The IUPAC Gold Book gives the following definition for substitution reactions: “A reaction, elementary, or stepwise, in which one atom or group in a molecular entity is replaced by another atom or group.” While the definition itself seems to serve its purpose, it is often too generalized for its own sake.
Furthermore, reactions are not defined as one simple transformation, but rather can be two or more transformations, leading to more complexities on how we would define a substitution reaction. I think it is apt to begin the introduction to substitution reactions by observing how overly generalized it is, such that many reactions can be included within its scope.
In the same way, we could argue, for most substitution mechanisms, that substitution is not in fact a reaction or transformation in itself, but rather a more specific version of addition or elimination. Undergraduates are typically taught the two most common so-called substitution reactions, SN1 and SN2 reactions.
Nitpicking can once again be done on the idea of naming SN1 and SN2 as ‘reactions’, since reactions are defined: “A process that results in the interconversion of chemical species.” It is, then, apt to suggest that a substitution could be considered a reaction; however, since SN1 and SN2 lead to the same products from similar reactants, but with different mechanisms, it should be that both of these are not reactions, but rather, mechanisms. In scientific literature, addressing SN1 and SN2 mechanisms as reactions seems to be fairly standard, and as such the rule will be followed.
One problem with the broad definition of substitution reactions is that many mechanisms are covered under such reactions, other than the SN1 and SN2. For instance, substitution reactions can be first separated broadly into ‘aliphatic’ (nonaromatic; there is really no other short or better description of aliphatics) and ‘aromatic’ substitution reactions.
Furthermore, reactions are not defined as one simple transformation, but rather can be two or more transformations, leading to more complexities on how we would define a substitution reaction. I think it is apt to begin the introduction to substitution reactions by observing how overly generalized it is, such that many reactions can be included within its scope.
In the same way, we could argue, for most substitution mechanisms, that substitution is not in fact a reaction or transformation in itself, but rather a more specific version of addition or elimination. Undergraduates are typically taught the two most common so-called substitution reactions, SN1 and SN2 reactions.
Nitpicking can once again be done on the idea of naming SN1 and SN2 as ‘reactions’, since reactions are defined: “A process that results in the interconversion of chemical species.” It is, then, apt to suggest that a substitution could be considered a reaction; however, since SN1 and SN2 lead to the same products from similar reactants, but with different mechanisms, it should be that both of these are not reactions, but rather, mechanisms. In scientific literature, addressing SN1 and SN2 mechanisms as reactions seems to be fairly standard, and as such the rule will be followed.
One problem with the broad definition of substitution reactions is that many mechanisms are covered under such reactions, other than the SN1 and SN2. For instance, substitution reactions can be first separated broadly into ‘aliphatic’ (nonaromatic; there is really no other short or better description of aliphatics) and ‘aromatic’ substitution reactions.
Among both of these categories, a further distinction can be made between nucleophilic and electrophilic substitutions, although electrophilic substitutions usually take place for aromatics, most of which are nucleophilic. Our focus here would be on aliphatics.
Of aliphatic nucleophilic substitutions specifically, there are many mechanisms that can take place to give the same product from the same starting material, excluding the solvent, reagents or reaction conditions. Firstly, there are the SN1 and SN2 reactions, which involve unimolecular and bimolecular substitutions respectively.
I would feel that it is acceptable to consider these two mechanisms the ‘main’ class of mechanisms in aliphatic nucleophilic substitution reactions, since a good percentage of them follow either mechanism. In general, tertiary substrates prefer the SN1 mechanism while primary and secondary substrates tend towards the SN2 mechanism (yet another complexity exists in which the preference of some secondary substrates will lean towards the SN1 mechanism under certain conditions).
The explanation for this is that SN1 reactions begin with solvolysis, meaning the dissociation of the substrate into a cation (the electrophile) and anion (the leaving group). This is also the rate-determining step, and thus the rate-determining factor is how stable the carbocation is.
The tertiary substrate is able to stabilize the negative charge better due to hybridization. As for the SN2 mechanism, there is only one step, which is also the rate-determining step, and involves a backside attack and transition state. The rate-determining factor here is attributed to steric hindrance, and thus the least sterically-hindered substrate, the primary and secondary ones, proceed with substitution via the SN2 mechanism. Substitution is easy, right?
Well, no. Unfortunately, there are several other mechanisms which also fit into the definition of substitution reactions, and can also be considered within this field. The simplest example of such a mechanism is, in fact, another class of mechanisms all in itself, which includes the tetrahedral, elimination-addition, and addition-elimination mechanisms.
Well, no. Unfortunately, there are several other mechanisms which also fit into the definition of substitution reactions, and can also be considered within this field. The simplest example of such a mechanism is, in fact, another class of mechanisms all in itself, which includes the tetrahedral, elimination-addition, and addition-elimination mechanisms.
The tetrahedral mechanism is less well-known, but the addition-elimination and elimination-addition mechasnisms are both very simple: they simply involve a two step, with either addition or elimination first. A reaction commonly encountered in undergraduate chemistry, the hydrolysis of nitriles, can be summarized into two parts: conversion of the nitrile to an amide, followed by hydrolysis of the amide.
The effect at the end is that the nitrogen is substituted with an oxygen, and thus it can be considered a substitution reaction. However, it is in fact composed of several ‘mini-reactions’ within itself, specifically, two nucleophilic additions and one elimination; however, it is yet still considered a part of aliphatic nucleophilic substitution reactions.
We could go on and on with other, completely different mechanisms, such as SET (single electron transfer) mechanism, where radicals are involved (they can be seen as nucleophiles) and the ‘neighbours’ of SN1 and SN2 reactions, SNi (intimate ion pair) mechanisms, but the end point of my argument is that substitution reactions are so broad that they can encapsulate even other types of reactions within them, such as nucleophilic addition or elimination.
In the end, SN1 and SNi reactions - solvolysis (according to Wikipedia, solvolysis can be considered nucleophilic substitution or elimination) and nucleophilic addition. Addition-elimination and elimination-addition reactions - literally the name of the reaction. These reactions and mechanisms have their merits, but do they really deserve to be in the same category as substitution reactions, when they are simply constituted of different combinations of transformations?
In the end, SN1 and SNi reactions - solvolysis (according to Wikipedia, solvolysis can be considered nucleophilic substitution or elimination) and nucleophilic addition. Addition-elimination and elimination-addition reactions - literally the name of the reaction. These reactions and mechanisms have their merits, but do they really deserve to be in the same category as substitution reactions, when they are simply constituted of different combinations of transformations?
I would regard the SN2 reaction as a ‘true’ substitution reaction, since there is strong evidence that it proceeds both in one step, and therefore cannot be sufficiently explained as the sum of elimination or addition reactions. In such cases, there really is a true substitution, taking away a leaving group and replacing it with a nucleophile - all in one step.