Introduction the mechanism of SN2 Reaction
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 –
Neutral 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
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 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.
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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 reactions 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.
FAQs on SN2 Reaction Mechanism
1. State the difference between SN1 and SN2 chemical reactions in brief.
SN1 reactions are chemical reactions in which the phase that decides the rate of reaction is unimolecular. For SN2 reactions, this rate-determining compound is a bimolecular reaction. SN1 is known to be a 2-stage catalytic chemical process, while SN2 is a one-step catalytic reaction. A carbocation form acts as an intermediary during the SN1 reaction. SN2 reactions do not have any such intermediates in the catalytic process
2. State in brief the process of an SN2 reaction.
The mechanism is quite simple as it is a one-step process. In this reaction, a nucleophilic substitution occurs involving two constituents which determine the rate of the chemical process. There are two molecules involved with the bond formation and bond-breaking step occurring in cascade. To know more, visit Vedantu and understand the mechanism.
3. What happens to the stereochemistry of molecules in the SN2 reaction process?
SN2 reactions are stereospecific and can sometimes lead to a change in the stereochemical properties of the end product. This is because SN2 reactions can occur primarily in two ways:
(i) If the substance is an R enantiomer, then in the SN2 reaction, the nucleophile will attack the substrate from the front end. This does not cause any change in stereochemistry and the substrate is known to have retained its configuration.
(ii) However if the same nucleophilic reaction occurs from the back end of the substrate, it leads to the inversion of the R- configuration to an S configuration. This is said to have stereospecifically altered the substrate. In such a case, the R isomer changes to S type while the S configuration changes to R type. A similar concept is applied to the cis and trans isomers of a compound. This type of inversion is also known as Walden inversion.
4. What denotes the type of molecular reaction and if it is SN1 or SN2?
The type of reaction can be easily known by careful examination of the number of steps of the reaction. All SN2 reactions essentially occur in a single step. While SN1 reactions are always known to be a multiple-step process. Secondly one can check the molecules involved in the rate-determining step of the reaction. Unimolecular are for SN1 while two molecules in the rate-determining step are SN2 reactions.
5. What do SN1 and SN2 stand for?
SN1 and SN2 are simply Nucleophilic Substitution reaction mechanisms hence the abbreviation “SN '' while the numerical 1 & 2 represent whether the reacting species in the rate-determining step is one molecular or bimolecular. SN1 means unimolecular nucleophilic substitution reaction and in the same way, SN2 means bimolecular nucleophilic substitution reaction.
