In this article, we are going to have a closer approach towards the nucleophilic substitution reaction mechanism i.e.SN2 using the appropriate sn2 examples and why there is a need for us to analyse this mechanism.
We will study the mechanism in detail using the SN2 examples, the factors affecting the mechanism such as nucleophile strength, carbon skeleton, leaving group, solvent etc. The stereochemistry involved in the reaction, rate-determining factors and the healthy ways to carry out the reaction to achieve the desired result.
First of all, why there is a need for us to know which mechanism a reaction will follow? It is simply because it helps in predicting the type of conditions that are required for a substance to react in order to obtain good yield of products. So we shall have closer approach using SN2 examples.
Note: Rate of the reaction depends on the concentration of both nucleophile and substrate.
The name SN2 stands for nucleophilic substitution – second-order reaction. In this reaction mechanism, a nucleophile attacks a substrate and a leaving group leaves and this occurs simultaneously. The reaction occurs in one single step only.
In the above reaction of SN2 example OH acts as a nucleophile attacks and simultaneously chlorine which is a good leaving group leaves, OH comes and attaches at that point.
Factors affecting SN2 mechanism:
Nucleophile: It plays a very important role in the mechanism as it determines the rate of the reaction and the stronger the nucleophile faster will be the reaction. Negatively charged species are more nucleophilic as compared to neutral molecules. OH is preferred over other nucleophiles as it is an anion and hence very reactive.
Carbon skeleton: It always prefers primary carbon over tertiary carbon because if the carbon is more substituted(tertiary) than due to steric hindrance it becomes difficult for a nucleophile to attack the substrate.
Note: Methyl and primary alkyl group always react by SN2 mechanism and never by SN1 mechanism because it cannot form carbocation.
Leaving group : If the leaving group is good then the reaction will proceed faster resulting in an increase in the rate of reaction. Usually, the weak bases are good leaving groups which include ions of halides I-, Cl- and Br- also H2O. The important factors for leaving groups such as halide is the strength of the C-halide bond and the stability of the ion of halide.
Stereochemistry : When the leaving group is attached to a chiral carbon, inversion of configuration of substrate takes place. It happens because the nucleophile attacks just opposite to the leaving group .
Transition state tells us the type of structures that react reliably and stereochemistry of the reaction.
Effect of solvent : It is mostly carried out in polar aprotic solvent as the polar solvent aids in dissociation of the C-X bond where X is the leaving group and aprotic solvents solvate the leaving group thus accelerating the reaction.
Other factors influencing reaction mechanism using sn2 examples :
When adjacent C=C or C=O π systems are present, they increase the rate of reaction mechanism. Consider the SN2 examples of Allyl bromide ,it reacts with alkoxides and forms ethers . It is observed that in the SN2 mechanism compounds of Allyl bromide react rapidly as the π system which is adjacent to the double bond stabilizes the state of transition by conjugation .
The p orbital present at the centre of the reaction makes two partial bonds having only two electrons (electron-deficient) so additional electron density can be gathered from adjacent π system which stabilizes the state of transition and hence the rate of reaction increases.
SN2 Example :Benzyl bromide and alkoxides react to give Benzyl Ethers
In the above reaction of SN2 examples , amine reacts to form aminoketone which plays vital role in the synthesis of drugs .
Mostly alcohols are not good leaving group, so to resolve this problem :
• We can protonate the OH group with strong acid. This converts alcohol to it’s respective oxonium ion which can be lost as water.
• We know that sulfonate’s are good leaving group, so we can use the Tosyl group such as Tosyl chloride or Mesyl chloride replace the H with the Tosyl group thus resulting in alkyl sulfonate .
Read more about: Monomer examples
Reaction with Nitrogen nucleophile :
Amines are great nucleophiles but the reaction of ammonia and alkyl halides will not always form single products. This is because the product formed from the substitution is almost equally nucleophilic like the starting material and hence competes with alkyl halide in the reaction.
This alkylation continuous leading to the formation of secondary, tertiary and stops only on formation of non-nucleophilic tetra-alkylammonium ion. This extra groups of alkyl push the electron density onto N resulting the product to be more reactive than the previous one.
This problem can be overcome by replacing ammonia with the azide ion. It is a triatomic species which is nucleophilic at both it’s ends. It is a slender rod consisting of electrons which is capable to insert itself in all kinds of electrophilic site. It’s availability is in the form of water soluble sodium salt NaN3.
Azide reacts only once with alkyl halides as the product formed i.e alkyl halide is no longer in nucleophilic state.
Read more about :CARBOHYDRATES
Let’s have a closer approach towards the potential energy profile of the SN2 reaction.
When we go from left to right in the periodic table the nucleophilicity decreases followed by increase in electronegativity from left to right. Hence high electronegativity is unfavourable as the tightly held electrons are relatively less available for the donation to the substrate in the SN2 reaction. Therefore OH is said to be more nucleophilic than F- and NH3 is more nucleophilic than H2O.
Note: Harder nucleophiles increase the rate of reaction as compared to soft nucleophiles.
The potential energy profile of the SN2 mechanism above shows that only one transition state exists and there is no formation of intermediate between reactant and product as it’s a single step reaction. The energy of reactants is slightly higher than that of products as the reactions is an exothermic reaction.
The energy of transition state is quite higher as it involves a five- coordinated carbon atoms consisting of two partial bonds. The hill which is on the topmost part corresponds to the transition state of the Sn2 reaction. The free energy of activation corresponds to the difference in the free energy between the reactants and the transition state.
The free energy change for a reaction corresponds to the difference in free energy between the reactants and the products. A reaction will occur faster when it has low free energy of activation as compared to the one having higher free energy of activation.
Hard nucleophiles include- H-, CH3-
Moderate nucleophile – R-O- , R-NH-
Soft nucleophile – Cl-,
Frequently Asked Questions–
1. Why is Sn2 reaction mechanism not favorable or slower in polar protic solvents ?
Ans.It is so because nucleophiles are solvated by polar protic solvents which inhibits it’s ability to participate in Sn2 mechanism.
2. In a reaction where CH3 is nucleophile which one below will proceed faster a) CH3-Br b) CH3-I ?
Ans.The reaction will be faster with CH3-I as I- is good leaving group also it is more stable as compared to Br- as it has less charge density.
3. Why Sn2 reactions prefer primary carbon ?
Ans. It is so because if the carbon is more substituted than due steric hindrance it becomes difficult for nucleophile to attack the substrate.