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SN1 versus SN2 Reactions, Factors Affecting SN1 and SN2 Reactions

Rate equations, Nucleophiles, Solvents, Leaving groups, Structure of alkyl halide, Effects of leaving group, Effects of solvents.

SN1 versus SN2 reactions

Rate equations

Sn1 and Sn2 reactions may at first seem paradoxical based on their numbers. Considering the number of steps involved, these reactions seem backward. In the process of determining the rate, only a limited number of reaction components are involved, and not all of the steps are involved. When a reaction occurs, the slowest step dictates the rate of the whole reaction, just as the bottleneck dictates how quickly liquids can be poured out.

The leaving group leaves the electrophile to slow down an Sn1 reaction. The concentration of the nucleophile does not affect this process since the nucleophile participates only in the second step. It can be represented by the rate equation R = k[electrophile], which means that the rate depends only on one reactant, namely the electrophile, because k depends on one reactant. The reaction is called Sn1: Substitution - nucleophilic - unimolecular because it takes place on one atomic level. Therefore, we call the equation bimolecular and write it as R = k[electrophile][nucleophile], because the two reactants you must bring together in the rate-determining (and only) step have to be mentioned. This is a bimolecular nucleophilic substitution, thus the name Sn2.

Nucleophiles

SN1 - SN1 reactions usually have a weaker nucleophile because the nucleophile is "attacking" a carbocation. Therefore, the second step, the nucleophilic attack, is likely not to require a great deal of strength - the electrophile's charge tends to spur it on. Nucleophiles are usually the solvents in which SN1 reactions take place.

Common nucleophiles in sn1 reactions include CH3OH and H2O

SN2 - Sodium nitrate must displace the leaving group if it is to act as a nucleophile in sn2. When this occurs, nucleophiles have a charge. If not, there must be a strong neutral nucleophile present. Despite this, one should also consider steric factors, as a bulky nucleophile can prevent the SN2 reaction from taking place.

In SN2 reactions, nucleophiles such as KOEt and NaCN are common

Since these molecules contain ionic bonds, they are charged nucleophiles. When NaCN reacts with CN–, it becomes the charged nucleophile, CN–.

Solvents

SN1 - Solvated Sn1 reactions are more likely to occur in polar, protic solvents due to their ability to stabilize the carbocation charge. Charges are stabilized by prototic solvents, which surround them and interact with them. Protic solvents form hydrogen bonds, though they are stabilized by sn1 reactions mediated by dipole interactions. Protic solvents form hydrogen bonds, but are stabilized through sn1 reactions mediated by dipole interactions.

Solutions like water, alcohols, carboxylic acids, and ketones are common solvents for SN1 reactions

SN2 - Sn2 reactions tend to be induced by polar, aprotic solvents. As they are polar enough to dissolve nucleophiles, the reaction is able to proceed. But SN1 reactions cannot form hydrogen bonds, nor do their solvents have the same solvating power. It is not necessary to stabilize a carbocation in sn2. Solvents with too strong a solvating power, such as polar and protic solvents, will solvate the nucleophile so that it cannot attack the electrophile.

Acetone, DMSO (dimethylsulfoxide), and acetonitrile are examples of solvents commonly used in sn2 reactions

Leaving groups

It does not matter what type of leaving group is used because both SN1 and SN2 reactions require good leaving groups. It is possible, however, for either reaction to being prevented by a very poor leaving group.

A good leaving group will take electrons from its bond in order to leave, so it needs to be highly electronegative. Electronegative species attract electrons more readily, especially those from bonded pairs.

Cl–, Br–, I–, and H2O are among the leaving groups that are found in both sn1 and sn2 reactions.

Factors affecting SN1 and SN2 reactions

  • Nature of substrate
  • The nucleophilicity of the reagents
  • Solvent polarity
The alkyl halide and leaving group structures must be taken into account when calculating the unimolecular transition state of SN1. The formation of stable carbocations by alkyl halides with SN1 is easier than for alkyl halides without SN1. Solvation also plays an important role in stabilizing the carbocation besides carbocation stability being the primary consideration energetically.

Structure of alkyl halide

SN1 mechanism produces stable carbocations from alkyl halides that can ionize. There are four main stability types:

In order of carbocation stability, 3o is followed by 2o, 1o, and methyl.

As a result, alkyl halides in the tertiary position are more reactive toward SN1 in comparison to those in the secondary and primary positions. The SN1 mechanism almost never reacts with methyl halides. The order of reactivity here is exactly opposite to the SN2 reaction order.

Effects of leaving group

An SN1 reaction is also accelerated by a good leaving group. Leaving groups are crucial for the rate of the reaction. Bonds are broken more quickly by a good leaving group than by a bad leaving group. When the bond breaks, the carbocation is immediately formed, allowing the nucleophile to enter immediately, speeding up the reaction.

Weak bases are an excellent way to leave groups, since weak bases are capable of holding the charge. The leaving group will be glad to leave with both electrons as long as they are able to accept electrons. The reason why strong bases cannot be good leaving groups is that they donate electrons. On the periodic table, electron-donating properties decrease and leaving groups to become more attractive as they move left. When the Halide's ability to form a group decreases with each column down, it makes a good leaving group.

Effects of solvents

In order to form ions, a polar solvent is required. The polar solvent is crucial to the SN1 reaction because it stabilizes the transition state and carbocation intermediate, thus speeding up the rate of the reaction. Considering that the carbohydrate is unstable, any stabilizing agent will help speed up the reaction. Despite their dipole moment, polar aprotic solvents do not have highly polarized hydrogen.

Nucleophile

The strength of the nucleophile has no effect on SN1 since it does not determine the reaction rate. The SN1 mechanism, therefore, tends to favor weak nucleophiles. Nucleophilic SN1 reactions typically involve the solvent as the nucleophile. The following examples illustrate the difference between H2O, alcohols (ROH), and CH3CN.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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