Difference Between Sn1 and Sn2

Definition 

Sn1 and Sn2 are the two forms of nucleophilic substitution reaction. SN1 involves one molecule while Sn2 involves two molecules.

To understand SN1 and SN2, it is imperative 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 Sn1 and Sn2 becomes easier. Here we discuss nucleophiles, substitution reactions, nucleophilic substitution reactions and difference between Sn1 and Sn2. Read on...

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:

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

Take NH3 for example. Yes, its octet 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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  • Anions - Chemical species that have negative charge are known as anions. Example - hydroxide ion or OH negative.


Special Note

While reading about this topic in your books or on the internet, you will come across the term - Chemical Species. While chemical species has its own definition, here it is used in a generic manner. So don’t fret over it.


What is Substitution Reaction?

Any chemical reaction where an atom or the functional group of a molecule is SUBSTITUTED by another atom or functional group. Substitution reactions can be nucleophilic or electrophilic. We will limit ourselves to nucleophilic substitutions.

Nucleophilic Substitution Reaction

Any substitution reaction that involves replacing of 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 pair 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:

Let us look at 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 will look like this -

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-

What is happening in each of the two reactions is - 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

  • Sn2

Basically Sn1 involves unimolecular reaction and Sn2 involves bimolecular reaction. Let us delve deeper and understand the differences between them.

Difference Between Sn1 and Sn2:

    Sn1

          Sn2

Sn1 is a unimolecular reaction

Sn2 is a bimolecular reaction

It follows a 1st order kinetic mechanism.

It follows the 2nd order Kinetic mechanism.

Sn1 involves two steps

Sn2 is a single-step process

In Sn1, the rate of reaction depends on the concentration of the substrate.


In Sn2, the rate of reaction depends on the concentration of both the substrate and the nucleophile.

In Sn1 as the leaving group leaves, the substrate forms a carbocation intermediate. 

In Sn2, the reaction happens in a single transition state.

Optically active substrate becomes optically inactive and half of the optically active substrate becomes similar.

Sn2 involves inversion reaction.



Now let us 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 negative 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.


Therefore, to Recap -

  • 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 nucleophile is an electrophile. An electrophile is the 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. Isn’t this similarity in line with nature’s law of survival of the fittest?

FAQ (Frequently Asked Questions)

1. What is a Nucleophilic Substitution Reaction?

Ans. The reaction where the negatively charged atoms or molecules in a compound get replaced by another negatively charged atom or molecule is called the nucleophilic substitution reaction. It is called substitution because one functional group or atom/molecule gets replaced by another functional group or atom or molecule in this type of reaction. It is called nucleophilic because of the involvement of negatively charged atoms or molecules.

2. How Many Kinds of Nucleophilic Substitution Reactions are there?

Ans. There are two types of nucleophilic substitution reaction -

  • Sn1 reaction, and

  • Sn2 reaction

3. What is Sn1 Reaction?

Ans. Sn1 is a unimolecular substitution reaction. It involves the separation of negatively charged functional groups or atoms first. This results in the formation of a carbocation. The anion or another negatively charged functional group or atoms then gets attached to the carbocation.

4. What is the Sn2 Reaction?

Ans. Sn2 reaction is a bimolecular nucleophilic substitution reaction. The detachment of the original negatively charged functional group and the attachment of the new negatively charged functional group or anion happen simultaneously forming an intermediate.  There is step 2. The replacement gets completed at the end.

5. What do Sn1 and Sn2 Rates of Reactions Depend Upon?

Ans. The rate of reaction in Sn1 depends on the concentration of the substrate. On the other hand, the rate of reaction in Sn2 depends on the concentration of the substrate as well as the concentration of the functional group. 

6. What is the Difference Between Nucleophile and Electrophile?

Ans. A nucleophile is that chemical species that is electron-rich. Therefore it is negatively charged. On the other hand, the electrophile is that chemical species that have electron deficiency and hence it is positively charged. 

7. Why can’t CH3Br React with OH Negative Following the Sn1 Reaction Pattern?

Ans. CH3BR, if follows the Sn1 pattern, will get separated into CH3 and BR-. However, CH3 is extremely unstable and it cannot wait for the OH to form a bond. Hence it can only form a bond in Sn2 reaction because Sn2 is a transitional reaction and the detachment and attachment process happens at the same time.