Introduction
We all have heard about the three ‘classic’ organic chemistry reactions that form the crux of chemistry itself. These are the substitution, addition, and elimination reactions. These types of reactions are then fashioned into questions, usually in the form of reaction schemes and diagrams, or the simpler types of questions that only require basic knowledge of what exactly substitution, addition, and elimination reactions are. Although many people on the web have differing opinions on the less conceptual and more memory-based type of questions, I believe that such questions require both the understanding of how these three types of reactions work, and also the memory of a reaction mechanism and how the reaction will proceed. However, in this article, we will not be looking at the more complex, memory-based questions; rather, we are going to focus on the easier questions that can be answered just by understanding the concepts of the different types of reactions that exist.
A common type of question I have seen says the following: classify the mechanism as a substitution, elimination, or addition reaction. A mechanism is then provided. Before we start looking at what exactly substitution, elimination, or addition reactions are, let us analyze the question first. While the question is generally quite broad, there may be some modifications such that the questions begin asking for the specific type of mechanism encountered (i.e. E1 or E2 for elimination reactions, and SN1 and SN2 for nucleophilic substitution reactions). For the broader and simpler type of question, the fastest way to differentiate between the substitution, elimination and addition reactions is simply by looking at the reactants and products of a reaction. In fact, the name itself gives us an idea of what the reaction is about: in a substitution reaction, the key moieties are substituted with other species; in an addition reaction, a substituent is ‘added’ onto the molecule; and in an elimination reaction, a substituent is removed from the molecule. But the definitions themselves may be slightly confusing; let us first understand the defining characteristics of each type of reaction, in order to answer the question.
Substitution Reactions
We will begin with the substitution reactions first. When the question is broad, we can know whether a reaction is a net substitution by studying the substituents involved. We need to observe the reactants and the products, and compare the differences between them. In the simplest case, where we observe that a single substituent (leaving group) is replaced by another substituent (nucleophile or electrophile), it is likely to be a substitution reaction (Fig. 1). Take note that it is not always obvious that a substituent is on the molecule, because it could be a hydrogen atom which can participate in substitution reactions (notably electrophilic aromatic substitutions). Hydrogen atoms are often ‘concealed’ in such questions when the structures are drawn out in the skeletal formulae, which is a simplification where hydrogens are implied. For those of you who are not very exposed to these types of questions, I would advise you to practice by drawing out the implied hydrogens (this will probably be in a guide that is coming soon) in skeletal formulae. After getting used to it, you should be able to visualize the implied hydrogens without having to draw them out, because in exams it is too time-consuming to draw out the hydrogens, especially on such questions.
Fig. 1: General scheme of a substitution reaction.
However, it is not likely that the question will be so simple as to just ask whether the reaction is a substitution. We should understand that there are broadly two types of substitution reactions: electrophilic substitution, and nucleophilic substitution, and this can be useful when the question is more specific and does not just want to ‘know’ whether the reaction is a substitution. In organic chemistry, when electrophilic substitution is mentioned, it will almost always be at a molecule of benzene (i.e. benzene is the reactant), in the electrophilic aromatic substitution reaction. This is a defining characteristic. Thus, when benzene is shown as one of the reactants in a reaction and we are asked to figure out whether it is a substitution, addition or elimination reaction, you should already know that it is highly likely to be the substitution reaction. However, when the question asks whether it is nucleophilic or electrophilic, the benzene does not necessarily have to undergo the electrophilic one - we will talk more about this later.
As for nucleophilic substitution reactions, there are many more cases where they may occur, but this is usually at the aliphatic carbon. A rule of thumb when I try to figure out whether the reaction is a nucleophilic substitution is to look for electronegative atoms, as these will typically be substituted in the substitution reaction. Electronegative atoms are also bonded to electrophilic carbons. So, what are electrophilic carbons? These are basically carbon atoms that lack electron density, and for this reason, such carbon atoms are usually bonded to other electronegative atoms, which take away electron density from the carbon atom, making it have less electron density, and therefore suitable for nucleophilic attack (to understand more in-depth about why nucleophiles attack electrophilic sites, see here). We should know how to identify electronegative atoms; elements become more electronegative as we progress to the top right of the periodic table. In most cases this electronegative atom should be bonded to a carbon. Now, if we look at the mechanism of the reaction and see that another molecule (the nucleophile) uses a pair of electrons to attack that carbon atom, and the leaving group bonded to that carbon atom gets substituted by the nucleophile, we know that it is definitely a nucleophilic substitution reaction. Alternatively, a faster way would be to directly identify a nucleophile, and this can be done especially when the reagent in the reaction is negatively-charged, making it highly likely to be a nucleophile. It is less clear for neutral compounds, unfortunately.
Nucleophilic substitutions can also occur for a benzene molecule, but this is usually obvious to tell, and it is likely that it will not be tested. The most obvious identifier that an aromatic substitution reaction is nucleophilic is when there is the presence of a nucleophile that attacks an aromatic carbon on the benzene. By the time we progress to the exam, it is likely that a large amount of progress has been made of nucleophilic substitution reactions in general, and that we can easily identify a nucleophile; when benzene is the reactant and a nucleophile is present, it is likely for a nucleophilic aromatic substitution reaction to occur (we will be making a separate article on the specific mechanisms and types of nucleophilic aromatic substitution reactions). In most cases, nucleophilic aromatic substitutions do not involve benzene itself, but instead involve monosubstituted or polysubstituted benzenes. In such cases, it is likely that nucleophilic attack will occur on the ipso carbon, i.e. the carbon already bonded to a substituent), forcing the initial substituent to leave. This will happen in the majority of nucleophilic aromatic substitution reactions, although exceptions are of course known.
Addition Reactions
We have previously discussed the compounds of addition reactions in three articles, the first of which is here. In those articles we noted that the definition of addition reactions is quite unclear, but the most general definition refers to addition reactions as simply involving the reaction of two molecules into a single product. However, on organic chemistry exams, I personally only saw one type of addition reaction: addition to multiple bonds, meaning that the reactants are molecules with double or triple bonds. Usually, the multiple bonds are carbon-carbon multiple bonds. Below, we show a general scheme of an addition reaction; notice that the multiple bond in ethene, the reactant, has been removed. When we see that one or more pi bonds which are present in the reactant are removed in the product (either from a triple bond to a double or single bond or from a double bond to a single bond), it is likely to be indicative of an addition reaction. This should be enough for us to distinguish addition reactions from substitution or elimination reactions.
Fig. 2: General scheme of an addition reaction.
Elimination Reactions
Elimination reactions are relatively similar to their addition counterparts; they are just the opposite. Usually, an elimination reaction removes a substituent (that may be a hydrogen) each from two neighboring atoms (usually both carbons), which results in the formation of a multiple bond between these two atoms. Elimination reactions are easy to spot, especially if we are already experienced at interpretation of skeletal formulae. If we know where the hydrogens are, we can see that some substituents that are present in the reactant are completely not present in the product, and this provides strong evidence that an elimination reaction has occurred. We should note that in elimination reactions, many different substituents may be eliminated, and this includes even hydrogens. Hydrogens are most commonly eliminated along with another larger substituent at the adjacent carbon, such as a hydroxyl substituent or a halide substituent. It is also possible for two hydrogens to be eliminated, one at each carbon, which is known as alkane dehydrogenation and a common method for alkene (olefin) preparation. The general scheme of an elimination reaction is shown below.
Fig. 3: General scheme of an elimination reaction.
Examples
Finally, we will do a walkthrough of one question with the format: classify the mechanism as a substitution, elimination, or addition reaction. Most conventional questions should be relatively easy to solve, based on personal experience as well as the tips outlined in this article. However, we will raise just one example so as to facilitate understanding of the article.
Fig. 4: What type of reaction is this?
Firstly, we see the reactants and the products. Immediately, we can discern that the chloride is substituted by the hydroxyl (OH) substituent. Clearly, this is a substitution reaction. This would all be good if the question did not require a more specific answer, but I believe it is generally better for us to list down more specific and accurate reaction types. So, is it an electrophilic or nucleophilic substitution? Now, we cannot simply rely on the reactants and the products. If we did so, we may have thought that the reaction is an electrophilic aromatic substitution reaction, but this is simply not true. Looking at the reagent in this reaction, it appears to be OH-. Since it is negatively charged, it is likely to be a nucleophile. So now, there are two options for the type of reaction: either a nucleophilic aliphatic substitution or a nucleophilic aromatic substitution. However, we see that the electrophilic site (bonded to the electronegative chlorine atom) is not an aromatic carbon, and this means that it is an aliphatic substitution. As such, the reaction is a nucleophilic aliphatic substitution.
Conclusion
So in this article, we have explained the three types of reactions, substitution, addition, and elimination reactions, as well as their defining characteristics. Hopefully, now we know how to classify the mechanism as a substitution, elimination, or addition reaction.