Introduction
When drafting this post, I honestly had no idea what to put as the title. In essence, what this article looks at are reactions which occur by mechanisms of addition, such as electrophilic addition, nucleophilic addition, or other mechanisms which we have highlighted in our previous post. However, that would prove to be too long a title for an article.
I also thought about simply naming the article ‘Addition Reactions’, but this fails for a few reasons. Firstly, it is too short, and secondly, because it is short, it is also vague. Addition reactions could refer to the topic of addition reactions, which includes the mechanisms, reactivity and all key aspects. It may also describe reactions of addition, which, of course, is what we are looking for.
Further thoughts on this would not be productive. Eventually, I settled on the name ‘Addition Reactions of Multiple Bonds’. This title is able to describe specifically what the article would be elaborating upon, and also makes it clear that we are not considering addition reactions as a whole topic.
So, let us begin to look at the addition reactions of multiple bonds. The simplest way for us to look at these addition reactions is to classify them. In addition reactions, two sites are attacked, and the products of the reaction are different depending on which attacking reagents attack. A good way to classify reactions would be by which reagents attack the multiple bond.
A lot of reactions proceed with a hydrogen, specifically, a proton, as the attacking reagent in the first step. Most of these reactions would then be electrophilic additions, because a proton is an electrophile, and the second step would require a nucleophile. Thus, whether hydrogen adds to one side of the multiple bond is a good starting point for us to classify addition reactions. We will look at how such a mechanism works first (Fig. 1) before we move on to study the reactions in detail. Note that it is H3O+ and not H+ that is actually involved.
Fig. 1: Electrophilic addition mechanism involving protons.
In general, there are two options, or ways, for hydrogen to add to one side of a multiple bond. The first way is for the attacking reagent itself to be a source of that proton. Of course we should be familar with the hydrogen halides. These are acids and therefore sources of protons; they are able to dissociate in solvent to form H+ (or more specifically H3O+ ions) and X- ions.
The attack by H+ results in the π bond being broken and a carbocation being formed. This carbocation has to be attacked by a nucleophile, and a suitable nucleophile in the reaction mixture is the X- anion, which attacks the carbocation to form the stable, neutral product, which is an alkyl bromide.
We note that it is also possible for some not-so acidic reagents, such as H2O, to add to multiple bonds (this produces an alcohol, as we will see later). The reaction typically calls for an acid catalyst to expedite the reaction, with one common acid catalyst being H2SO4 (sulfuric acid).
As we may have guessed, the H2SO4 or other acid catalyst is present as the source of the proton, the electrophile which attacks the multiple bond first. Without this acid catalyst, there would be very little protons, only being formed by the autoionisation of water, thus the reaction would be very slow. After the carbocation is formed, the oxygen of the water molecule can then attack it and form a bond to it. Subsequently it is deprotonated to form the alcohol. Another mechanism can also take place simultaneously, involving the acid catalyst, which we will discuss later.
Addition of Oxygens
With the examples above of electrophilic attack by protons, it makes sense to begin with these reactions first. The first reaction we will discuss would be addition of hydrogen halides to a multiple bond. This can happen with both alkenes and alkynes. For alkenes, simple addition of the hydrogen halide is enough for reaction to occur, forming the alkyl halide.
The mechanism for the above reaction has already been discussed in the introduction. In that mechanism, the Markovnikov product is predominantly formed, because of stabilization of the carbocation by inductive effects. When the proton attacks at the less substituted side, the carbocation will be formed at the more substituted side. The more substituted side usually has more electron-donating alkyl groups, which can stabilize the carbocation formed.
Since the nucleophile attacks the carbocation directly, it is very likely that the nucleophile will be attached to the more substituted carbon. This allows the product to obey Markovnikov’s rule. However, a different mechanism is followed when peroxides are added. We may already have guessed that the mechanism is free radical addition, because peroxides are able to spotaneously cleave homolytically to form radicals.
When peroxides are added, addition of the hydrogen halide occurs via the free radical addition mechanism, and the anti-Markovnikov product is formed instead. Although the electrophilic addition mechanism may still be operating, a far higher percentage of the free radical addition product is produced.
Fig. 2: Addition of hydrogen halides.
Next, let us take a look at the addition of hydroxyls to double bonds (a hydration reaction). This is relatively similar to the mechanism of hydrogen halides, but another mechanism may be faster and is sometimes followed. A general way for the preparation of alcohols invovles the addition reaction between alkenes, water (H2O) and sulfuric acid (H2SO4). Sulfuric acid is the acid catalyst, as we have noted previously.
Sulfuric acid provides the proton which performs electrophilic attack upon the multiple bond, similarly to that of the first reaction (thus the product obeys Markovnikov’s rule). This forms a carbocation which is attacked by a nucleophile in the second step. At this point, the question arises: what nucleophile is present to attack the carbocation?
In most cases, the carbocation is unstable enough such that even the water molecule (which does possess the electronegative oxygen atom) is enough to attack it and form a bond to it. It can then be subsequently deprotonated to form the final compound, the alcohol. However, there is evidence that another mechanism may operate simultaneously.
In this mechanism, the nucleophile is not the water molecule, but rather the HSO4- ion, which is the conjugate base of H2SO4. It is formed after sulfuric acid donates a proton. Since it carries a negative charge, it makes sense that it is nucleophilic. It may attack the carbocation (Fig. 3), which seems to result in a different product.
However, the same alcohol is still formed! The substituent is subsequently hydrolyzed by the many water molecules in the reaction mixture, which still results in the same intermediate, with a water molecule bonded to the carbon atom. Subsequent deprotonation of this still forms the same alcohol.
Fig. 3: Second step of the addition of hydroxyls.
A similar reaction involves the addition of hydroxyl substituents to alkynes. Based on the above mechanism, we would expect an enol (an alkene with a vinylic hydroxyl substituent) to be formed, but in reality this usually tautomerizes to the ketone or aldehyde, which should be generally more stable. The alkyne reacts with mercuric sulfate (HgSO4) to generate the enol.
There is only one alkyne that can react to form aldehydes, although we may believe that all terminal alkynes may do so. Only acetylene may react to form aldehydes, as in its case both carbons are terminal. In any other terminal alkyne, the nucleophile will preferably attack the internal carbon, because of carbocation stabilization, and it thus obeys Markovnikov’s rule (the product formed would be a methyl ketone). It is possible for anti-Markovnikov hydration if different reagents are used.
The mechanism of this reaction is worth discussing because it is quite interesting. The Hg ion here possesses a 2+ charge as a free atom. When dissociation occurs to HgSO4, the Hg ion may form bonds to both carbons in the triple bond (Fig. 4). This will break one π bond. Attack then occurs by nucleophiles such as a water molecule, breaking one of the Hg-C bonds. Tautomerism occurs in the final step, forming the ketone (or aldehyde).
Fig. 4: Mercury-alkyne complex.
We will also take a look at another addition reaction involving oxygen nucleophiles. It is the addition of OR (alkoxy) substituents to multiple bonds. The reagents used are an alcohol as well as the an catalyst. Here the attacking species is the oxygen atom of the ROH group; it is comparable to the addition of hydroxyls, except that the intermediate formed after the second step is RR’OH instead of RH2O+.
The reaction proceeds via acid catalysis, as is typically of many electrophilic addition reactions, because it provides the protons needed for the first step, where electrophilic attack occurs. Although the alcohol itself is also an acid, it is still weak and the reaction would proceed at a slow pace without the presence of a strong acid, which is why the acid catalyst is required.
In the first step of the mechanism, the multiple bond is attacked by the proton. This forms the carbocationic intermediate that we have already seen many times in previous reactions. The carbocation is then attacked by the nucleophile, in this case the electronegative oxygen atom of ROH. The subsequent intermediate is then deprotonated at the oxygen to form the final product, an ether. The intramolecular variation is known and results in cyclic ethers.
The last reaction we will discuss of oxygens is the addition to carboxylic acids. Usually it is the double bond that is attacked. The reaction follows acid catalysis and obeys Markovnikov’s rule, and the typical electrophilic mechanism is followed except that attack is by the carboxylate ion (which has a negatively-charged oxygen). The same reaction may be performed on triple bonds, and the product is an enol ester. The reagent HgSO4 may be employed to faciltiate the reaction by forming the same complex as in Fig. 4.
We leave out the reactions of other similar compounds, such as addition of thiols and amines, because they also follow the same electrophilic addition mechanism.
Hydrogenation
We separate hydrogenation, which appears to be a single reaction, from the others in this article because it follows a different mechanism, which seems surprising because it involves hydrogens. Hydrogenation in our case describes the reduction of multiple bonds by the breaking of a π bond. However, it can sometimes reduce other functional groups depending on the reagents used.
In hydrogenation reactions, one of the reactants is molecular hydrogen (H2). What varies between different hydrogenation reactions is the catalyst used. There are two classes of catalysts - they are heterogeneous and homogeneous catalysts. Heterogeneous catalysts refer to catalysts that are not in the same state as the solvent, while homogeneous catalysts refer to catalysts that are in the same state as the solvent.
In particular, we will be looking at the mechanism occurring when heterogeneous catalysts are used. Given that the heterogeneous catalyst is in a different state as the solvent, it is not possible for a direct reaction to occur. Instead, something special known as adsorption happens. This means that the multiple bond ‘sticks’ to the solid catalyst. It is not clear whether a chemical bond is indeed present between the catalyst and the multiple bond.