Addition reaction: Definition, Electrophilic addition reaction, Nucleophilic addition reaction, Practice problems, FAQs

As the word “addition” suggests adding a moiety to another moiety. Let us consider an analogy to understand this better. Until your schooling, your world was your family but as soon as you entered college you spent more time with new friends. So, now being in college you get to see your parents less, but yet your bond is stronger and at the same time you are making a new bond with your college people. So, from the knowledge of organic chemistry can you think of compounds which have such a tendency to break its bond with previously bonded atoms, yet not completely break and at the same time form a new bond with other atoms? In organic chemistry, compounds containing a double bond, triple bonds like alkene(-C=C-), alkyne (-CC-), and carbonyl compounds(-C=O) are known to show addition reactions. Let's see what all types of addition reactions are there. 

Table of content

• Definition
• Electrophilic addition reaction
• Nucleophilic addition reactions
• Free Radicals addition reaction 
• Cycloaddition reaction
• Practice problems
• Frequently Asked Questions-FAQs

Definition We can define an addition reaction as the reaction involving the addition of two reactants resulting in a larger product, and the product of the addition reaction is known as ‘Adduct’. The chemical compounds possessing multiple bonds undergo an addition reaction, as the basic principle behind the reaction is the compound’s ability to break double or triple bonds. 

Depending on the nature of the reactant (alkene, alkyne, carbonyl) and the substrate(electrophile and nucleophile) addition reactions are majorly of two types, Electrophilic and nucleophilic addition reaction. Also, some other types of addition reactions are Free radical and Cycloaddition reactions. Let's study all these types of addition reactions in detail. 

Electrophilic addition reaction

The process of electrophilic addition with an alkene takes place in such a manner that the added electrophile generates the most stable intermediate. During this reaction, the most stable carbocation is formed in the compound when the π electron attacks an electrophile followed by an attack on the carbocation by the nucleophile which leads to the formation of the final product. 

The general mechanism of electrophilic addition in alkenes is given below:

One such example of addition of HCl to alkene is: 

The general mechanism of electrophilic addition in alkynes is given below:

One such example of addition of H2O to alkyne is: 

Nucleophilic addition reaction

Nucleophilic addition reaction involves a chemical compound possessing an electron-deficient double or triple bond or an electron deficient π bond that reacts with a nucleophile (an electron-rich reactant). Thus, generally the double bond disperses and results in the formation of two new σ bonds. Such reactions are shown by carbonyl compounds.

 Free Radicals addition reaction

This concept can be easily understood by referring to the following example of the Free Radical Addition reaction of HBr to alkenes.

 Mechanism Peroxides contain an O-O bond, which is comparatively weaker than a C-H bond. When the compound is provided sufficient heat, a homolytic fragmentation of this O-O bond takes place leaving behind an unpaired electron on each atom. The alkoxy radical produced is reactive enough to extract the hydrogen from the compound H-Br. This results in the formation of a Bromine radical that will now add onto the alkene in a manner that results in the formation of the most stable radical. Now, bromine gets attached to the least substituted carbon of the alkene followed by the removal of hydrogen from H-Br, thus, releasing bromine radical. This cycle goes on.

Cycloaddition reaction

In a cycloaddition reaction, two molecules containing bonds react to form a ring, at the expanse of the bonds. This is the most common pericyclic reaction observed. A Cycloaddition reaction is characterized by Δσ = 2, (where Δσ is the difference between the number of sigma bonds in the product and the number of sigma bonds broken in the reactant). The most well-known cycloaddition process is the Diels-Alder reaction, which is shown here. Below given is the general example of cycloaddition reaction. 

Practice problems

Q 1. Which of the following compounds give addition reactions?

a. Alkene
b. Alkyne
c. Carbonyl compounds 
d. All of the above

Answer: D
Compounds having unsaturation are said to give addition reactions. Example- alkene, alkyne, carbonyl compounds. 

Q 2. Addition of electrophile to alkenes gives which of the following intermediate?

a. Carbocation 
b. Carbanion
c. Free radical
d. None of the above

Answer: A
During electrophilic addition on alkenes, a carbocation is formed on the most stable carbon in the compound when the π-bond attacks an electrophile. 

Q 3. During free radical reaction, which type of cleavage occurs?

a. Homolytic cleavage
b. Heterolytic cleavage
c. Both of the above
d. All of the above

Answer: A
During the free radical reaction, -C=C- along with other substrates like Br2, HBr etc. undergo homolytic cleavage.

Q 4. During cycloaddition the difference in the number of sigma bonds of reactant and product is:

a. 1
b. 2
c. 3
d. 4

Answer: B
Cycloaddition reaction can be characterized by Δσ = 2, (where Δσ is the difference of the number of sigma bonds formed in the product and the number of sigma bonds that were broken in the reactant).

Frequently Asked Questions-FAQs

Q 1. Why do alkenes give addition reaction?
Answer: Alkene consists of a strong sigma bond(bonding energy = 397 kJ mol^-1) and weak pi bond(bonding energy = 284 kJ mol^-1). Because of the loosely held pi electrons, alkenes undergo addition reactions.

Q 2. Why are electrophilic addition reactions named so?
Answer: Electrophilic addition reactions are given by electron-rich species like alkene and alkyne. They have a tendency to attract electron deficient species towards them. Therefore, whenever they react with species like HCl, Br₂, etc. the first step which is the rate-determining step involves the attack of the electrophile, H⁺, Br⁺ respectively. Due to this reason, they are known as electrophilic addition reaction.

 Q 3. Why do carbonyl compounds give nucleophilic addition reaction?
Answer: Carbonyl compounds consist of -C=O bond. Carbon and oxygen develop a partial positive and negative charge respectively because of the difference in electronegativity. Thus, carbon having a partial positive charge is prone to be attacked by some electron rich nucleophile. 

Q 4. What are the reaction conditions of cycloaddition reaction?
Answer: Cycloaddition reactions are a type of pericyclic reactions. Such reactions occur in the presence of heat or sunlight. They don’t require any solvents for reactions to take place. 

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