What is Substitution Reaction?
The substitution reaction can be described as a reaction in having the functional group of one chemical compound substituted by another group. It is also defined as a reaction that involves the replacement of one molecule or an atom of a compound with another molecule or an atom.
What happens in a Substitution Reaction?
Chemical reactions happen when two or more particular types of substances come in contact with each other under certain circumstances. These reactions change the whole physical as well as chemical properties of the substances reacting with each other. To understand this the best example is illustrated by the combination of oxygen and hydrogen to make water. Hydrogen which is a fuel itself burns in the presence of oxygen, the element that assists in the process of combustion. Contrastingly, when these two elements combine chemically then they form water. And water is a substance that doesn't and is used in putting off the fire.
There are many types of chemical reactions occurring in our surroundings to break down different substances or produce new substances. These reactions occur at the atomic and molecular levels; the chemical bonding of the atoms changes with it. There are also certain chemical reactions that produce two or more substances. This type of reaction occurs by regrouping the atoms in the molecules of different elements and is known as a substitution reaction. During this reaction, the functional group present in the molecule of one compound gets detached from it and combines with the molecule of another element or compound. This transfer of functional groups is determined by the reactivity of the two substances with respect to the functional group.
Substitution Reaction Example
These types of reactions are referred to as the nucleophilic and possess major importance in the field of organic chemistry. Let us say, for example, when the CH3Cl compound is reacted with the hydroxyl ion (OH-), it leads to the formation of the original molecule, which is called methanol with that hydroxyl ion. The chemical reaction for this can be given as follows:
\[CH_3Cl + (^-OH) \rightarrow CH_3OH \text{(methanol)} + Cl^- \]
Another example would be the Ethanol reaction with the hydrogen iodide, which produces iodoethane along with water. The chemical reaction for this can be given as follows:
\[CH_3CH_2OH + HI \rightarrow CH_3CH_2OI + H_2O \]
Substitution Reaction Conditions
In order for the substitution reaction to take place, there are some conditions that have to be used. They are given below.
Maintaining low temperatures such as room temperature
The strong base like NaOH has to exist in the dilute form. For suppose, if the base is with a higher concentration, there are high chances of dehydrohalogenation occurring the solution is required to be in an aqueous state like water
Types of Substitution Reactions
Substitution Reactions are given as two types, which are named as nucleophilic reactions and the electrophilic reactions. These both reactions primarily differ in the kind of an atom, which is attached to its original molecule. And, in the nucleophilic reactions, the atom is referred to as electron-rich species. On the other hand, in the electrophilic reaction, the atom is said to be an electron-deficient species. A detailed explanation of these two types of reactions can be given below.
Nucleophilic Substitution Reaction:
What are Nucleophiles?
Nucleophiles are defined as the species either in the form of a molecule or an ion, which are strongly attached to the positive charge region. These are known to be either fully charged or contain negative ions, present on a molecule. The common examples of these nucleophiles can be given as water, cyanide ions, ammonia and hydroxide ions.
What is the Nucleophilic Substitution Reaction?
In organic chemistry, a Nucleophilic substitution reaction can be defined as a type of reaction, where a nucleophile gets attached either to the positively charged molecules or atoms of the other substance.
The Nucleophilic Substitution Reaction Mechanism
Let us discuss the mechanism of nucleophilic substitution reaction(s), which are SN1 and SN2 reactions. Here, S represents the chemical substitution, N represents nucleophilic, and finally, the number is the kinetic order of a reaction.
SN2 Reaction – Mechanism of SN2 Reaction
In this particular reaction, the addition of the nucleophile and the elimination of the leaving group takes place simultaneously. Also, SN2 reaction occurs where the central carbon atom has easy access to the nucleophile.
In these SN2 reactions, the reaction rate is affected by some conditions. They can be listed as follows:
The numerical value two present in the SN2 states that there exist two concentrations of the substances which affect the rate of reaction, which are nucleophile and substrate.
The rate equation for the reaction given above can be written as
Rate = k Sub Nuc.
An aprotic solvent like DMSO, DMF, or acetone is suited best for the SN2 reaction because they do not add the H+ ions for the solution.
If in case, there are protons available, they react with the nucleophile to limit the rate of reaction critically. This is a one-step reaction, and the speed of reaction is driven by the steric effects. During this intermediate step, the position of the leaving group can be inverted, whereas the nucleophile is given as 180°.
Also, nucleophilicity affects the rate of reaction.
SN1 Reaction – Mechanism of SN1 Reaction
There are some factors that affect the SN1 reaction also. A few of them are discussed below:
Instead of two concentrations, only one concentration, that is, the substrate affects the rate of reaction.
The rate equation for the reaction, which is given above can be written as
Rate = k Sub.
The reaction rate can be defined by its slowest step. Thus, the leaving group leaves at a specific rate that helps in defining the reaction speed.
It can be considered that the weaker the conjugate base, the better is considered as the leaving group.
SN1 reactions are defined by the bulky groups which are attached to the carbocations.
The tertiary carbocation reaction is faster to that of the secondary carbocation, which is faster than the primary carbocation.
In the rate-determining step, the nucleophile is not needed.
Example of Nucleophilic Substitution Reaction:
One of the good examples of a nucleophilic substitution reaction is given as the hydrolysis of alkyl bromide (R-Br), under the basic conditions. Whereas, the nucleophile is the base OH−, and the leaving group is the Br−. The reaction for this can be given as follows:
\[R-Br + ^-OH \rightarrow R-OH + Br^− \]
Nucleophilic reactions are the most important ones in the field of organic chemistry, and these are broadly divided as to take place at the position of a carbon atom of a saturated aliphatic carbon compound.
Electrophilic Substitution Reactions:
What Are Electrophiles?
The electrophilic substitution reaction involves electrophiles. Electrophiles are the ones which donate an electron pair in the covalent bond formation. The Electrophilic reactions take place mostly with the aromatic compounds. And, these compounds contain up to about excess electrons which can be shared on the whole system of reaction.
What is Meant by the Electrophilic Substitution Reaction?
Basically, the Electrophilic substitution reactions can be described as those chemical reactions, where the electrophile replaces the compound’s functional group, but not the hydrogen atom. A few examples of the electrophiles species include hydronium ion (H3O+), and the halides of hydrogen, like HBr, HCl, sulphur trioxide (SO3), HI, and nitronium ion (NO2+).
Types of Electrophilic Substitution Reaction:
There exist two types of electrophilic substitution reactions. They are the aromatic electrophilic substitution reaction and the aliphatic electrophilic substitution reaction.
Electrophilic Aromatic Substitution Reaction
In this electrophilic substitution type, an atom which is attached to the aromatic ring, mostly hydrogen, is substituted by an electrophile. The reactions that take place are said as aromatic halogenation, aromatic acylation and sulfonation, and aromatic nitration. Also, it is further composed of alkylation and acylation.
Electrophilic Aliphatic Substitution
In this electrophilic substitution reaction type, an electrophile dislocates one of the functional groups. The four electrophilic aliphatic substitution reactions that are the same as to the counterparts of nucleophile SN1 and SN2 are given as – SE1, SE2, SE2 and SEi (which are called Substitution Electrophilic). During the SE1 reaction, the substrate ionizes to the carbanion and recombines with the electrophile. And, during the SE2 reaction, only a single transition state takes place, where the old and newly formed bonds are present.
Other types of substitution reactions are organometallic substitution reactions, radical reactions.
FAQs on Substitution Reaction
1. What is a substitution reaction in chemistry?
A substitution reaction is a type of chemical reaction in which one functional group in a chemical compound is replaced by another. In simple terms, an atom or a group of atoms is swapped for another. This process is fundamental in organic chemistry for creating new molecules from existing ones. For example, in the chlorination of methane (CH₄), a hydrogen atom is substituted by a chlorine atom to form chloromethane (CH₃Cl).
2. How can you identify a substitution reaction?
You can identify a substitution reaction by observing the reactants and products. The key indicator is that an atom or group attached to a central atom (usually carbon) is replaced by a new atom or group, without changing the overall saturation of the molecule. For instance, if a saturated alkane reacts to form a saturated haloalkane, it's a substitution. This is different from an addition reaction, where atoms are added across a double or triple bond, or an elimination reaction, where atoms are removed to form a double or triple bond.
3. What are the main types of substitution reactions?
Substitution reactions are primarily classified into two main types based on the nature of the attacking species:
- Nucleophilic Substitution: This reaction is initiated by a nucleophile, which is an electron-rich species that attacks an electron-deficient carbon atom, replacing a leaving group.
- Electrophilic Substitution: This reaction is initiated by an electrophile, an electron-deficient species that attacks an electron-rich area, such as an aromatic ring, and replaces a functional group, typically a hydrogen atom.
4. Can you provide a simple example of a substitution reaction as per the CBSE syllabus?
A classic example from the CBSE syllabus is the reaction of a haloalkane with an aqueous alkali. When chloroethane (CH₃CH₂Cl) is treated with aqueous potassium hydroxide (KOH), the chlorine atom is substituted by the hydroxyl group (-OH) to form ethanol (CH₃CH₂OH). The reaction is: CH₃CH₂Cl + KOH(aq) → CH₃CH₂OH + KCl. This is an example of a nucleophilic substitution reaction.
5. What is the key difference between SN1 and SN2 mechanisms in nucleophilic substitution?
The primary difference between SN1 and SN2 mechanisms lies in their reaction kinetics and pathway:
- SN1 (Substitution Nucleophilic Unimolecular): This is a two-step reaction. First, the leaving group detaches to form a stable carbocation intermediate. Then, the nucleophile attacks the carbocation. Its rate depends only on the concentration of the substrate. It is favoured by tertiary substrates and polar protic solvents.
- SN2 (Substitution Nucleophilic Bimolecular): This is a one-step concerted reaction where the nucleophile attacks the substrate at the same time as the leaving group departs. Its rate depends on the concentration of both the substrate and the nucleophile. It is favoured by primary substrates and polar aprotic solvents and results in an inversion of stereochemistry.
6. Why are electrophilic substitution reactions common in aromatic compounds like benzene?
Electrophilic substitution is characteristic of aromatic compounds like benzene because of its unique electronic structure. Benzene has a stable, delocalized pi-electron cloud above and below the plane of its ring. This electron-rich system is highly attractive to electron-deficient species (electrophiles). A substitution reaction allows benzene to react with an electrophile while preserving the aromaticity and stability of the ring system, which would be lost in an addition reaction.
7. How do substitution reactions differ from addition and elimination reactions?
These three reaction types change the structure of a molecule in distinct ways:
- Substitution Reaction: An atom or group is swapped for another, maintaining the molecule's saturation level (e.g., R-Cl + OH⁻ → R-OH + Cl⁻).
- Addition Reaction: Atoms are added across a double or triple bond, breaking a pi (π) bond and increasing saturation (e.g., CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br).
- Elimination Reaction: Two atoms or groups are removed from adjacent carbons to form a double or triple bond, decreasing saturation (e.g., CH₃-CH₂Br → CH₂=CH₂ + HBr).
8. What factors determine whether a nucleophilic substitution follows an SN1 or SN2 pathway?
Several factors influence the choice between an SN1 and SN2 mechanism:
- Substrate Structure: Tertiary (3°) substrates favour SN1 due to the formation of a stable carbocation. Primary (1°) substrates favour SN2 as there is less steric hindrance for the nucleophile to attack.
- Nature of the Nucleophile: A strong nucleophile favours the bimolecular SN2 reaction, while a weak nucleophile (like water or alcohol) favours the SN1 pathway.
- The Leaving Group: A good leaving group (the conjugate base of a strong acid, like I⁻ or Br⁻) increases the rate of both SN1 and SN2 reactions.
- The Solvent: Polar protic solvents (e.g., water, ethanol) stabilize the carbocation intermediate, favouring SN1. Polar aprotic solvents (e.g., acetone, DMSO) favour SN2.
9. What are some real-world applications of substitution reactions?
Substitution reactions are crucial in synthetic organic chemistry for producing a vast range of useful products. They are widely used in the industrial manufacturing of pharmaceuticals (like aspirin), agrochemicals (herbicides and pesticides), polymers, and dyes. For example, the synthesis of many drugs involves replacing a functional group on a starting molecule to create the final active compound with the desired biological activity.
