Types of Organic Reactions: Reactions take place all around us. There are different types of reactions in organic chemistry. Organic reactions are defined as chemical reactions in which organic compounds are involved. Organic compounds are molecules with strong covalent bonding. Organic reactions are used to construct new organic molecules in organic synthesis.

Several man-made chemicals, like plastics, drugs, food additives, and fabrics, depend on organic reactions. Some of the types of organic reactions are substitution, addition, elimination, rearrangement, etc. This article will provide detailed information about types of organic reactions, their mechanism, examples, etc. Scroll down to learn more!

Types of Organic Reactions

Organic reactions are chemical reactions in which organic compounds are involved. Because the number of organic compounds is so large, the chemical reactions that occur are also expected to be quite diverse. However, all organic reactions have been classified into different types.

1. Substitution reactions
2. Addition reactions
3. Elimination reactions
4. Rearrangement reactions
5. Condensation reactions
6. Isomerization reactions
7. Pericyclic reactions
8. Polymerization reactions

Though the organic reactions are classified into different types, the major types of organic reactions are as follows:

1. Substitution reactions
2. Addition reactions
3. Elimination reactions
4. Rearrangement reactions

Substitution Reactions

The direct replacement (displacement or substitution) of an atom or group of atoms in an organic molecule by another atom or group of atoms without any alteration in the remaining part of the molecule is known as a substitution reaction. The product obtained as a result of the substitution is called the substitution product and the new atom or group of atoms that enter the molecule is called the substituent.

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For example,

\(\begin{gathered} {\text{C}}{{\text{H}}_3} – {\text{C}}{{\text{H}}_2} – {\text{Cl}} + {\text{KOH}}\left( {{\text{aq}}} \right) \to {{\text{C}}_2}{{\text{H}}_5}{\text{OH}} + {\text{KCl}} \hfill \\ {\text{Ethyl}}\,{\text{Chloride}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{Ethyl}}\,{\text{alcohol}} \hfill \\ \end{gathered} \)

Types of Substitution Reactions

Depending on the nature of the attacking reagent, substitution reactions are further categorized into three categories they are electrophilic, nucleophilic, and radical.

A. Nucleophilic Substitution Reactions

When the attacking reagent is a nucleophile (\({\text{N}}{{\text{u}}^ – }\) or \({{\text{Z}}^ – }\)), the substitution reaction is called a nucleophilic substitution reaction. These reactions are typically of alkyl halides.

For example, hydrolysis of an alkyl halide with an aqueous base;

Mechanism of Nucleophilic Substitution Reactions

There are two types of nucleophilic substitution reactions: They are

a) \({{\text{S}}_{\text{N}}}{\text{2}}\) (Substitution, nucleophilic, bimolecular)
b) \({{\text{S}}_{\text{N}}}{\text{1}}\) (Substitution, nucleophilic, unimolecular)

a) \({{\text{S}}_{\text{N}}}{\text{1}}\) Reaction or \({{\text{S}}_{\text{N}}}{\text{1}}\) Mechanism

The \({{\text{S}}_{\text{N}}}{\text{1}}\) reaction is a two-step process. The first step is a slow ionization to form a carbocation. The second step is a fast attack on the carbocation by the nucleophile.

\({\text{R}} – {\text{X}} \to {{\text{R}}^ \oplus } + {{\text{X}}^\Theta };\,{\text{rate}} – {\text{determining}}\,{\text{step}}\)
\({{\text{R}}^ \oplus } + {\text{N}}{{\text{u}}^\Theta } \to {\text{R}} – {\text{Nu;}}\,{\text{Very}}\,{\text{fast}}\)

The carbocation is a very strong electrophile, it reacts very fast with both strong and weak nucleophiles.

Factors affecting the \({{\text{S}}_{\text{N}}}{\text{1}}\) reaction

The factors which affect the \({{\text{S}}_{\text{N}}}{\text{1}}\) reaction are:

a. Effect of the substrate structure
b. Effect of leaving group
c. Substitution effect
d. Effect of the attacking nucleophile

b) \({{\text{S}}_{\text{N}}}{\text{2}}\) Reaction or \({{\text{S}}_{\text{N}}}{\text{2}}\) Mechanism

\({{\text{S}}_{\text{N}}}{\text{2}}\) mechanism is a one-step process and involves no intermediate. In this reaction, the nucleophile attacks the substrate from the just opposite (back) side (at \({180^ \circ }\)) to leaving group.

A typical example of this process is the hydrolysis of methyl bromide in the presence of \({\text{NaOH}}.\)

\({\text{C}}{{\text{H}}_3} – {\text{Br}} + {\text{NaOH}} \to {\text{C}}{{\text{H}}_3} – {\text{OH}} + {\text{NaBr}}\)

B. Electrophilic Substitution Reactions

In these reactions, the attacking reagent is an electrophile. These reactions are typical of arenes and other aromatic compounds.

For example, halogenation, nitration, sulphonation, and Friedel-Crafts reactions.

C. Radical Substitution Reaction

The attacking reagent in radical substitution reactions is a radical. These reactions take place at high temperatures or in the presence of \({\text{UV}}\) radiation.

For example, chlorination of methane to form chloromethane. In this reaction, the

attacking reagent is a radical \(\left({\overset{ \bullet }{\mathop {\text{C}}} {\text{l}}} \right).\)

The reaction occurs by radical mechanism involves the following steps:

a. Initiation: In this step, radicals are formed.

b. Propagation: The propagation phase describes the ‘chain’ component of chain reactions.

c. Termination: In this process, two radical species react with each other to form a stable, non-radical adduct.

\(\overset{ \bullet }{\mathop {\text{C}}} {\text{l}} + \overset{ \bullet }{\mathop {\text{C}}} {\text{l}} \to {\text{C}}{{\text{l}}_2}\)

\(\mathop {\rm{C}}\limits^ \bullet {{\rm{H}}_3} + \mathop {\rm{C}}\limits^ \bullet {{\rm{H}}_3} \to {\rm{C}}{{\rm{H}}_3} – {\rm{C}}{{\rm{H}}_3}\)

Addition Reaction

Addition reactions are defined as reactions in which two reactive molecules combine to form a single product molecule. Compounds with many (double and triple) bonds are susceptible to such reactions.

For example,

Addition reactions are classified into three categories based on the nature of the attacking species. They are electrophiles, nucleophiles, or radicals.

A. Electrophilic Addition Reaction

Electrophilic addition reactions are addition reactions caused by electrophiles. These are typical alkene and alkyne reactions.

For example, the addition of halogen acids to alkenes proceeds by an electrophilic mechanism.

B. Nucleophilic Addition Reaction

Nucleophilic addition reactions are those that are caused by nucleophiles. These are typical aldehyde and ketone reactions.

For example, consider the base-catalyzed addition of \({\text{HCN}}\) to aldehydes or ketones.

C. Radical Addition Reaction

radical addition reactions are addition reactions caused by radicals.

For example, consider the addition of \({\text{HBr}}\) to alkenes in the presence of peroxides:

radical addition reaction occurs by the following mechanism:

a. Initiation: In this step, radicals are formed.

b. Propagation: The propagation phase describes the ‘chain’ component of chain reactions.

c. Termination: In this process, two radical species react with each other to form a stable, non-radical adduct.

\(\mathop {\rm{C}}\limits^ \bullet {\rm{l + }}\mathop {\rm{C}}\limits^ \bullet {\rm{l}} \to {\rm{C}}{{\rm{l}}_{\rm{2}}}\)

Elimination Reactions

The reaction in which two atoms or groups either from the adjacent positions or from the same position get eliminated or removed, leading to the formation of multiple bonds (i. e., double or triple bond) is known as an elimination reaction.

These reactions are of two types. They are;

a. \(\beta \)– Elimination reactions: In these reactions, the loss of two atoms or groups takes place from the adjacent carbon atoms in the molecule.

For example,

1. Dehydration of alcohols in the presence of concentrated sulphuric acid upon heating releases water.

Types of \(\beta \) – Elimination Reactions

Depending on the structure of the alkyl halide, the strength of the base, and the polarity of the solvent, \(\beta \)-elimination reactions occur via one of two mechanisms. They are:

1. \({{\text{E}}_2}\) (Elimination, bimolecular) Reactions: Primary alkyl halides and some secondary alkyl halides undergo elimination or dehydrohalogenation by the \({{\text{E}}_2}\) mechanism. \({{\text{E}}_2}\) reactions like \({\text{S}}{{\text{N}}_2}\) reactions are concerted reactions, i.e., occur is one step through a transition state.

2. \({{\text{E}}_1}\) (Elimination unimolecular) Reaction: When heated in an ionizing solvent, many secondary and tertiary alkyl halides undergo elimination reactions. For example, tert-butyl bromide, when heated in alcohol, undergoes dehydrobromination to form \(2\)-methylpropene or isobutylene.

b. \(\alpha \)– Elimination reactions: In these reactions, there is a loss or elimination of two atoms or groups from the same carbon atom in the molecule.

For example,

1. Dehydrogenation of primary or secondary alcohols with reduced copper at \(573\,{\text{k}}\)
2. Dehydrohalogenation of chloroform with \({\text{NaOH}}\) to form dichlorocarbene

Rearrangement Reactions

Reactions involving the migration of an atom or a group from one atom to another within the same molecule are called rearrangement reactions.

For example,

But-\(1\)-ene is the expected dehydration product in this reaction. However, but-\(2\)-ene is formed as a result of a rearrangement reaction. As a result of rearrangement, the less stable primary carbocation has been converted to the more stable secondary carbocation.

2. Wohler’s synthesis of urea from ammonium cyanate is also an example of a rearrangement reaction.

Polymerisation Reactions

Polymerisation is a basic addition reaction in nature. It may be defined as a process in which a large number of same or different molecules of unsaturated compounds combine to form a bigger molecule of higher molecular mass.

Pericyclic Reactions

Numerous organic reactions do not use ionic or radical intermediates. These reactions, on the other hand, occur in a single step via a cyclic transition state. Bond formation and bond breaking occur simultaneously in these reactions. These reactions do not require a catalyst and can be started with either heat or light. All of these reactions are referred to as pericyclic reactions.

Isomerisation Reactions

Reactions that involve interconversion of one isomer into another keeping the molecular formulae as well as the carbon skeletons of the reactant and the product intact are called isomerisation reactions.

For example, \(1\)-bromobutane isomerizes to \(2\)-bromobutane in the presence of anhydrous \({\text{AlC}}{{\text{l}}_3}\) at \(575\,{\text{k}}.\)

Condensation Reactions

Two or more molecules of the same or different reactants combine to form a product, with or without the elimination of simple molecules such as \({{\text{H}}_2}{\text{O}},{\text{HCl}},{\text{N}}{{\text{H}}_3},{\text{ROH}},\) and so on. For example, in the presence of dilute alkali, two molecules of acetaldehyde condense to form \(\beta \)-hydroxybutanal.

Summary

From this article, we can conclude that Organic reactions are chemical reactions in which organic compounds are involved. All organic reactions have been classified into different types. The major types of organic reactions are as follows: Substitution, Addition, elimination and rearrangement reaction.

FAQs on Types of Organic Reactions

We have provided some frequently asked questions on types of organic reactions here:

Q.1. Is sublimation in organic reaction?
Ans: No, sublimation is a method of purifying solids.

Q.2. How do you classify reaction types?
Ans: Reactions can be classified as
A. Synthesis reaction
B. Decomposition reaction
C. Single displacement reaction
D. Double displacement reaction
E. Combustion reaction

Q.3. How do you classify a reaction as endothermic or exothermic?
Ans: Chemical reactions that release energy are called exothermic reactions, and the reactions that absorb energy are called endothermic reactions.

Q.4. How do you determine if a reaction is exothermic?
Ans: If the reaction takes place with the release of heat, then that reaction is considered an exothermic reaction.

Q.5. What are three examples of endothermic reactions?
Ans: Three examples of endothermic reactions are;
1. Melting of ice
2. Sublimation of ammonium chloride
3. Evaporation of water

Q.6. What are the \(4\) types of organic reactions?
Ans: Four types of organic reactions are;
1. Substitution reactions
2. Addition reactions
3. Elimination reactions
4. Rearrangement reactions

Q.7. How will you differentiate the four reactions of organic compounds?
Ans: If the reaction in which two reactive molecules combine to form a single product molecule, then that reaction is known as an addition reaction. If the reaction in which the replacement of one atom or group of atoms is involved, then that reaction is known as a substitution reaction. If the reaction in which two atoms or groups either from the adjacent positions or from the same position get eliminated or removed, leading to the formation of multiple bonds, then that reaction is known as the elimination reaction. If the reactions involve the migration of an atom or a group from one atom to another within the same molecule, then that reaction is known as a rearrangement reaction.

Q.8. What are the \(5\) types of organic reactions?
Ans: Following are the \(5\) types of organic reactions
1. Substitution reactions
2. Addition reactions
3. Elimination reactions
4. Rearrangement reactions
5. Pericyclic reactions

Q.9. Is polymerization in organic reaction?
Ans: Yes, polymerization is an organic reaction.

Q.10. Why do organic reactions take a long time?
Ans: Organic compounds are molecules with strong covalent bonding. With such strong bonds, it is more difficult for these molecules to react; thus, it takes a long time.

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