Types of Organic Reactions: Substitution Reactions Types, Example and Condensation reaction Example

We all know that proteins play a vital role in our life as they help in the growth and maintenance of our body. Proteins are made from the removal of water molecules from the various amino acids to form a long polypeptide chain constituting proteins. This is an example of a condensation reaction.

We can see dehydration of grapes is taking place which occurs by loss of water, the same happens in our chemistry world where dehydration reaction occurs by loss of water molecules from starting substrate (i.e. alcohol in our present case).

Organic chemistry is essential because life and all the chemical processes associated with life are majorly investigated in this segment of chemistry. These reactions are important in the manufacture of various compounds which play an active role in living beings. So let's study what all types of reactions are there in organic chemistry.

Table of content

• Introduction
• Types of reactions
• Substitution Reaction
• Elimination Reaction
• Addition Reaction
• Condensation Reaction
• Rearrangement Reaction
• Practice Problems
• Frequently asked questions-FAQs

Introduction

Chemical reactions involving organic molecules are known as organic reactions. Several of these reactions are correlated to functional groups. A detailed, step-by-step explanation of how an organic compound reaction occurs is called an organic reaction mechanism.

A reaction is a general explanation of how a reaction takes place. A mechanism is a detailed description of how the bonds are broken and in what sequence, which bond is formed and in what order at each phase of a chemical reaction.
It is a thorough description of how electrons flow during chemical processes.
In a reaction mechanism, all the reactants, intermediates and products are involved.

Types of Organic reactions

There are four different types of organic reactions that can occur. Let us understand them.

Substitution Reactions

In a substitution reaction, one atom or a group of atoms is replaced by another atom or group of atoms, resulting in the formation of a completely new chemical compound. 

Example: 
 X - Y + Z X-Z + Y

In the above reaction Y is substituted by Z. Such kinds of reactions in organic chemistry are said to be substitution reactions.

Substitution reactions are of two types:

1. Nucleophilic substitution reaction

If a substitution reaction is brought about by a nucleophile, then it is known as a nucleophilic substitution reaction. The general reaction represents the nucleophilic substitution reaction. 
R-lg + Nu   R-Nu + lg
Where Nu is Nucleophile and lg is the leaving group.

Nucleophilic substitution reactions can be further classified into three types:

(I) SN1 reaction:

The SN1 reaction is a unimolecular nucleophilic substitution reaction.

The SN1 reaction mechanism follows a step-by-step process wherein first, the carbocation is formed by the removal of the leaving group. Then the carbocation is being attacked by the nucleophile. Finally, the deprotonation of the protonated nucleophile takes place to give the required product. 

Example:

(II) S_N2 reaction:

The SN2 reaction is a bimolecular nucleophilic substitution reaction because the rate-determining step involves two reacting species, i.e. the substrate and the nucleophile. It is always a single-step reaction in which a bond is broken and a new one is created simultaneously.

The S_N2 reaction mechanism follows a single step in which a nucleophile attacks the substrate from the backside, which initiates the reaction. At an angle of 180⁰ to the carbon-leaving group bond, the nucleophile approaches the substrate through a transition state, the carbon-nucleophile bond is formed and the carbon-leaving group bond breaks at the same time. Now, on the other side of the carbon-nucleophile bond, the leaving group is forced out of the transition state, generating the corresponding product.

Example: Nucleophilic substitution of chloroethane with hydroxide as nucleophile.

Below is a diagram of the S_N2 reaction mechanism for the above reaction.

(III) S_Ni reaction:

In S_Ni, S stands for substitution, N stands nucleophilic, i stands for intramolecular. 

In the S_Nimechanism ( internal nucleophilic substitution), a part of the leaving group must be able to attack the substrate detaching itself from the rest of the leaving group in the process. The S_Ni reaction proceeds with the retention of configuration. 

Example: Reaction of alcohol with SOCl₂ Thionyl chloride (SOCl₂) converts primary and secondary alcohols to alkyl chlorides with the retention of configuration.

2. Electrophilic substitution reaction

If a substitution reaction is brought about by an electrophile, then it is known as an electrophilic substitution reaction.

The general reaction represents the electrophilic substitution reaction. 

R-lg ⁺ E+   R-E + lg

Where E⁺ is Electrophile and lg is leaving group.

Benzene being electron-rich, is susceptible to electrophilic attack, therefore it generally gives electrophilic aromatic substitution reactions.

Electrophilic substitution of benzene involves the reaction of an electrophile with an aromatic compound. Therefore, it is more precisely called an electrophilic aromatic substitution reaction, denoted by the term SNAr.

S_NAr of benzene takes place in three steps:

1. Generation of an electrophile
2. Formation of a carbocation intermediate
3. Removal of a proton from the carbocation intermediate

Various types of electrophilic substitution reactions are:

Halogenation, Nitration, Sulphonation, Friedel-craft alkylation, and Friedel-craft acylation. 

Example: In halogenation of benzene, reaction of bromine or chlorine occurs in the presence of a lewis acid to give the corresponding halogenated substitution products in good yield.

Elimination Reactions

In some organic reactions, atoms from two adjacent carbon atoms are eliminated to form a product containing stronger bond. Such organic reactions are known as elimination reactions. And E1 is a unimolecular elimination reaction.

As a result of the formation of numerous bonds at the same time, tiny molecules like H₂O, HCl, etc. are released as products.

^Example:

Converting ethyl alcohol to ethene is one of the common example of an elimination reaction.

In the above reaction H2O molecule is eliminated. 

E1, E2 and E1cB are the commonly studied elimination reactions. 

1. E1 elimination reaction

In E1 elimination, the leaving group leaves the substrate first to form a carbocation intermediate. Then, the proton abstraction takes place to form an alkene. Example: Dehydrohalogenation of an alkyl halide in the presence of water. 

2. E2 elimination reaction

In E2 elimination, two groups/atoms depart simultaneously from adjacent carbons along with the proton being abstracted by a base. And E2 is a bimolecular elimination reaction.

Example: Dehydrohalogenation of an alkyl halide in presence of base, RONa. 

3. E1cB elimination reaction

In E1cB, E stands for elimination, 1 for unimolecular, and cB for conjugate base. In E1cB, the proton is abstracted to form the conjugate base. The anion that results is stable enough to exist because it can

 be delocalised on to the electron-withdrawing group. Although the anion is stabilized by the electron-withdrawing group, it still prefers to lose a leaving group and become an alkene.

Example:

Conditions for E1cB Elimination

There are two conditions for any molecule to give E1cB elimination reaction:

1. A good electron withdrawing group must be present at the β-position to the leaving group. 

Example: Carbonyl (- C = O), nitro (- NO₂), cyano (-C = N), sulphonyl (- SO₂ -),

Phenyl (- Ph), ester (- COOR), and other carbonyl stabilizing groups. 

2. Generally, the poor leaving group shows E1cB reaction. 

Example: ─F, ─OH, etc.

Note-There is a subclass of elimination reactions known as condensation reaction. 

Condensation reaction

 Condensation reaction are those reactions in which small molecules like H₂O, NH₃, etc. are removed when two similar or different reactants are combined. It is a versatile group of reactions that may occur in acidic or basic environments and in the presence of a catalyst.

The condensation reaction is important in the formation of peptide bonds from amino acids and the production of fatty acids as they play a vital role in our life. The aldol condensation, Claisen condensation, Knoevenagel condensation, and Dieckmann condensation (intramolecular Claisen condensation). 

Example: Peptide formation from amino acids.

Addition Reactions

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. An addition reaction is just the reverse of an elimination or reduction reaction. For example, when HCl is added with ethylene in the process shown below, it produces ethylene chloride.

HCl + CH₂ = CH₂ → CH₃CH₂Cl

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.

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 carbocation intermediate. During this reaction, a carbocation is formed on the most stable carbon in the compound when the π electron attacks an electrophile. This is followed by an attack on the carbocation by the nucleophile leading to the formation of the required product. 

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

One such example is of addition of HCl to alkene. 

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.

Rearrangement reaction

Organic reactions in which atoms, groups (alkyl or aryl), double bonds, or functional groups migrate within the molecule are known as rearrangement reactions.

It is a broad category of chemical processes in which the carbon atoms in a molecule are rearranged to produce a structural isomer of a parent molecule. There are also intermolecular reorganizations of atoms.

Some rearrangement reactions are Beckmann rearrangement, Baeyer–Villiger oxidation, rearrangement of carbocation. 

In rearrangement of carbocation, there are two types rearrangement:

1) Cyclic rearrangement: They are of two types: Ring expansion and ring contraction. 
Example: Ring contraction will only happen if a more stable carbocation is formed after contraction. Thus ring contraction occurs in order to form the most stable carbocation. A cyclopropyl methyl carbocation is formed, which is highly stable.

2) Acyclic rearrangement: They are of three types: 1,2-hydride shift, 1,2-alkyl shift, and 1,2-aryl shift
Example- In the below example 1,2 methyl shift occurs to form more secondary carbocation from unstable primary carbocation.

Practice problems

Q1. Which of the following compounds give addition reactions?

1. Alkene
2. Alkyne
3. Carbonyl compounds 
4. All of the above

Answer: 4 

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

Q2. Addition of electrophile to alkenes gives which of the following intermediate?

1. Carbocation 
2. Carbanion
3. Free radical
4. None of the above

Answer: 1

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

Q3. What are the characteristic features of a free radical?

1. Presence of an unpaired electron
2. Presence of an odd number of electrons
3. Produced by homolytic fission of a bond
4. All of the above

Answer: 4

Solution: Free radical has unpaired electrons, so option A is correct.
Since a free radical has an unpaired electron, the total number of electrons must be odd. So, B is a correct option.

Since radicals are produced when a covalent bond is cleaved homolytically, So D is a correct option. Hence, options (B), (C), and (D) are the correct answers.

Frequently Asked Questions-FAQs

Question 1. Is arenium intermediate formed in the electrophilic substitution of benzene, aromatic in nature?
Answer: Sigma complex/arenium ion is not aromatic due to the presence of an sp³ hybridized carbon. To restore the aromatic character again, σ complex loses H⁺ from the sp³ hybridized carbon.

Question 2. Why do alkenes give addition reaction?
Answer: Alkene consists of a strong sigma bond(bond energy = 397 kJ mol^-1) and weak pi bond(bond energy = 284 kJ mol^-1). Because of the loosely held pi electrons, alkenes undergo addition reactions.

Question 3. Which all compounds give addition reactions?
Answer: Compounds like alkene, and alkyne give electrophilic addition reactions and compounds like carbonyl give nucleophilic addition reactions.

Question 4. Name a reaction which involves a free radical.
Answer: Anti-Markovnikov's addition of HBr or peroxide effect or kharasch effect involves a free radical mechanism.

Question 5. Give one name reaction which involves the carbocation intermediate formation.
Answer: In Wagner-Meerwein rearrangement carbocation is formed as an intermediate.

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