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NEET Study notes for Reaction Mechanism, Important Formulas, and Sample Questions
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Nikkil Visha

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A reaction Mechanism is the sequence of some simple steps that lead to a chemical reaction. At times when more than two steps get involved the reaction gets complicated. The slowest steps out of all determine the final reaction. The last step is the slowest it does not make a huge difference but if the first step is slow then the subsequent steps need to wait. The weightage to this chapter in NEET is around 5%. 

Chemistry is among those toughest subjects in the NEET Exam that becomes difficult to score. If the candidate becomes successful to score at least 140 marks by attending 40 questions minimum then the student can qualify. Check NEET Chemistry Syllabus

Points to Remember

Points to Remember in Reaction Mechanism

  • The detailed study of the reaction mechanism is important as it points out different aspects of chemical reactions. 
  • The study helps in understanding and controlling different chemical reactions. The reactions themselves involve different molecules, ions, electrons, and free radicals at different interfaces. 
  • Based on the reaction mechanism one can find out different co-relations between systems but it is not obvious. 
  • Detailed information about the reaction mechanism permits unification and helps in the understanding of large bodies of unrelated phenomena. 
  • Several techniques and methods are using which the reaction mechanism takes place. The best part of the reaction mechanism is- it changes the understanding of the substance. 
  • To understand the energy requirement of the reaction, the methods are important. 
  • When the change of molecule or substance takes place due to the reaction mechanism, the overall route of change is called the course of the reaction, and the detailed process it went through is called the pathway.

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General Principles

General Principles

  • It is important to understand the flow of electrons to get the chemical transformation mechanism. 
  • One needs to get through the drivers and the key levers which are associated with the models. 
  • The lever includes the flow of electrons, their solvent effect, polarity, stability, and the balancing reactions in organic chemistry.

The reaction of Nucleophiles and Bases

  • The nucleophile is an electron donor (that has an electron pair for bonding) that can easily bond with an atom apart from hydrogen. 
  • The action of bases or nucleophiles can give rise to numerous and varied transformations. 
  • These transformations follow a set of principles. These transformations can be categorized differently based on the level of understanding that is applicable in many situations. 
  • Different types of nucleophiles and electrophiles can have different transformation effects.

Sample Question

Ques: The increasing order of hydrolysis of the following compounds:-

Ques 1

  1. (i)> (iii)>(ii)
  2. (i)<(iii)<(ii)
  3. (ii)<(i)<(iii)
  4. (i)<(ii)<(iii)

Ans:- Option A

Reaction with Acid and Other Electrophiles

  • Electrophiles are categorized as electron-deficient species that attract electron-rich species. 
  • When an Electrophile accepts an electron pair then it reacts to bond with the nucleophile that also includes the interactions of proton and a base. 
  • Electrophiles are normally a positive electron that does not contain octet of electrons and it is because it contains an atom. 
  • The addition of nucleophiles to electrophiles and rearrangements, hydrolysis, and substitution processes are affected by the reaction of an electrophilic species and it is described by Lewis Acid.

Radical and radical Anions

  • The species like radicals contain one or more than one unpaired electron. Although radical species are also electron-deficient, they are usually uncharged and have properties that are distinct from carbenes.
  • Early radical chemistry possesses high-energy and is generally unpredictable processes. But with time radical chemistry became a well-developed set of tools that are both versatile and predictable and represent a contribution to the overall understanding of chemical processes. 
  • Key aspects of the mechanisms and outcomes of a variety of radical processes emphasize understanding the forces which drive these reactions and their outcomes.

Multistep Reaction Mechanism

It is believed that some reaction occurs through two elementary steps. For example

Multistep Reaction Mechanism

After adding the overall reaction 

Multistep Reaction Mechanism Ex2

Pericyclic Reaction

  • An organic reaction in a cyclic transition state leads to a concerted transformation. 
  • A majority of these reactions are rearrangements and are predictable. It uses systems that were developed upon many observations that are associated with molecular orbitrary theory. 
  • These transformations are used for their predictability and consistent performance.

Sample Question

Ques: Which of the following statement is true?

  1. The configuration of the chiral carbon is the same
  2. The configuration of the chiral carbon gets inverted.
  3. The compound changes into a Dextro isomer.
  4. The reactant is optically inactive but the product is obtained as a Levo isomer.

Ans:- Option A

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Oxidation and Reduction

  • Reductions and oxidations are chemical transformations where electrons are added or removed from a molecule or atom. 
  • Oxidation or reduction avoids involving electron transfer, but it involves a change in oxidation number which is associated with the number of bonds an atom has to minimize electronegative atom like hydrogen or to an atom that is more electronegative. 
  • There are many methods developed to effect the change of oxidation number or to change the state of oxidization which will give desired structural change. 
  • At times oxidations and reductions constitute a powerful tool in organic chemistry and provide us with a great opportunity to understand the driving forces in a better manner that will help to prepare the used in our daily lives.

Sample Question

Ques: The reactivity order of Halides for dehydrohalogenation is:-

  1. R-F>R-CI>R-Br>R-I
  2. R-I>R-Br>R-CI>R-F
  3.  R-I>R-CI>R-Br>R-F
  4. R-F>R-I>R-Br>R-CI

Ans: Option C

Quick Notes on Different types of Reactions

When the chemical reactions take place it changes the substance of the article. When one atom breaks and another atom are created it results in a different equation and product. There are majorly four types of reaction:-

  1. Synthesis- The process is also called a combination reaction. The process involves two or more elements to create one complex substance. The synthesis also converts large molecules into small ones. The reactions can take place between different kinds of the compound. When multiple equations form a single equation, it results in synthesis.
  2. Decomposition- when a complex molecule divides into a simple one it is called decomposition. The reaction takes place between two molecules or substances. In simple words, it is the opposite of a Synthesis reaction. Eg- Hydrogen oxide divides into Hydrogen and water and Oxygen. Soft drinks also get their carbonation from decomposition. Most of the decomposition reactions are endothermic. More energy is required to form a new chemical bond and different products. But few of them are also exothermic. Instead of absorbing they release more heat. The different types of decomposition are- Thermal decomposition, electrolytic decomposition, photolytic decomposition. Catalyst also results in decomposition. The process is used in extracting metals from the oxides, treating acid indigestion, obtaining hydrogen which is compounded, and identifying different simple decomposed products.
  3. Single Displacement- It is also called Single Replacement Reaction. The formula stands out to be A+B= B+AC. The types of replacement are- Cation Replacement, Anion Replacement, etc. If the reactant is not more reactive other than cation, the reaction does not occur. The reaction action is good but the forward reaction is not good. One can easily predict the single replacement reaction. All the halogens have a simple substitution with oxidation as (-1). The element is not halogen, it replaces the cation in the compound. Two cations will never have the same oxidation state. The reactivity series for cation is no longer and not obvious.
  4. Double Replacement- the cation and anion of two compounds switch their places. By exchanging the ions two new products are formed. The double replacement may contain those reactants which contain either ionic or covalent bonds but mostly one can find ionic bonds in the compound. Usually, the solvent used in double replacement is water. One can simply take the example of silver nitrate and sodium chloride in the water solvent as a result of double replacement. One can identify the double replacement by witnessing the exchange of anions with each other. The types of double replacement are neutralization, alkylation, acid-carbonation reaction, counter-ion exchange, and double decomposition, etc. 

Important Formulas

Important Formulas for exam

  1. Molecularity and the Rate-Determining Step

Examine the much simpler reaction of carbon monoxide with nitrogen dioxide


The experimentally determined rate law for the reaction is


  1. Using Molecularity to Describe a Rate Law
Elementary Reaction Molecularity Rate Law Reaction Order
A → products unimolecular rate = k[A] first
2A → products bimolecular rate = k[A]2 second
A + B → products bimolecular rate = k[A][B] second
2A + B → products termolecular rate = k[A]2[B] third
A + B + C → products termolecular rate = k[A][B][C] third

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Tips to solve questions from this chapter:

  1. One should prepare their notes for better understanding
  2. One should get all the important formulas in one place for last-minute revision.
  3. Understand the concept of atom, molecule and their bonding process.
  4. Try solving previous year's questions. Few of the samples are given in this particular article.
  5. Read the questions multiple times so that the concept can be understood.

Past Yr Questions

Past year questions related to the Reaction mechanism

Ques: In an SN1 reaction on chiral centers, there is

  1. 100% racemization
  2. 100% retention
  3. 100% inversion
  4. 100% partial racemization

Ans:- Option C

Ques: Which are the best-suited method for the separation of para and ortho-nitrophenols from1:1 mixture?

  1. Crystallization
  2. Sublimation
  3. Steam Distillation
  4. Chromatography

Ans:- Option C

Ques: What is the incorrect statement about nucleophiles?

  1. A nucleophile is a Lewis acid
  2. Nucleophiles do not seek electron
  3. Ammonia is a nucleophile
  4. Nucleophile attacks low electron

Ans: Option A

Ques: How many structural Isomers are possible if one hydrogen is replaced by chlorine?

  1. 8
  2. 4
  3. 7
  4. 6

Ans: Option B

Ques: Which of the following methods is best suited for the separation of a mixture containing Naphthalene and benzoic acid?

  1. Crystallization
  2. Sublimation
  3. Distillation
  4. Concentration

Ans: Option B

Ques: What will be the major product of the following reaction?

Ques 5

Ans: Option C

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