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 ​→ RCOOCH3 ​+ 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​ + CH3​OH
Hydrazine (NH2​NH2) replaces the methoxy group (-OCH3​) in an ester (RCOOCH3​) to form a hydrazide (RCOONHNH2​) and methanol (CH3​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.
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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:
Tertiary Butyl Chloride → Tertiary Butanol (tert-butanol) + Chloride ion (Cl-)
Chloromethane (CH3Cl) + Hydroxide ion (OH-) → Methanol (CH3OH) + Chloride ion (Cl-)
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​H5​CH3​ + HCl
Halogenation of Alkanes
Reaction: R−H + X2​ → R−X + HX
Example: CH4​ + Cl2 ​→CH3​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 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:
| To remove two groups from a molecule. |
To replace one group with another. |
| Often involves a base as a reagent. |
Involves a nucleophile or electrophile. |
| Typically involves one reactant group. |
Typically involves two reactant groups. |
| Produces a new π bond (alkenes or alkynes). |
Produces a different compound entirely. |
| E1, E2, or E1cB mechanisms are common. |
SN1, SN2, or SNi mechanisms are common. |
| Can lead to the formation of stereoisomers. |
Often leads to inversion or retention. |
| Often requires strong bases and elevated temperatures. |
Varies depending on the specific reaction. |
| Dehydrohalogenation, dehydrations, dehalogenations. |
Nucleophilic and electrophilic substitutions. |
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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 SN1 and SN2 Substitution Reactions?
SN1 is a two-step nucleophilic substitution with a carbocation intermediate, while SN2 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.
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