SN2 Reaction Mechanism: Energy Level Diagram, Nucleophile, Electrophile

S_N2 reaction mechanism involves the nucleophilic substitution reaction of the leaving group with a nucleophile in a given organic compound. In the acronym S_N2 , S stands for Substitution, N stands for Nucleophilic and 2 stands for bimolecular. The S_N2 reaction mechanism is one of the most important reaction mechanisms of Organic Chemistry.

Key Terms: Nucleophiles, isomers, compounds, Electrophile, carbon atom, S_N2 mechanism, S_N2 reaction, Electron, Stereoisomers, Organic Chemistry

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S_N2 Reaction: Important Terms

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In the S_N2 reaction, the nucleophile attacks from the rear side of the carbon atom. As a result, the product takes on a stereochemical position opposite to the leaving group. This is known as configuration inversion. The sn2 Reaction is a stereospecific reaction in which various stereoisomers react to produce different stereoisomers of the Product. Before moving to the S_N2 mechanism, let’s discuss some important terms. 

• Nucleophile

The nucleophile is an electron-rich negatively charged or neutral species. It usually donate a pair of electrons. Nucleophiles attack the 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 and positively charged species. It can accept a pair of electrons.

Examples of Electrophile-

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

• Leaving Group

A leaving group is that anion or neutral atom that leaves 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. 

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S_N2 Reaction Mechanism

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Let’s take CH₃Br and OH^– reaction as an example of the S_N2 mechanism.

S_N2 Reaction Mechanism

The S_N2 mechanism involves the exchange of two different electron pairs simultaneously. In this, the addition of the nucleophile (red arrow) and the elimination of the leaving group (blue arrow) takes place at the same time. The nucleophile OH^– reaches the electrophilic carbon in the opposite direction as the leaving group Br does. As the nucleophile OH^– approaches, the Br begins to leave as well. The new C—OH bond is formed and the old C—Br bond is broken. 

The carbon atom is partially coupled with both OH and Br for a very brief transient moment, resulting in the highest energy level state of the entire process, known as the transition state. There are five groups around the carbon in the transition state of S_N2 reaction and the carbon is referred to as "pentacoordinate." 

The Br moves further away from the carbon as the OH^– approaches it with the bonding electron pair. When the new bond is completely formed and the old bond is completely broken, the product CH₃OH is formed.

According to the mechanism, the reaction occurs in a single step that involves both the nucleophile and the substrate. So, increasing the concentration of either of them increases the chance of collision, which further explains the S_N2 reaction's second-order kinetics. 

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Energy Level Diagram of S_N2 Mechanism

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The change in energy for the above reaction is represented in the energy level diagram given below. SN2 is a single-step reaction, so the diagram only shows one curve. The lower energy of the products CH₃OH and Br^– compared to the reactants CH₃Br and OH^– suggests that the reaction is exothermic and that the products are more stable.

Energy Level Diagram of SN2 Mechanism

The transition state is at the top of the curve. Transition states always comprise incomplete, partially formed, and partially broken bonds. Due to this, they are extremely unstable and have a short lifespan. As a result, the transition state can never be separated. The transition state structure is typically shown in square brackets. 

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Examples of S_N2 Reactions 

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• The reaction between 2-bromobutane and OH^- (nucleophile from KOH)
• The reaction between methyl chloride and nucleophile OH^- 
• The reaction between methyl chloride and bromide ion 
• The reaction between benzyl bromide and sodium cyanide 

Stereochemistry of S_N2 Reactions

In the majority of S_N2 reactions, the conformation of the substrate is completely inverted. After the S_N2 reaction, when a nucleophile hits the substrate from the opposite side or rear side of the leaving group linked to the substrate, we get an inverted product. This process is known as Walden inversion.

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Factors Affecting S_N2 Reactions

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• The S_N2 reaction mechanism is used to process strong nucleophiles. A weak nucleophile, on the other hand, will use the sn1 Reaction Mechanism.
• The reaction is S_N2 if the carbocation is unstable. 
• Less substituted systems favour S_N2 reactions, while if the central carbon is linked to a smaller group or element, such as H then it will favour S_N2 reactions more than carbon coupled to larger groups, such as CH₃CH₂.

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Things to Remember

• In the S_N2 reaction, the nucleophile attacks from the rear side of the carbon atom.
• The S_N2 reaction is a nucleophilic substitution reaction of the leaving group with a nucleophile.
• In the S_N2 mechanism, the addition of nucleophiles and the elimination of the leaving group takes place simultaneously.
• Increasing the concentration of nucleophiles and substrates will increase the chance of collision.

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