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Substitution reaction can be defined as the reaction in which we substitute the functional group of one chemical compound by another group or in other words it can be said as a reaction in which one atom or a molecule is replaced of a compound with another. Here, we will be learning more about substitution reaction and discussing some important questions.
| Table of Contents |
Key takeaways: substitution reactions, organic chemistry, hydroxyl ion, nucleophilic substitution reaction, electrophilic substitution reaction.
Substitution Reaction Examples
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Substitution reactions possess primary importance in the field of organic chemistry. For example, when CH3Cl is reacted with the hydroxyl ion (OH-), it will result in the formation of the original molecule called methanol with that hydroxyl ion. The following reaction is shown below-
CH3Cl + (OH−) → CH3OH( methanol) + Cl–
One more example can be the reaction of Ethanol with the hydrogen iodide which forms iodoethane along with water. The reaction is-
CH3CH2OH + HI → CH3CH2I + H2O
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| Chapter Related Concepts | ||
|---|---|---|
| Ester Hydrolysis | Fehling Test | Iodoform Test |
| Hell-Volhard-Zelinsky Reaction | Friedel-Crafts acylation reaction | Butyric Acid |
Types of Substitution Reaction
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Substitution Reactions can be categorized into two types, namely nucleophilic reaction and electrophilic reactions. These two types of reactions mainly differ from each other based on the kind of atom which is attached to its original molecule. In the nucleophilic reactions, the atom is said to be electron-rich species, while, in the electrophilic reaction, the atom is an electron-deficient species. A brief explanation of the two types of reactions is as given below.
Nucleophilic Substitution Reaction
(a) What are nucleophiles?
Nucleophiles which are in the form of an ion or a molecule, are strongly attached to the region of a positive charge. These species are said to be fully charged and have negative ions present on a molecule. The common examples of nucleophiles include cyanide ions, water, hydroxide ions, and ammonia.
(b) What is a Nucleophilic substitution reaction?
A Nucleophilic substitution reaction in organic chemistry is referred to as a type of reaction where a nucleophile gets attached to the positive charged atoms or molecules of the other substance.
(c) Mechanism of Nucleophilic substitution reaction:
There are two mechanisms of nucleophilic substitution reaction, SN1 reaction and the SN2 reaction, where S represents chemical substitution, N represents nucleophilic, and the number stands for the kinetic order of a reaction.
SN1 Reaction – Mechanism of SN1 Reaction
There are certain factors that affect the SN1 reaction as well. A few are discussed below:
- Instead of two concentrations only one i.e., the substrate affects the rate of reaction.
- The rate equation for the above reaction is written as Rate = k[Sub].
- The reaction rate is determined by its slowest step. Therefore, the leaving group leaves at a particular rate which helps in determining the reaction speed.
- It is considered that the weaker the conjugate base, the better is the leaving group.
- SN1 reactions can be determined by bulky groups that are attached to the carbocations.
- The reaction of the tertiary carbocation is faster than secondary carbocation which is faster than primary carbocation.
- The nucleophile is not required in the rate-determining step.
SN2 Reaction – Mechanism of SN2 Reaction
In this reaction, the elimination of the leaving group and the addition of the nucleophile occur simultaneously. SN2 takes place where the central carbon atom has easy access to the nucleophile.
In SN2 reactions, the rate of the reaction is affected by a few conditions. They are discussed below:
- The numerical value 2 in SN2 states that there are two concentrations of substances which affect the rate of reaction, viz. nucleophile and substrate.
- The rate equation for the above reaction is written as Rate = k [Sub][Nuc].
- An aprotic solvent such as acetone, DMSO, or DMF is suited best for the SN2 reaction as they do not add the H+ ions in the solution.
- In case if there are protons present, they react with the nucleophile to critically limit the rate of reaction. It is a one-step reaction and the reaction speed is driven by the steric effects. During the intermediate step, the position of the leaving group is inverted whereas the nucleophile is 180°.
- Nucleophilicity also affects the reaction rate.
Difference between SN1 and SN2
The major difference involved between these two types of reactions is to study the different properties of the departure group that helps us in finding out the pathway of the group. Understanding the major differences between these two will give us the key differences between one and the other. For a full list of differences between the two, check out the tabular column below:
| Difference between SN1 and SN2 | |
|---|---|
| SN1 | SN2 |
| The rate of reaction is unimolecular. | The rate of reaction is bimolecular |
| It is a two-step mechanism | It is only a one-step mechanism |
| Carbocation is formed as an intermediate part of the reaction. | No carbocation is formed during the reaction. |
| There is no partial bond formed with the carbon during this reaction. | Carbon forms a partial bond with the nucleophile and the leaving group. |
| There are many steps in this reaction which start with the removal of the group while attacking the nucleophile. | The process takes place in only one cycle, with a single intermediate stage. |
Thus, these are the key differences between SN1 and SN2.
Electrophilic Substitution Reactions
(a) What are electrophiles?
The electrophilic substitution reaction involves the electrophiles. Electrophiles are those which donate a pair of electrons in the formation of a covalent bond. The Electrophilic reactions occur mostly with the aromatic compounds. These compounds have about an excess of electrons that can be shared throughout the system of reaction.
(b) What is an Electrophilic substitution reaction?
Electrophilic substitution reactions are basically defined as those chemical reactions where the electrophile replaces the functional group in a compound but not the hydrogen atom. Some examples of species of electrophiles include hydronium ion (H3O+), halides of hydrogen such as HCl, HBr, HI, sulphur trioxide (SO3), the nitronium ion (NO2+), etc.
(c) Types of Electrophilic substitution reaction:
Two types of electrophilic substitution reactions are discussed here. Aromatic electrophilic substitution reaction and aliphatic electrophilic substitution reaction.
Electrophilic Aromatic Substitution
In this type of electrophilic substitution, an atom attached to the aromatic ring which is mostly hydrogen is substituted by an electrophile. The reactions that occur are aromatic halogenation, alkylating Friedel-Crafts reactions, aromatic nitration, and aromatic sulfonation and acylation. It further comprises acylation and alkylation.
Electrophilic Aliphatic Substitution
In this type of electrophilic substitution reaction, an electrophile dislocates one functional group. The four electrophilic aliphatic substitution reactions which are similar to counterparts of nucleophile SN1 and SN2 are as follows – SE1, SE2(front), SE2(back) and SEi (Substitution Electrophilic). During the SE1 reaction, the substrate ionizes to a carbanion and recombines with the electrophile. During the SE2 reaction, a single transition state occurs where the old and newly formed bonds are present.
The other types of substitution reactions include radical reactions and organometallic substitution reactions. For more details on aromatic and aliphatic electrophilic substitution checkout the Electrophilic substitution reaction mechanism.
Things to Remember
- The application of anesthetic agents,which are produced by the process of substitution reaction into your body reduces the pain.
- They are primarily important in organic chemistry.
- Substitution Reactions can be categorized into two types, namely nucleophilic reaction and electrophilic reactions.
- These two types of reactions mainly differ from each other based on the kind of atom which is attached to its original molecule.
- Nucleophiles which are in the form of an ion or a molecule, are strongly attached to the region of a positive charge.
- The electrophilic substitution reaction involves the electrophiles. Electrophiles are those which donate a pair of electrons in the formation of a covalent bond.
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Sample Questions
Ques: How does SN1 reaction occur? [3 Marks]
Ans: The SN1 reaction is a nucleophilic substitution reaction where the rate determining step is unimolecular. It is a type of organic substitution reaction. SN1 stands for substitution nucleophilic unimolecular. Thus, the rate equation (which states that the SN1 reaction is dependent on the electrophile but not on the nucleophile) holds in situations where the amount of the nucleophile is far greater than the amount of the carbocation intermediate.
This reaction involves the formation of a carbocation intermediate. It is generally seen in the reactions of tertiary or secondary alkyl halides with secondary or tertiary alcohols under strongly acidic or strongly basic conditions. The SN1 reaction is often referred to as the dissociative mechanism in inorganic chemistry. Given below are some examples of an SN1 type of nucleophilic substitution reaction.
Ques: How does SN2 reaction occur? [2 Marks]
Ans: SN2 reactions involve a backside nucleophilic attack on an electrophilic carbon. As a result, less steric congestion for this backside attack results in a faster reaction, meaning that SN2 reactions proceed fastest for primary carbons. In addition, beta-branching next to a primary carbon results in a slower reaction, as does a poorer leaving group (i.e. chloride instead of bromide).
1-bromopentane has a good leaving group (bromine) attached to a primary carbon with no beta-branching, meaning it will proceed the fastest.
Ques: What is a substitution reaction? Give examples [3 Marks]
Ans: Substitution reaction is defined as the chemical reaction in which a more reactive element displaces a less reactive element from its chemical reaction. It is also known as single displacement reaction
The reactivity of metal is determined by a series known as reactivity series. The metals lying above in the series are more reactive than the metals which lie below in the series.
A + BC → AC + B
Metal A is more reactive than metal B.
For Example: When zinc granules are added to a solution of copper sulfate solution, the blue color of copper sulfate solution gets decolorized because of the substitution of copper ions with zinc ions.
The chemical equation follows:
Zn + CuSO4 → ZnSO4 + Cu
Ques: Explain: The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkenes are major products. [2 Marks]
Ans:
R−Cl+KOH(aq)→R−OH+KCl
The ionization of aqueous KOH produces hydroxide ions which are strong nucleophiles. Hence, alkyl chlorides undergo substitution to form alcohol.
R−CH2−CH2−Cl+KOH(alc)→R−CH=CH2 + KCl+H2O
Alcoholic KOH solution gives alkoxide ion which is a strong base. It abstracts β hydrogen atoms of alkyl chloride. A molecule of HCl is eliminated and an alkene is formed.
Note: The basicity of hydroxide ion is much lower than the basicity of alkoxide ion as hydroxide ion is significantly hydrated in aqueous solution.
Hence, hydroxide ions cannot abstract β hydrogen atoms of alkyl chloride.
Ques: Out of C6H5CH2Cl and C6H5CH(Cl)C6H5, which is more easily hydrolysed by aqueous KOH? [2 Marks]
Ans: C6H5CH(Cl)C6H5 is more easily hydrolysed by aqueous KOH. It involves formation of a secondary carbocation which is stabilized by resonance with two phenyl groups. On the other hand, during hydrolysis of C6H5CH2Cl, a primary carbocation is formed which is stabilized by resonance with only one phenyl group and is less stable and less readily formed.
Ques: How is the reaction of iodoethane and sodium hydroxide a hydrolysis reaction? [2 Marks]
Ans: When iodoethane reacts with sodium hydroxide, iodine is substituted by the hydroxyl group to form ethyl alcohol.
Above reaction is a substitution reaction, but it is referred to as a hydrolysis reaction, it is a special case of substitution reaction.
Because when the nucleophile is water or OH- and the product is alcohol then such reactions are also referred to as a hydrolysis reaction.
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