Nucleophilic substitution reactions are those in which an electron-rich nucleophile approaches a positively charged electrophile in order to replace a leaving group.
SN2 reaction mechanism
SN2 stands for nucleophilic substitution bimolecular reaction. The term “biomolecular” implies that there are two reacting species in the rate-determining step of the reaction. When the rate of nucleophilic substitution reactions depends upon the concentration of substrate (e.g., alkyl halide) and nucleophile, then the reaction is of second order.
Kinetics of the SN2 reaction
The kinetic data show that the rate of the reaction is determined by the concentration of both reactants.
i.e. Rate directly proportional to: [alkyl halide] [nucleophile]
Rate = k [CH3Br] [OH]–
Since this reaction is of second order, it occurs by a direct displacement mechanism in which both reactants are present at the rate-determining step.
Mechanism of the SN2 reaction
The nucleophile is supposed to target the side of the carbon atom opposite to that of bromine. This is known as a “backside attack.” As a result of this approach, a transition state with carbon atoms partially bonded to both the OH and Br groups is formed. The center carbon is sp2 hybridized in the transition state, and the three hydrogens connected to it are in the same plane with a bond angle of 120 °C. The reaction may be illustrated as follows:
It should be observed that the hydroxide ion’s negative charge has decreased in the transition state since it began sharing electrons with carbon. Similarly, bromine develops a partial negative charge when it moves away from the bonding electrons.When the carbon-oxygen link is fully formed, the carbon-bromine bond is totally broken. The energy expended to break a bond is balanced by the formation of a new bond. Thus, the entire reaction is a coordinated process that occurs in a single step due to the presence of a single transition state. The C-Br link is broken and the C-OH bond is formed simultaneously in a single step, resulting in the formation of an alcohol molecule.
Steriochemistry for SN2 reaction
It is expected in the SN2 reaction that the nucleophile attacks the side of the carbon opposite the leaving group. As a result, the finished product’s configuration is inverted. In other words, the SN2 reaction involves stereochemical inversion. This has been confirmed in practice.
The configuration of the reactant and the products are different in such SN2 reaction. A reaction that gives a product whose configuration is opposite to that of configuration and is known as walden inversion.
SN1 reaction mechanism
The SN1 reaction is a first-order nucleophilic substitution process. It stands for the nucleophilic substitution unimolecular reaction. The rate-determining step of the SN1 reaction depends upon the concentration of substrate only.
Here the rate-determining step of the reaction is:
It involves only one substance on the reactant side.
Kinetics of the SN1 reaction
Chemical kinetics studies reveal that the rate of this reaction depends only on the concentration of alkyl halide.
i.e. Rate = k [alkyl halide]
The reaction is of the first order and thus it is believed to occur in two steps.
Mechanism of SN1 reaction
It is obvious that the slow stage, which involves ionization of tert-butyl bromide to generate tert-butyl carbocation, is the rate-determining stage of the process. Hence, step (I) is the rate-determining step. The formation of many ion-dipole bonds between the ions produced and the polar solvent molecule provides the energy required for ionization. The second step, which involves combining the carbocation with the hydroxide ion to generate alcohol, is quite fast.
Steriochemistry of SN1 reaction
The formation of a carbocation is the rate-determining step in unimolecular nucleophilic substitution. The sp2 hybridized state of the carbon atom in the carbocation. As a result, the carbocation has a flat structure, with all three components bonded to carbon lying in the plane and producing 120 ° C angles between them. The plane lies perpendicular to the empty p-orbitals.
The nucleophile attaches to the flat carbocation most likely from the rear of the carbonium ion. The nucleophile will attach to the carbonium ion with 50% probability from the front and 50% probability from the back. In a basic case, this may not make a difference, because it does when the halogen bearing carbon is chiral, meaning it is connected to four distinct groups. When an alkyl halide is hydrolyzed with a chiral halogen carrying carbon, a racemic product is formed, including 50% of each of the d and l forms.
One configuration is formed when the nucleophile is attached from the front, whereas the other configurations are formed when the nucleophile is attached from the back. As a result, the final product should be racemic, including an equal number of molecules with configuration retention and inversion. In actuality, however, the product as a whole is not racemic. In most cases, there are more molecules with inverted configurations than those with the same configuration.
This can be explained by the fact that alkyl halide ionization does not result in free carbonation. In reality, when an alkyl halide is ionized, it initially forms an ion pair in which the leaving halide ion is still adjacent to the carbocation. As a result, the attack on the carbocation’s front side, which leads to a product with configuration retention, is slightly hindered.
On the other hand, an attack on the backside that results in a product with an inverted configuration is preferable. As a result, the final product is somewhat racemized, with the inverted enantiomer predominating. for example, when (-)-2-bromoctane is hydrolyzed at SN1 conditions, a largely recmized product is produced.
Factors Affecting the SN Mechanism
The following are the elements that affect SN1 and SN2 reactions:
- Nature of alkyl groups:
The relative reactivity of alkyl halides in SN1 reactions is equal to the ease of formation and stability of carbocation. The stability of the carbocation ion is in order.
Tert. > sec. > primary > methyl carbocation
Therefore, the reactivity of alkyl halides is also in the same order.
In SN2 reactions, the steric factors are to be taken into consideration. Lesser the crowding around halogen-carrying carbon, the greater the reactivity. For such reactions, the reactivity follows the order.
Methyl alkyl halide > primary > secondary > tert. alkyl halide
- Nature of halogen atom
In SN1 reactions, the ease of elimination of the leaving group is I– > Br– > Cl– > F– , therefore the order of alkyl halides is RI > RBr > RCl > RF. In SN2, the reaction is also the same as SN1.
- Nature of the nucleophile
The nature of the nucleophile has no effect on the SN1 reaction since it is not involved in the rate-determining step. In the SN2 reaction, the stronger the nucleophile, the greater the rate of reaction. The increasing strength of a few nucleophiles is shown below.
H2O < OH– < RO– < CN–
- Nature of the solvent
The polarity of the solvent plays an important role in the ease of formation of carbocations for the SN1 reaction. The greater the polarity of the solvent, the greater the rate of reaction.
The polarity of the solvent has a role in the SN2 reaction but in the opposite way. The higher polarity of the solvent is likely to destroy the less polar intermediate product. Hence, the less polar the solvent, the greater is the reactivity.
Difference Between SN1 and SN2 Reactions
|SN1 reaction||SN2 reaction|
|SN1 reactions follow the first-order reaction.||SN2 reactions follow the second-order reaction.|
|The nucleophile attacks the carbocation from both sides although the backsides attack dominates.||The attack of nucleophiles takes place from the backside only.|
|Partial racemization of optically active halides takes place.||Inversion of configuration take place.|
|It is favored by solvents of high polarity.||It is favored by solvents of low polarity.|
|Rearrangement of products takes place.||There is no possibility of rearrangement.|
|The reactivity follows the order:|
Tert. > sec. > primary > methyl halide.
| The reactivity follows the order:|
methyl > primary > sec. >Tert. halide
What is the order of reactivity that follows by SN2 reaction?
The reactivity follows the order: methyl > primary > sec. >Tert. halide