This article will highlight what is SN1 and SN2 reaction, their features, difference between them and more.
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SN1 and SN2 are two of the most common reactions involved in organic chemistry. SN1 is a substitution, nucleophilic addition reaction, which often occurs with carbonyl compounds as well as benzene. SN2 is another type of nucleophilic substitution reaction that more commonly occurs with alkyl halides and also benzoins.
What is SN1?
SN1 reaction is a substitution, nucleophilic addition reaction. It occurs with carbonyl compounds as well as benzene compounds. So, in other words, this process involves electrophiles, which are attacked by an electron rich nucleophile. These reactions are also known as the SN1 reactions or first order reactions and they proceed through three major steps.
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Features of SN1:
First, in the first step the nucleophile attacks the proton of the electrophilic center (H) at an angle of 180 degrees.
The second step involves a transition state where both of the atoms involved in this process are highly reactive.
During this transition state, there are two possible mechanisms that can occur. The nucleophile and electrophile may form an intermediate complex or they may undergo dissociation. The dissociation is a fast reaction and it proceeds to form an intermediate complex. This happens because a hydrogen atom is transferred to the nucleophile, which then gives it stability.
What is SN2?
The SN2 reaction is another type of nucleophilic substitution reaction that more commonly occurs with alkyl halides and also benzoins. This process involves the electrophile, which is attacked by the nucleophile in a concerted fashion employing an angle of 180 degrees. The key feature of this reaction is that it proceeds through a single transition state in which both of the atoms are highly reactive.
Feature of SN2:
The first step involves a free rotation about the carbon-carbon single bond in which the nucleophile attacks. This step also involves hydrolysis as well as carbocation formation.
The second step involves the formation of a transition state in which both the atoms involved in this process are highly reactive.
During this step, there are two possible reactions that occur. The nucleophile and electrophile may form an intermediate complex or they may undergo dissociation. The dissociation is a fast reaction and it proceeds to form an intermediate complex. This happens because a hydrogen atom is transferred to the nucleophile, which then gives it stability.
Difference between SN1 and SN2:
| SN1 | SN2 |
| Substrate in SN1 reaction is uncharged | Substrate in SN2 reaction is charged. |
| Transition state of the first step in both processes involves a 180 degree bond rotation | But, in the SN2 process this rotation occurs about an alkyl fragment while in the SN1 process this rotation occurs about a carbonyl or aromatic ring. |
| There are two possible mechanisms for the first step of the reaction | However, this does not happen for the second step as it proceeds through a single transition state with both atoms being highly reactive. |
| SN1 reaction doesn’t proceeds through a single transition state with both the atoms being highly reactive | SN2 reaction proceeds through a single transition state with both the atoms being highly reactive |
| In case of the SN1 reaction, both the atoms involved in this process have a free rotation about each other. | But, with respect to SN2 reaction, there is a free rotation about carbon-carbon single bond and no rotation about a carbonyl or aromatic ring. |
The common feature of SN1 and SN2 is that they both involve a concerted, concerted process. However, this process occurs in different forms in each of these processes. For example, in case of SN2 process this step occurs through a concerted attack by an activated nucleophile while it occurs through a single transition state in case of SN1 process where both the atoms are highly reactive.
How to test for SN1 and SN2 reactions:
The reaction mechanism of SN1 and SN2 reactions can be tested with the help of a spectrometer. A spectrometer is a tool that measures the wavelengths of light emitted or absorbed by an atom or molecule. The difference between the two is that while testing for SN1, one has to measure frequencies while testing for SN2 one has to measure wavelengths. The simplest way to test for either nucleophilic substitution reaction is by using silver nitrate solution as a reference standard. Another way is by using 1% HCl solution as a reference standard. The test is done by mixing the two solutions together and then titrating with silver nitrate solution. The increase in the absorbance (wavelength) of light indicates the formation of an adduct.
Conclusion
SN1 and SN2 are two of the most common reactions involved in organic chemistry. SN1 is a substitution reaction which often occurs with carbonyl compounds as well as benzene, while SN2 is another type of nucleophilic substitution reaction that more commonly occurs with alkyl halides and also benzoins. SN1 reaction is a faster process than SN2, because it proceeds through multiple steps involving transition state instead of one.
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Frequently asked questions
Get answers to the most common queries related to the JEE Examination Preparation.
What is SNH?
Answer: This stands for Substitution by Nucleophilic Halide.
What is the mechanism of SN1 reaction?
Answer: The first step involves the nucleophile attacks at an angle of 180 degrees to form a species known as transi...Read full
How to explain SN2 reaction?
Answer: This involves the electrophile is attacked by the nucleophile involving a concerted process. The second step...Read full
What is the difference between SN1 and SN2 reactions?
Answer: The difference between these two reactions is that in case of action by nucleophilic halide, the first step ...Read full
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