NEET Chapter Page - Reaction Mechanism

In this article, we will discuss the chapter-Reaction Mechanisms and its important points for NEET aspirants. We'll look at how an organic reaction actually happens. Organic chemists use a variety of laboratory procedures and sophisticated gear to anticipate how an organic reaction may have occurred, much like detectives analyse a crime after it has occurred and come to a fair prediction about how the crime may have occurred. Many of these methods are based on the Redox, Equilibrium, Chemical Kinetics, and Thermodynamics concepts of Physical Chemistry. Polarimeter, NMR Spectroscopy, Electric Dipole Measurement Device, isotopic tracer, pH metre, and other instruments were employed.

We will also solve reaction mechanism NEET questions for the students to get familiar with the type of questions asked in the exams. 

Important Topics for Reaction Mechanism

• SN1 Reaction Mechanism

• SN2 Reaction Mechanism

• Free Radicals

• Elimination Reaction

• Organic Rearrangement Reaction

Important Concepts for Reaction Mechanism

Organic Reaction Classification

• Organic reactions in higher secondary classes can be divided into the following categories:

1. Substitution reactions include (i) free radicals, (ii) nucleophilic reactions such as SN-1, SN-2, and SNi, and (iii) electrophilic reactions.

2. E1 and E2 Elimination Reactions.

3. Electrophilic, Nucleophilic, and Free Radical Addition Reactions.

4. Rearrangements.

Substitution Reactions

• A substitution reaction is one in which one group or atom is replaced by another. 

• The arriving group binds to the same carbon that the outgoing group binds to. 

• The type of the substituents involved in substitution reactions has been categorised.

• Substitution reactions occur when one atom or a group of atoms in a substrate is replaced by another atom or group.
  Electrophilic substitution reactions, nucleophilic substitution reactions, and free radical substitution reactions are the three types of substitution reactions.

Free Radical Substitution

• In the gas phase or in nonpolar liquids, radicals initiate radical substitution processes.

• Homolytic fission of chlorine is caused by light or heat, resulting in chlorine radicals that attack methane to create methyl chloride.

Nucleophilic Substitution

• One of the two primary reactions of alkyl halides is nucleophilic substitution. On an sp3 hybridised carbon, a nucleophile replaces a leaving group.

• One sigma connection is broken, while another is established.

• SN1 and SN2 are two proposed mechanisms.

• The rate and/or mechanism of a reaction will be affected by any factors that alter the energy of activation.

• These factors can help determine whether SN1 or SN2 is present. 

• The type of the leaving group, the nature of nucleophiles, the participation of neighbouring groups, and the action of the solvent are all factors to consider.

• The rate and/or mechanism of a reaction will be affected by any factors that alter the energy of activation. 

• These factors can help to determine whether SN1 or SN2 is present. 

• The type of the leaving group, the nature of nucleophiles, the participation of neighbouring groups, and the action of the solvent are all factors to consider.

Elimination Reaction

• Alkyl halide elimination reactions are essential processes that compete with substitution reactions. 

• The fragments of a molecule (YZ) are removed (eliminated) from adjacent atoms of the reactant in an elimination process.

• An elimination reaction is a reaction in which two atoms or groups are eliminated from a substance. 

• The method of elimination is determined by the strength of the base: strong bases favour E2 reactions, while weak bases favour E1 reactions.

Elimination vs Substitution Reactions

• All bases are potential nucleophiles, and all nucleophiles are prospective bases. This is due to the fact that both nucleophiles and bases have unshared electron pairs in their reactive parts. 

• It's no surprise, then, that nucleophilic substitution and elimination reactions frequently compete with one another.

• As a result, substitution reactions are frequently associated with elimination reactions. 

• If the reagent is a good base, it absorbs protons to produce elimination products (alkenes), while if it is a good nucleophile, it attacks the carbon to produce substitution products.

• The percentage of elimination and substitution is determined by the substrate's structure, the nature of the base, the nature of the solvent, and the temperature effect.

• When a strong base with a high concentration is combined with a low polarity solvent, the proportion of elimination increases. 

• Using a weak base with a low concentration and a solvent with high polarity, on the other hand, enhances the proportion of substitution.

Rearrangement Reactions

• The structural changes within a species are referred to as rearrangement. In general, there are two sorts of rearrangements: transferring a group from one atom to the other and rearrangement due to resonance.

• Transferring a Group from One Atom to the Other: Although a group can be shifted from one atom to another, the majority of group changes occur between nearby atoms. The existence of six valence electrons and an empty orbital on an atom is the primary cause of this shift. The group from the next atom migrates with the bonding electrons as such an atom tends to complete its octet.

• Rearrangement Due to Resonance: In these circumstances, no rearrangements occur, but due to the resonance in the intermediate, many products are generated. Pseudo rearrangements are another name for these types of rearrangements. These rearrangements are conceivable when intermediates such as carbocation, carbanion, or carbon-free radical are resonance stabilised, allowing for many resonance structures.

Solved Examples Based on the Chapter: Reaction Mechanism

Question 1: In the SN2 reaction, how many transition stages are involved?

(a) 1

(b) 2

(c) 3

(d) 0

Solution: 

• There is only one transition state because it is a one-step second-order reaction.

• As a result option (a) is the correct answer. 

Key point to remember: The steps of any order of reaction matter the most and hence the number of steps determines the transition states. 

Question 2: Arrange the nucleophilic strength of the following Nus in the correct order.

(a) OH- > CH3COO- > CH3O- > C6H5O-

(b) CH3COO- < C6H5O- < CH3O- < OH- 

(c) OH- > CH3O-  > CH3COO- > C6H5O-

(d)  CH3COO- < C6H5O- < OH- > CH3O- 

Solution: 

• Acetic acid is a kind of acetic acid that acidic compounds include CH3COOH and phenol C6H5OH. 

• Resonance is used to keep anions stable. 

• There is no resonance between CH3O- and OH-.

• As a result, option (d) is the correct answer.

Key points to remember: Understand the resonance structures of the molecule. 

Solved Examples from Previous Years’ Question Papers

Question 1: Due to the presence of an unpaired electron, free radicals are

(a) Chemically reactive

(b) Chemically inactive

(c) Anions

(d) Cations

Solution: 

• Free radicals are chemically active due to the presence of an unpaired electron.

• As a result, option (a) is the correct answer.

Question 2: Which one has the highest SN1 reaction reactivity?

(a) C6H5CH(C6H5)Br

(b) C6H5CH(CH3)Br

(c) C6H5C(CH3)(C6H5)Br

(d) C6H5CH2Br

Solution: 

• Out of all the given compounds, C6H5C(CH3)(C6H5)Br is the one which provides the most resonance on leaving the Br- ion.

• Hence, the compound C6H5C(CH3)(C6H5)Br has the highest reactivity undergoing SN1 reaction. 

• As a result, option (c) is the correct answer.

Question 3: Formation of ethylene from acetylene is an example of

(a) Elimination reaction

(b) Substitution reaction

(c) Addition reaction

(d) Condensation reaction

Solution: 

• The reaction as given in the question is as follows:
  CH≡CH + H2 → CH2=CH2.

• The above reaction is called the addition reaction.

• As a result, option (c) is the correct answer.

Practice Questions

Question 1: The SN1 mechanism for nucleophilic substitution reaction is preferred by

(a) Low nucleophile concentration 

(b) Weak nucleophile 

(c) Polar solvent 

(d) All of the above

Answer: (d) all of the above

Question 2: Which of the following reactions is an example of elimination?

(a) Methane chlorination 

(b) Ethanol dehydration 

(c) Benzene nitration 

(d) Ethylene hydroxylation

Answer: (b) Ethanol dehydration 

Conclusion

The reaction process, as we all know, is governed by chemical kinetics. Intermediates are species that are created in one step and eaten in a subsequent step. The disintegration of ozone indicates that two ozone molecules react to produce three oxygen molecules; nevertheless, the mechanism of the reaction does not entail two ozone molecules colliding and reacting.