SN2 Reaction Mechanism

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

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

Examples of Nucleophiles – Neutral Nucleophiles- ammonia (NH3), water (H2O), carboxylic acid (RCOOH) etc. 

Negatively Charged Nucleophiles

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

Electrophile

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 reactions are bimolecular. As two reactants are involved in the rate determining step. The slow step in the reaction is called rate determining step. In these reactions addition of nucleophile occurs with detachment of a leaving group. For SN2 reaction, rate of reaction can be expressed as – r = [Nu][R-LG] (General representation)

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

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

What is SN2 Reaction Mechanism?

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

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

Now let’s understand 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 – 

  • Reaction between 2-bromobutane and OH- (nucleophile from KOH)

 

  • Reaction between methyl chloride and nucleophile OH- 

 

  • Reaction between methyl chloride and bromide ion 

 

  • Reaction between benzyl bromide and sodium cyanide 

 

Stereochemistry of SN2 Reactions 

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

 Factors Affecting SN2 Reactions 

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

  • If carbocation is unstable, reaction is SN2 while if carbocation is stable reaction is SN2

  • 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 SN2 reaction mechanism more than carbon attached to larger groups such as CH3CH2 etc. 

 

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