction

• SN2 Reaction

Basically SN1 involves unimolecular reaction and SN2 involves bimolecular reaction. Let’s understand the differences between them.

Difference Between SN1 and SN2:

The difference between SN1 and SN2 reactions are given below.

Now let’s analyse the differences carefully.

→ Look at the example of Bromomethane given above. From the alkyl group the Br gets separated and the CN- ion replaces Br. However there are two steps involved -

At first, the Br gets separated - 

CH3Br → CH3 + BR

This is known as the slow step or the rate-limiting step. As the Br ion gets separated, the CN- ion around gets attracted to CH3 and it attacks CH3 to form CH3CN. Furthermore, the first step is considered the main step in SN1. Since the first step involves only one kind of molecule, it is a unimolecular reaction.

So you see that it is a two-step process where the first one involves the formation of a cation.

In the SN2 reaction as the Br tries to leave the CH3Br compound, the OH anion simultaneously attacks the CH3Br. This results in a partially attached OH- and a partially detached Br- and it is a transition stage. Ultimately the process finishes with the complete separation of Br and the complete attachment of OH anion. 

Special Note

Did you notice that we used the example of CH3Br in both examples? It was for teaching purposes only. In reality, in SN1 CH3Br forms methyl carbocation. This is unstable and cannot result in stage 2 of SN1 . So the CH3Br reaction happens in the form of SN2 substitution and not SN1.

Conclusion or Summary of SN1 and SN2

Therefore we can conclude it as : SN1 and SN2 are two kinds of nucleophilic substitution reaction

• A nucleophile is an electron-rich atom or molecule. Sometimes it is in the form anion and sometimes it is in the form of a compound or atom that has at least one lone pair of electrons.

• The opposite of a nucleophile is an electrophile. An electrophile is a positively charged chemical species. The nucleophile attacks the positively charged area of a compound or atom.

• A nucleophilic substitution reaction is a reaction that involves the replacement of one functional group or atom with another negatively charged functional group or atom.

• SN1 is a unimolecular reaction while SN2 is a bimolecular reaction.

• SN1 involves two steps. SN2  involves one step.

• In SN1, there is a stage where carbocation forms. The anion or the negatively charged atoms or compounds then gets attracted to the carbocation. In SN2 , there is only a transition stage and no formation of intermediates.

• Just like in real life, there is power-play in Chemistry too. There is the fight for existence. What we see in this chapter is that the weaker nucleophile gets displaced by a stronger nucleophile.