[Chemistry Class Notes] on SN2 Reaction Mechanism Pdf for Exam

In the term SN₂, S stands for Substitution, N stands for Nucleophilic and 2 stands for bimolecular. So, SN₂ reactions are nucleophilic substitution reactions. These are very important substitution reactions of Organic Chemistry. Before understanding the SN₂ reaction and its mechanism, you need to understand the terms like nucleophile, electrophile and leaving group. So, let’s start to understand these terms first.

 

Nucleophile

Nucleophiles are negatively charged or neutral and electron-rich species. It can donate a pair of electrons. Nucleophile attacks positively charged species.

Examples of Nucleophiles – 

ammonia (NH₃), water (H₂O), carboxylic acid (RCOOH) etc. 

• Negatively Charged Nucleophiles

Bromide (Br–), iodide (I–), chloride (Cl–) etc. 

Electrophile

An electrophile is an electron-deficient species. It can accept a pair of electrons. It is generally a positively charged species. 

Examples of Electrophile

hydronium ion (H+), nitrosonium ion (NO+) etc.

 

Leaving Group

A leaving group is that anion or neutral molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. These can be neutral, negative, or positively charged. 

Examples of leaving groups – Cl–, water, H+, etc.

 

SN₂ Reaction 

This type of nucleophilic substitution reaction is bimolecular as two reactants are involved in the rate-determining step. The slow step in the reaction is called the rate-determining step. In these reactions, the addition of nucleophiles occurs with a detachment of a leaving group. For SN₂ reaction, the rate of reaction can be expressed as:

                   R = [Nu][R₁-LG] 

Where Nu = Nucleophile, R₁ = alkyl group or group attached to leaving group, LG = leaving group.

As the nucleophile is either negatively charged or neutral so here, we are giving examples of SN₂ reactions with a negatively charged nucleophile and neutral nucleophile.

 

What is the SN₂ Reaction Mechanism?

SN₂ reaction mechanism takes place by single step only. First, a nucleophile attacks an electrophile or partially positively charged element attached to the leaving group. Simultaneously, the leaving group starts getting detached from electrophile or positively charged elements. 

As the reaction is a single step, it is the rate-determining step as well and has one transition state. 

Now let’s understand the SN₂ reaction mechanism by an example of SN₂ reaction- bromide (nucleophile, Br–) attacks on ethyl chloride (the electrophile) and results in ethyl bromide and chloride ions as products. 

Examples of SN₂ Reactions 

 

Stereochemistry of SN₂ Reactions 

In most of the SN₂ reactions, a complete inversion of the configuration of the substrate takes place. When a nucleophile attacks the substrate from the opposite side or backside of the leaving group attached to the substrate then we get an inverted product after completion of the SN₂ reaction. This process is known as Walden inversion.

 

Factors Affecting SN₂ Reactions 

• Strong nucleophiles will proceed by the SN₂ reaction mechanism. While a weak nucleophile will proceed through the SN₁ reaction mechanism. 

• If carbocation is unstable, the reaction is SN₂ while if carbocation is stable, the reaction is SN₁. 

• SN₂ reactions are favored by less substituted systems means if central carbon is attached to a smaller group or element such as H then it will favour the SN₂ reaction mechanism more than carbon attached to larger groups such as CH₃CH₂ etc. 

If you are still not confident about the SN₂ reaction’s mechanism and want to solve some problems based on this then register yourself on and unlock free PDFs of NCERT Solutions, sample problems, etc. You can also download Learning App for Class 6-10, IIT JEE, and NEET for mock tests, online classes, revision notes, and much more.

 

Common mistakes and misconceptions about SN₂ reactions 

There are some common fallacies on the part of the students in their understanding of chemical reactions. Some of them are:

• Students often misinterpret chemical reactions in that they do not understand that a chemical reaction can give a mixture of products. The same reaction can yield or follow the SN₁ mechanism as well as the SN₂ mechanism based on other factors. For example in the case of bulky secondary alkyl halides, the two stereoisomers are SN₁ products but if some water molecules in the reaction enter equilibration with ethanol molecules, SN₂ products may also be simultaneously achieved. 

• Solvents are one of the most crucial factors. For SN₂  reactions, simply avoid protic solvents. This is because the nucleophile in the reaction is known to gain a proton from the solvent and deactivate itself.

• Under high heat, the reaction may produce both elimination products (more likely the E₁ elimination products as ethanol has weak basicity) and substitution products.

• Under practical conditions, even SN₁ reacti
  ons are known to give a stereochemical mixture as the carbocation intermediate is planar and nucleophile attack can occur from above and below the plane.

• The reaction rate for SN₂ reaction increases with an increase in temperature (in non-biological mediums) and with an increase in either substrate or nucleophile concentration. But at the same time, a very high temperature will alter the mechanism altogether. Instead of the desirable SN₂, an elimination reaction takes place.

• Lastly, as a pre-emptive concern, students must be careful how and where the arrow of the progressing reaction is placed. It is important in chemical conventions.