There are a great many molecules in organic compounds that have double bonds. When these molecules react with other reagents, addition reactions occur. When addition reactions occur, the double bonds of the alkene are transformed into single bonds, giving the product.

In organic chemistry, the addition reaction of alkenes is the basis of the reaction. However, there are several types of addition reactions of alkenes, each with a different reaction mechanism. Therefore, it is necessary to distinguish how organic chemical reactions proceed.

In addition reaction of alkenes, we have to learn many reactions, such as the Markovnikov’s rule, anti-Markovnikov’s rule, hydroboration, and halogen addition.

If we just go through the reasons why organic reactions occur in sequence, we will understand why and how the addition reaction of alkenes occur. In this section, we will review the mechanism of addition reactions to double bonds.

Electrophilic Addition Reaction Caused by Double Bonds

One of the organic chemical reactions that occur for alkenes (double bonds) and alkynes (triple bonds) is an addition reaction.

In organic chemistry, there are several reaction mechanisms. Among them, an addition reaction is an organic chemical reaction in which all of the reactive compounds are contained in a molecule. The following synthetic reactions are examples of addition reactions.

These are all addition reactions. All the reacted compounds are present in a single molecule. That is why they are called addition reactions.

Addition reactions are also referred to as electrophilic addition reactions. Double and triple bonds hold more electrons than single bonds. They are electron-rich and provide electrons to an electron-deficient reagent (electrophilic reagent) to cause a chemical reaction.

Reaction Mechanism of the Addition Reaction of HX (HBr) to Alkenes

So what is the reaction mechanism for these addition reactions to alkenes? Since the HX (hydrogen halide) addition reaction is the simplest of the electrophilic addition reactions to alkenes, it is common to explain this reaction mechanism first.

X stands for halogen. Therefore, understand that HX is HCl (hydrogen chloride) and HBr (hydrogen bromide). In addition reactions to alkenes, HBr in particular is frequently used as an example.

The presence of double and triple bonds, as mentioned above, results in an abundance of electrons. Double bonds are formed by π bonds (π-electrons); π bonds are weak bonds and can give electrons to other molecules.

Halogens, on the other hand, are known as atoms with strong electronegativity. Therefore, when a hydrogen bond is attached to a halogen, the molecule becomes highly polarized. HCl (hydrogen chloride) and HBr (hydrogen bromide) have both positive and negative charges within the molecule.

The hydrogen atoms of these HCl and HBr are electron deficient because the electrons are attracted to the halogens. Since HCl and HBr are deficient in electrons, they can be considered as electrophiles.

These electrophiles and alkenes react chemically with each other. Specifically, the π-electrons forming the double bond attack the hydrogen atoms (electron-deficient atoms). A proton (hydrogen) is added to the alkene. The result is a carbocation as an intermediate.

For example, when an alkene reacts with hydrogen bromide, the result is as follows.

The bromine ions just generated then attack the carbocation. The bromine ion produced is negatively charged and also has an unshared electron pairs (lone pair). In other words, they have a lot of electrons.

In contrast, the carbocation is positively charged and requires electrons. Therefore, the bromine ion attacks and adds to the carbocation. Thus, an alkyl halide compound is formed.

Markovnikov’s Rule (Markovnikov Addition): Stability of Carbocation

The addition reaction of HX to alkenes proceeds in this way. For the sake of clarity, we have just used symmetrical alkenes to explain this. However, asymmetric alkenes seem to produce more than one compound.

For example, in the following case, which compound will be formed?

However, in the electrophilic addition reaction to alkenes, the compounds that are formed are determined. In the addition reaction, hydrogen is added to the lesser carbon atom of the alkyl group. This rule of thumb is called the Markovnikov’s rule (Markovnikov addition).

Why does the Markovnikov’s rule occur? The reason for this is the carbocation of the intermediate. The carbocation has a ranking of stability, as shown below.

Therefore, when the intermediate carbocation is formed, the tertiary carbocation is preferentially formed. If the tertiary carbocation is not formed, the secondary carbocation is formed.

checking the addition reaction just described, the intermediates are as follows.

If we check the intermediates of carbocation, one produces tertiary carbocation and the other produces primary carbocation.

In terms of carbocation stability, the tertiary carbocation is the most stable carbocation. Therefore, tertiary carbocations are preferentially formed instead of primary carbocations. As a result, the addition reaction to alkenes proceeds according to the Markovnikov addition.

-A Different Rule than the Saytzeff Rule

There are several rules in organic chemistry. One of them is the Markovnikov’s rule. Markovnikov addition that should be taken into account in addition reactions of alkenes.

However, some people confuse the Markovnikov’s rule with the Saytzeff rule. The Saytzeff rule is a rule that should be taken into account for elimination reactions. Elimination reactions are representative of synthetic reactions to create alkenes (double bonds) from alkanes. It is important to understand that the Markovnikov addition and the Saytzeff rule are completely different rules.

Proton Transfer in the Carbocation Rearrangement

Understanding the stability of the carbocation also helps us to understand other organic chemical reactions. We can understand why carbocation rearrangement occurs in addition reactions as well as the Markovnikov’s rule.

When an addition reaction occurs for a double bond, a proton (hydrogen atom) can be transferred to a neighboring carbon atom. This is called a carbocation rearrangement. As a result of the proton transfer, different compounds can be produced.

An example is the following organic chemical reaction.

When an addition reaction occurs according to the Markovnikov’s rule, it would seem that only the secondary product shown in the above figure would be produced. But in fact, it produces different compounds. Why does this kind of organic chemical reaction occur? The reason for this is the carbocation rearrangement.

The carbocation wants to be in a more stable structure. Therefore, after the carbocation intermediate is formed, a proton (hydrogen atom) is transferred to the neighboring carbon. In the present synthetic reaction, the secondary carbocation is converted to a tertiary carbocation by the carbocation rearrangement.

The main product can then be obtained by the attack of the halogen on the tertiary carbocation.

If a more stable carbocation is produced, a carbocation rearrangement occurs. Carbocation rearrangements also only involve the transfer of a neighboring hydrogen atom. It is important to understand that protons (hydrogen atoms) do not move from every part of the molecule.

Addition of Water to Alkenes (Hydration)

After learning the reaction mechanism with HX, you will be able to understand the addition reaction of water. Water can be thought of as a type of reagent, and the addition of water to an alkene occurs.

However, water is a weak acid and cannot produce carbocation. In other words, the addition of water (hydration) to alkenes does not occur. Therefore, by adding sulfuric acid (H2SO4) as an acid catalyst, the water addition reaction can proceed.

In the presence of an acid catalyst (e.g., sulfuric acid), an oxonium ion (H3O+) is generated. Oxonium ions are strong acids, so they can give H+ to alkenes.

The addition reaction then proceeds according to Markovnikov’s rule of regioselectivity. Understand that the addition reaction of water is the same as the addition reaction of HX, except that an acid catalyst is required.

Since it is a strong acid-catalyzed chemical reaction, there is no OH in the aqueous solution. Therefore, only oxonium ions (H3O+) or water molecules are involved in the chemical reaction in this hydration reaction.

Reaction Mechanism of Addition Reaction by Alcohol

If we understand what we have seen so far, we can understand that the reaction mechanism is almost the same for both the addition reactions of HX and water. In the case of water, an acid catalyst is required.

Once we understand this fact, we can understand the reaction mechanism of the addition reaction with alcohol. We have just explained the addition reaction (hydration) with water. If alcohol such as methanol or ethanol is used as a solvent instead of water, the addition reaction with the alcohol will proceed.

Like the water addition reaction, the addition reaction of alcohol uses an acid catalyst. Then, the reaction between the alcohol and the acid catalyst will cause H+ to be bonded to the alcohol.

Thereafter, the addition reaction of alcohol proceeds by exactly the same reaction mechanism as the addition reaction of water. It is as follows.

Once you understand the addition reactions to alkenes, you can synthesize any compound you want. Understand that you can change the product you get by simply changing the reagent you react with.

Hydroboration-Oxidation Reaction: Anti-Markovnikov Rule

We have discussed chemical reactions that follow the Markovnikov addition. In all of these synthetic reactions using water and alcohols, the Markovnikov’s rule takes precedence, and therefore, multi-substituted alkanes are synthesized. However, is it possible to synthesize alkanes with fewer substituents?

There is an addition reaction that does not follow the Markovnikov addition, namely the anti-Markovnikov rule. There are several types of anti-Markovnikov rule, and a typical organic chemical reaction is the synthesis of alcohols by hydroboration.

So what kind of chemical reaction is hydroboration, which is an anti-Markovnikov rule? Hydroboration is a reaction in which a borane (BH3) is added to an alkene.

Borane is known to be a Lewis acid. Boranes have empty p-orbital and can accept electrons. Borane is a Lewis acid and is a powerful electrophilic reagent that accepts electrons present in the double bond of alkenes.

However, when borane is added to an alkene, an opposite addition reaction to the Markovnikov addition occurs. It looks like the following.

When the borane undergoes an addition reaction, the intermediate carbocation is not produced. Therefore, the stability of the carbocation becomes irrelevant and the reaction becomes anti-Markovnikov rule.

Why does the anti-Markovnikov rule apply to the transition state? The reason is related to the steric hindrance. A carbon atom with fewer substituents has less steric hindrance. Therefore, boron atoms bond with carbon atoms that have fewer substituents.

Since the addition reaction takes place under conditions of low steric hindrance, the hydroboration is an anti-Markovnikov rule. Since no carbocation is produced, the stability of the carbocation is not relevant, as mentioned above. Naturally, the carbocation rearrangement does not occur. Due to steric hindrance, an anti-Markovnikov rule occurs.

After the addition of a borane by the anti-Markovnikov rule, the alkylborane is oxidized with hydrogen peroxide (H2O2).

Hydroboration is frequently used in alcohol synthesis. Alkylboranes are synthesized by hydroboration and then oxidized by hydrogen peroxide, which converts the alkylboranes to hydroxy groups (-OH). Thus, alcohol synthesis is possible.

Anti-Addition Reactions with Halogens

After understanding what we’ve discussed so far, we’ll move on to explain the addition reaction with halogens. Why is the addition reaction of alkenes with halogens so important? It is because stereochemistry is involved. Although we have not considered stereochemistry in our previous explanations, let’s consider stereochemistry using halogens as an example.

When halogens such as Cl2 and Br2 cause addition reactions, they are known to cause anti-additions. When they are added on the same side, it is called syn-addition. When the addition reaction occurs on the opposite side, it is called anti-addition.

Halogens are always anti-addition, not syn-addition. Why is it an anti-addition?

When a halogen is added to a double bond, the reaction proceeds without producing a carbocation. Like hydroboration, the reaction proceeds at the same time. The reaction of halogens also produces a cyclic ionic intermediate. For example, when bromo is reacted, cyclic bromonium ions are produced as intermediates.

The bromo ion then attacks from the other side. This allows the synthetic reaction to proceed by anti-addition rather than syn-addition. Since the halogen ion attacks stereoselectively, it should be understood that the chemical reaction takes stereochemistry into account.

As mentioned above, no intermediates of the carbocation are formed. Therefore, the addition of halogens does not cause a carbocation rearrangement.

-When a Chiral Center Is Created, Stereoselectivity Is Important

It is important in stereochemistry that the addition reaction by halogen is anti-addition. If a chiral center is formed as an optical isomer, we must consider what kind of stereochemistry the molecule will be in.

If a carbocation intermediate is formed, the compound with the chiral center will be racemic. This is because after the carbocation is formed, the compound (nucleophile) attacks it from above or below. Because of racemization, two optical isomers are mixed together in a compound with a chiral center.

On the other hand, in the addition reaction of halogens, as mentioned above, only anti-addition occurs. Therefore, no racemization occurs and only certain compounds can be obtained, even if there is a chiral center.

Water or Alcohol Is Used as a Solvent to Synthesize Trans Compounds

The application of this halogen reaction allows us to synthesize alcohol and alkoxy compounds. Anti-addition makes it possible to synthesize compounds while obtaining trans compounds.

In these syntheses, water or alcohols (e.g., methanol, ethanol, etc.) are used as solvents. That way, the water or alcohol, rather than the bromonium ion, attacks the intermediate, giving you the alcohol or alkoxy compound.

If water is used as the solvent, the hydroxy group (-OH) will become a substitute group. On the other hand, if methanol (CH3OH) is used as the solvent, the methoxy group (-OCH3) will be added. Depending on the solvent used, the compound you want to synthesize can be changed.

Why does water or methanol attack the molecule and not the bromonium ion? This is because water or methanol is used as the solvent.

As a solvent, there is an exceptionally large amount of water (or alcohol) in the surroundings of the compound. So it is more likely that the solvent will nucleophilic attack rather than the low concentration of bromonium ions. This is why we are able to obtain these compounds.

Hydroboration and Hydrogen Catalytic Reduction are Syn-Addition

In addition, when we consider stereochemistry, the synthesis of alcohols by hydroboration and oxidation is understood to be syn-addition. When a borane is used in the synthesis, it causes syn-addition without a carbocation intermediate. Subsequently, the alcohol is obtained by oxidation.

Another addition reaction, catalytic reduction with hydrogen, is known to be a syn-addition.

In catalytic reduction, the air is filled with hydrogen and carbon palladium is added to the solution to cause the reduction reaction. The hydrogen is adsorbed on the palladium carbon, and at this time, syn-addition occurs.

Since only syn-addition proceeds, racemization does not occur even in the presence of a chiral center. Also, the hydrogen addition reaction proceeds without the carbocation intermediate.

In the absence of a chiral center, there is no need to take into account the difference between the syn-addition and anti-addition reactions. However, if a chiral center is formed after the reaction, the stereochemistry becomes important.

Learn Why Addition Reactions of Alkenes Occur and Regioselectivity

In high school chemistry, you only need to learn the compounds that are formed after an organic chemical reaction occurs. However, in college organic chemistry, you have to understand the reaction mechanism. In such cases, the addition reaction of alkenes is the first step in organic chemistry.

Even though the content is basic, there are various types of addition reactions in organic chemistry. There are many organic chemists who use addition reactions when doing synthetic reactions. This is because it is possible to synthesize all kinds of compounds simply by changing the reaction reagent.

There are many types of reactions. In addition, you need to understand the Markovnikov’s rule and the difference between syn-addition and anti-addition. Furthermore, you should learn even regioselectivity. However, the reaction mechanisms are all similar.

Let’s understand how each reaction proceeds. We have described the addition reactions here in the simplest way possible to make them easy to understand, so you should try to understand these basics of organic chemistry.