Substitution Reaction as the name suggests tells us about the reaction in which some functional group is substituted by the other functional group. Substitution Reaction play a crucial role in the synthesis, modification, and functionalization of organic compounds, allowing chemists to form compounds with specific properties and reactivity of molecules for various applications.

In this article, we will discuss all things related to Substitution Reaction i.e., its definition, and examples including Alkane Substitution, Nucleophilic, Electrophilic, Free Radical as well as Organomatelic Substitution.

What is Substitution Reaction?

Substitution reactions are fundamental chemical reactions in chemistry, and they involve the exchange of one atom or group of atoms (functional group) with another in a molecule.

Considering the reaction between methane (CH₄​) and chlorine (Cl₂​),

CH₄ + Cl₂→CH₃Cl + HCl

This is an example of substitution reaction as one hydrogen atom in methane with a chlorine atom, which forms hydrochloric acid (HCl) and chloromethane (CH₃Cl).

Substitution Reaction Definition

A substitution reaction is a chemical reaction where one functional group or atom within a molecule is replaced by another functional group or atom.

Examples of Substitution Reactions

Some of the most common examples of substitution reactions are:

• Alkane Substitution Reactions
• Acyl Substitution Reactions

Let’s discuss these examples as follows:

Alkane Substitution Reactions

Alkanes experience substitution reactions because they are saturated hydrocarbons. One typical example is when UV light causes a halogen to replace a hydrogen atom in an alkane, creating a haloalkane.

Halogenation of Methane

Halogenation of methane involves the substitution of hydrogen atoms in methane (CH₄) with halogen atoms, typically chlorine (Cl₂) or bromine (Br₂).

CH₄+Cl₂ →CH₃Cl + HCl

Bromination of Ethane

Bromination of ethane involves the substitution of hydrogen atoms in ethane (C₂H₆) with bromine (Br₂) atoms. This reaction is a type of substitution reaction and typically requires the presence of heat or ultraviolet (UV) light to initiate it.

C₂H₆+Br₂→C₂H₅Br + HBr

Chlorination of Propane

The chlorination of propane involves the substitution of hydrogen atoms in propane (C₃H₈) with chlorine (Cl₂) atoms. This reaction is a type of substitution reaction and usually requires the presence of heat or ultraviolet (UV) light to initiate it.

C₃H₈ + Cl₂ → C₃H₇Cl + HCl

Acyl Substitution Reaction

An acyl substitution reaction is the process by which an acyl group is replaced in a compound. These are common reactions in organic synthesis, especially when amides and esters are formed.

Nucleophilic Acyl Substitution in Carboxylic Acids

Nucleophilic acyl substitution is a type of organic reaction that involves the substitution of an acyl group (RCO-) in a carboxylic acid or a derivative of a carboxylic acid with a nucleophile.

General reaction scheme for nucleophilic acyl substitution in carboxylic acids is given as:

R-COOH + Nu^– ​→ R-CO-Nu + OH^–

Let’s consider an example for the same.

RCOOH + NH₃ ​→ RCOONH₂ + H₂

In this reaction, ammonia (NH₃​) replaces the hydroxyl group (-OH) in a carboxylic acid (RCOOH) to form an amide (RCOONH₂​).

Nucleophilic Acyl Substitution in Acid Chlorides

Nucleophilic acyl substitution involves the substitution of the acyl group (RCO-) in an acid chloride (RCOCl) with a nucleophile.

General reaction for this is given as:

RCOCl + Nu^– → RCONu + Cl^–

Let’s consider and example for the same.

RCOCl + HOCH₃ ​→ RCOOCH_{3 ​}+ HCl

Methanol (HOCH₃​) reacts with an acid chloride (RCOCl) to produce an ester (RCOOCH₃​) and hydrogen chloride (HCl).

Nucleophilic Acyl Substitution in Esters

Nucleophilic acyl substitution involves the substitution of the acyl group (RCO-) in an ester (RCOOR’) with a nucleophile.

General reaction for the same is given as:

RCOOR’ + Nu^– → RCONu + R’O^–

Let’s consider an example for the same.

RCOOCH₃​ + NH₂​NH₂​ → RCOONHNH₂​ + CH_3​OH

Hydrazine (NH₂​NH₂) replaces the methoxy group (-OCH₃​) in an ester (RCOOCH₃​) to form a hydrazide (RCOONHNH₂​) and methanol (CH_3​OH).

Classification of Substitution Reactions

Substitution reactions can commonly be classified as follows:

• Nucleophilic Substitution
• Electrophilic Substitution

Other than this there are some more classification of substitution reaction given as:

• Free Radical Substitution
• Organometallic Substitution

Let’s discuss these classifications in detail.

Read More,

• Chemical Reactions
• Characteristics of Chemical Reactions
• Types of Chemical Reactions
• Chemical Changes

Nucleophilic Substitution Reactions

In a nucleophilic substitution reaction, a nucleophile (electron-rich species) replaces a leaving group (a group that can depart as a stable ion or molecule) from a molecule. The nucleophile attacks the electrophilic center of the molecule, leading to the substitution of the leaving group with the nucleophile.

Common examples of nucleophilic substitution reactions include SN1 (substitution nucleophilic unimolecular) and SN2 (substitution nucleophilic bimolecular) reactions.

Nucleophilic Substitution Examples

Some of the common example all types of substitution reaction are listed as follows:

• Examples of S_N1 Reaction

Tertiary Butyl Chloride → Tertiary Butanol (tert-butanol) + Chloride ion (Cl-)

• Examples of S_N2 Reaction

Chloromethane (CH₃Cl) + Hydroxide ion (OH-) → Methanol (CH₃OH) + Chloride ion (Cl-)

• Some Other Examples

Some other examples include halogenation of alkane i.e.,

• Methane (CH₄) + Chlorine Gas (Cl₂) → Chloromethane (CH₃Cl) + Hydrochloric Acid (HCl)
• Ethyl Bromide (CH₃CH₂Br) + Hydroxide ion (OH^–) → Ethanol (CH₃CH₂OH) + Bromide ion (Br^–)

Electrophilic Substitution Reactions

In an electrophilic substitution reaction, an electrophile (electron-poor species) replaces an atom or group in a molecule. This type of reaction commonly occurs in aromatic compounds, such as benzene rings. The electrophile attacks the aromatic ring, leading to the substitution of one atom or group with the electrophile.

An example of an electrophilic substitution reaction is the nitration of benzene, where a nitro group (-NO₂) replaces a hydrogen atom on the benzene ring.

Electrophilic Substitution Examples

Aromatic Substitution (e.g., Friedel-Crafts Alkylation)

Reaction: R−H+E⁺→R−E + H⁺

Example: C₆​H₆​ + CH₃​Cl + AlCl₃ ​→ C₆​H_5​CH₃​ + HCl

Halogenation of Alkanes

Reaction: R−H + X₂​ → R−X + HX

Example: CH_4​ + Cl_{2 ​}→CH_3​Cl + HCl

Electrophilic Substitution Reactions of Benzene

Electrophilic substitution reactions of benzene are a class of reactions where an electrophile (electron-deficient species) replaces one of the hydrogen atoms in a benzene ring. Benzene is a highly stable and aromatic compound, and electrophilic substitution reactions allow for the introduction of various functional groups onto the benzene ring.

Some common electrophilic substitution reactions of benzene include:

• Nitration of Benzene
• Sulphonation of Benzene
• Halogenation of Benzene
• Friedel-Crafts Alkylation
• Friedel-Crafts Acylation

Free Radical Substitution Reactions

Free radical substitution reactions are a type of chemical reaction in which a radical (a species with an unpaired electron) replaces another atom or group of atoms in a molecule. Free radical substitution reactions can be described as a three-step process:

• Initiation
• Propagation
• Termination

Examples of Free Radical Substitution

Some examples of free radical substitution are listed as follows:

• Halogenation of Alkanes
• Chlorination of Methane
• Bromination of Ethane
• Idonination of Propane

Organometallic Substitution Reactions

Organometallic substitution reactions involve the replacement of one or more ligands (organic groups or other atoms) in an organometallic compound with different ligands.

Organometallic Substitution Examples

Organometallic substitution reactions can be broadly categorized into several types, including ligand substitution, oxidative addition, and reductive elimination. Here are some examples:

Ligand Substitution

Ligand substitution reactions involve the exchange of one ligand for another. Commonly, a new ligand displaces an existing ligand from the coordination sphere of the metal center. For example, in the reaction of ferrocene (an organometallic compound) with iodine:

Fe(C₅H₅)₂ + I₂ → Fe(C₅H₅)I + C₅H₅I

Here, iodine (I₂) substitutes for one of the cyclopentadienyl (C₅H₅) ligands.

Oxidative Addition

Oxidative addition reactions typically involve the addition of a new ligand and the simultaneous increase in the oxidation state of the metal center. For example, in the reaction of a palladium complex with hydrogen gas:

Pd(PPh₃)₄ + H₂ → Pd(PPh₃)₂H₂ + 2PPh₃

In this reaction, hydrogen gas (H₂) undergoes oxidative addition to palladium (Pd) to form a dihydride complex.

Reductive Elimination

Reductive elimination reactions involve the removal of one or more ligands from the metal center and the simultaneous decrease in the oxidation state of the metal. For example, in the reductive elimination of ethylene from a platinum complex:

Pt(C₂H₄)₂ + Cl₂ → PtCl₂ + C₂H₄

In this reaction, the platinum complex loses ethylene (C₂H₄) to form platinum(II) chloride.

Substitution with a Nucleophile (Reaction with RMgX)

Organometallic compounds can also undergo nucleophilic substitution reactions, where a nucleophile replaces one of the ligands. For instance, in the reaction of a Grignard reagent (nucleophile) with a transition metal complex:

M(CO)₆ + RMgX → M(CO)₅R + MgXCl

Here, the Grignard reagent (RMgX) substitutes one of the carbonyl (CO) ligands to form a new organometallic compound.

Elimination Vs Substitution Reaction

The key diferences between elimination and substitution reaction are listed in the following table:

Read More,

• Decomposition Reactions
• Combination Reaction
• Precipitation Reactions
• Rate of Reaction

Frequently Asked Questions on Substitution Reaction

What are some Examples of Substitution Reaction?

Examples of substitution reactions in chemistry include:

• Chlorination of methane: CH₄ + Cl₂ → CH₃Cl + HCl
• Bromination of ethane: C₂H₆ + Br₂ → C₂H₅Br + HBr

What is the Product of Substitution Reaction of Alkane?

In a substitution reaction of an alkane, hydrogen atoms are replaced with different atoms or groups, yielding halogenated alkanes or other products depending on the reagents used.

Write balance Substitution Reaction of Methane and Chlorine.

The balanced chemical equation for the substitution reaction of methane (CH₄) with chlorine (Cl₂) to form chloromethane (CH₃Cl) is:

CH₄ + Cl₂ → CH₃Cl + HCl

What are the 3 main Types of Substitution Reactions?

Three main types of substituition are:

• Nucleophilic Substitution
• Electrophilic Substitution
• Free Radical Substitution

What is S_N1 and S_N2 Substitution Reactions?

S_N1 is a two-step nucleophilic substitution with a carbocation intermediate, while S_N2 is a one-step reaction without an intermediate, involving simultaneous nucleophilic attack and leaving group departure.

What is the difference Between Substitution and Displacement Reactions?

Substitution involves replacing one atom or group in a molecule, encompassing various types of reactions, whereas displacement primarily refers to inorganic reactions where one element replaces another, often involving metals.

Define Nucleophilic Substitution Reaction.

A nucleophilic substitution reaction is a chemical reaction in which a nucleophile displaces a leaving group in a molecule, leading to the formation of a new compound.