Organic Reaction Mechanisms MCQ Quiz in తెలుగు - Objective Question with Answer for Organic Reaction Mechanisms - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Concept:-

• The elimination reaction in a compound having a suitable leaving group, consists of three fundamental events, and they are;

  • Proton removal.
  • Formation of C-C pi bond.
  • Removal of the leaving group.
• Depending on the reaction kinetics, elimination reactions can occur mostly by two mechanisms namely E₁ or E₂ where the number represents the molecularity.
• An E₂ elimination reaction is a bimolecular elimination reaction that is basically a one-step mechanism. Here, the carbon-hydrogen and carbon-halogen bonds mostly break off to form a new double bond.
• In the E₂ mechanism, a base is part of the rate-determining step and it has a huge influence on the mechanism.
• The stereochemical condition for the E₂ elimination reaction is that the two eliminating groups must be in anti-periplanar configuration.
• 4-Toluenesulfonyl chloride or TsCl is an organic compound with the formula CH3C6H4SO2Cl. This reagent is widely used in organic synthesis. In presence of a base, it can transform a primary hydroxyl (-OH) group into a leaving group -OTs.

Explanation:-

• The reaction pathway is shown below:

• In the above reaction, the first step of the reaction involves the transformation of the primary hydroxyl (-OH) group into -OTs in presence of pyridine. It is a better-leaving group the hydroxyl (-OH) group.
• In the next step, to attain the anti-periplanar configuration between the H atom and -OTs group, the C-C bond is rotated through in-plane by 60 degrees.
• In the next step of the reaction, the base abstracts the H atom and it undergoes a E2 elimination reaction to finally give the product.

Conclusion:-

• Hence, the  major product formed in the following reaction is:

​

Concept:-

• Organic compounds can be transformed into products through multiple pathways and different processes.
• A detailed study of the sequence of steps involved in the conversion of reactant into the product is known as the reaction mechanism.
• One necessary condition for a chemical reaction is that a molecule must be activated before it enters into a chemical reaction. A reaction occurs if the collision of reactant molecules is accompanied by a minimum amount of energy called activation energy.
• Such a collision must occur with the proper orientation of reactant molecules. The particular arrangement of reactant molecules at the peak of the energy mountain is called the transition state.

Explanation:-

• The reaction pathway is shown below:

• In the above reaction, we can see that the reaction proceeds through three steps, which involve two reactive intermediates and three transition states.
• Every subsequent step is easier than the previous step and the product is more stable (exothermic) than the reactant.
• Thus, the energy of the intermediate product at the end of each step will be less compared to starting material.
• Hence, the correct energy profile diagram which best represents the progress of the reaction will be

Conclusion:-

• Hence, option 3 is the correct answer.

Concept:-

Nucleophilic substitution reactions-

Substitution reactions are the types of reactions where a nucleophile is an attacking reagent.

• There are three types of substitution reactions depending on the nature of the substrate.
  • Nucleophilic substitution at saturated carbon. 
  • Nucleophilic acyl substitution
  • Nucleophilic aromatic substitution.

Nucleophilic substitution reaction is mainly of two types. These are 

• SN1 or Unimolecular nucleophilic substitution and SN2 or bimolecular nucleophilic substitution

1. SN1 or Unimolecular nucleophilic substitution:

• Depends upon the concentration of the substrate.
• Is independent of the concentration of the nucleophile.
• Follows first-order kinetics.

2. SN2 or bimolecular nucleophilic substitution:

• The rate depends on the concentration of both the reactant and the substrate.
• It follows second-order kinetics.

Given below are the examples of SN2 and SN1 nucleophilic substitution reactions:

This reaction is a nucleophilic substitution reaction.

Here, Dimethyl formamide (DMF) is a polar aprotic solvent that stabilizes cation i.e. Na⁺ and gives the desired nucleophile PhS^- to which the reaction proceeds through the S_N2 mechanism with an inversion of configuration.

Polar aprotic solvents like DMF, DMSO, Acetone, etc. don't form a Hydrogen bond increasing the reactivity nucleophile.

As it occurs in a single step so the nucleophile attacks from the back side (the front side is blocked due to leaving the group). So it follows inversion stereochemistry.

Here is the diagram of the transition state. 

Below is an example of an S_N2  reaction.

Explanation:-

The Tosyl group is an excellent leaving group in substitution reactions. So the nucleophile phenyl sulfide (PhS^-) will attack on the back side as the tosyl group is bulky. It will follow the inversion stereochemistry. 

This reaction won't follow the S_N1 mechanism because

(i) The solvent should be polar protic like H₂O, ROH, etc. which increases the stability of carbocation formed as the intermediate in a SN1 mechanism.

Conclusion:-

• Hence, the major product formed in the following reaction is 
  

Concept:

• Methyl amine (MeNH₂) can act as a base as well as the nucleophile.  
• if substrate has acidic proton, it will preferably act as the base and abstracts the most acidic proton.
• the H is considered acidic if the negative charge formed after  its abstraction can be stabilized by conjugation or presence of some electronegative atom.

Explanation:

• In the first step, Methyl amine will abstract the most acidic proton attached to N in 5-membered ring.

          The generated negative charge is resonance stablized.

• In the next step, negative charge will move to C and will substitute Br^- to form 3 membered ring (SN²).

• Next step will follow up with  nucleophillic attack of another methylamine molecule at electrophillic carbon centre of Carbonyl bond.

• Finally, the back conjugation of negative change on O, will facilitate the breaking of  3-membered ring (which is unstable) in such a way that the aromaticity of 5-membered ring is regained.

Conclusion:

The final product of the reaction is :

Concept:-

• The kinetic isotope effect (KIE) describes the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes.
• It is the ratio of the rate constant of reactions involving the light (k_L) and the heavy (k_H) isotopically substituted reactants.
• A primary kinetic isotope (PKI) effect may be found when a bond to the isotopically labeled atom is formed or broken at the rate-limiting step.
• The difference in bond strength will be reflected in different rates of breaking of the two bonds under comparable conditions.
• Quantum mechanical calculation suggests a maximum rate difference observed when,

\({{{{\rm{k}}_{\rm{H}}}} \over {{{\rm{k}}_{\rm{D}}}}}{\rm{ = 7}}\)

Explanation:-

• Out of these four reactions, only the bromination of acetone in presence of acid (reaction C) involves the breaking of the C-H bond in the rate-limiting step of the reaction.
• The mechanism of bromination of acetone in presence of acid is given by,

​

• The first step involves acid-catalyzed enolization followed by the electrophilic attack of the bromine molecule on the nucleophilic carbon of the enol.
• As the rate-limiting step involves the breaking of the C-H bond,  this reaction is expected to show a primary kinetic isotope effect.

​Conclusion:-

Hence, the reaction that is expected to show a primary kinetic isotope effect for the indicated H‐atom (C‐H) is

Explanation:- 

• Diethyl aluminium chloride is used as a catalyst in  Ziegler-Natta catalysis.
• The organic compound is also used as a Lewis acid in various organic synthesis reactions.
• They are highly reactive and pyrophoric in nature.

Thus, the final product is an alpha-beta unsaturated carbonyl compound.

Reaction:-

Concept: 

NUCLEOPHILIC SUBSTITUTION REACTION

• A chemical reaction in which the displacement of a leaving group is taking place by a nucleophile.
• Nucleophilic substitution reactions mainly take place through two reaction mechanisms, S_N1 and S_N2.

Comparing the SN1 and the S_N2 reaction

Explanation:

Consider the reaction of 2-bromo-propanoic acid with OH^-.

• The reaction proceeds through S_N2 mechanism. 
• Backside attack of OH^- take place and product formed with inversion of configuration. 

So here the product P formed has an inversion of configuration. 

Consider the other reaction of 2-bromo-propanoic acid with OH^- in presence of Ag₂O/H₂O.

• The presence of Ag⁺ as a Lewis acid helps the ionization of substrate molecules. 
• The reaction proceeds through the intermolecular S_N2 mechanism with neighbouring group participation. 
• The result of the neighbouring group participation is the formation of substituted product with retention of configuration. 

So here the product Q formed has retention of configuration.

Concept:

Reaction of Alcohols with Thionyl Chloride (SOCl₂)

• Thionyl chloride (SOCl₂) is commonly used to convert alcohols into alkyl chlorides.

Explanation:

• When converting alcohols to alkyl chlorides using thionyl chloride (SOCl₂), the reaction generally proceeds with retention of configuration meaning the stereochemistry at the chiral carbon is maintained.
• The reaction of thionyl chloride with chiral 2º-alcohols has been observed to proceed with either inversion or retention. In the presence of a base such as a pyridine, the intermediate chlorosulfite ester reacts to form an "pyridinium" salt, which undergoes a relatively clean S_N2 reaction to the inverted chloride.

​Therefore, the correct major product is the chlorinated compound shown in option 3.

Concept:

Ozonolysis

Definition and Purpose: Ozonolysis is a reaction where ozone (O₃) cleaves the carbon-carbon double bonds (alkenes) to form carbonyl compounds. It is a widely-used method to break down alkenes into smaller, more functionalized fragments such as aldehydes or ketones.

Reaction Mechanism:

• Ozonide Formation: The reaction begins with the addition of ozone to the carbon-carbon double bond, forming an unstable intermediate called a molozonide, which rearranges to a more stable ozonide.
• Hydrolysis Step: The ozonide is then typically subjected to hydrolytic workup, which can be either reductive or oxidative.
• Reductive Workup: The ozonide can be cleaved under reductive conditions, often using zinc and acetic acid (Zn/HOAc) or dimethyl sulfide (DMS) as the reducing agents. This leads to the formation of aldehydes or ketones.
  • Example: RCH=CH₂ → RCHO (if hydrolysis conditions are reductive).
• Oxidative Workup: Using hydrogen peroxide (H₂O₂) under oxidative conditions results in the formation of carboxylic acids.
  • Example: RCH=CHR' → RCOOH + R'COOH (if hydrolysis conditions are oxidative).

Explanation:

Reaction Mechanism:

Conclusion:

Number of carbonyl groups present in the final product of the following reaction sequence is: 4

Concept:

The reactions P and Q involve dehydrohalogenation using i-PrOH (isopropanol) and a base at 43°C, leading to the formation of alkyne products. The reactions can proceed through different elimination mechanisms, such as E2 (bimolecular elimination) and E1cB (unimolecular elimination via a conjugate base intermediate). The pathway depends on the nature of the substrate and the reaction conditions.

Key Points on Elimination Mechanisms (E2 vs. E1cB):

• E2 Mechanism: A one-step mechanism where the base abstracts a proton as the leaving group departs. This reaction occurs in a concerted fashion and is favored by strong bases and good leaving groups.
• E1cB Mechanism: A two-step elimination where the base abstracts a proton, forming a carbanion intermediate. The leaving group is eliminated in a second step. This mechanism is favored when the leaving group is poor or when a strong base is used with a poor electrophile.
• The reaction rate constants k_P and k_Q reflect the relative reactivity of the substrates in these two elimination pathways.

Explanation: 

• P proceeds via the E2 mechanism: The structure of P allows for a concerted E2 elimination because the base can easily abstract a proton, leading to the rapid formation of the alkyne product. This is why the rate constant k_P is higher.
  • ​
• Q proceeds via the E1cB mechanism: In contrast, the structure of Q leads to an E1cB pathway due to a more difficult proton abstraction, forming a carbanion intermediate before the bromide leaves. This makes the reaction slower, reflected by k_Q being lower.
  • ​
  •  

Conclusion:

Thus, the correct statement is: k_P > k_Q; P proceeds via an E2 mechanism, and Q proceeds via an E1cB mechanism.