Common Catalysts and Reagents MCQ Quiz in தமிழ் - Objective Question with Answer for Common Catalysts and Reagents - இலவச PDF ஐப் பதிவிறக்கவும்

Concept:-

 Corey–Winter reaction:

• Elimination of thionocarbonates is termed the Corey–Winter reaction.
• Attack by the phosphorus reagent on the sulfur atom is followed by fragmentation to give the alkene.

Explanation:-

• The reaction pathway is shown below
  

Conclusion:-

• Hence, option 4 is the correct answer.

Concept:

→ The Corey–Winter olefin synthesis (also known as Corey–Winter–Eastwood olefination) is a series of chemical reactions for converting 1,2-diols into olefins.

→ The reaction mechanism involves the formation of a cyclic thiocarbonate from the diol and thiophosgene.

→ The second step involves treatment with trimethyl phosphite, which attacks the sulfur atom, producing S=P(OMe)3 (driven by the formation of a strong P=S double bond) and leaving a carbene. This carbene collapses with loss of carbon dioxide to give the olefin.

Mechanism:

Conclusion:
The correct answer is option 1.

Explanation:

1. In the first step selective oxidation using Mno₂ will take place at allylic alcohol to form enone.

2. In second step, second OH group will get protected using NaH and addition of MeI to get OMe.

3. The enone will undergo cyclopropanation using sulphur ylide.

4. In the last step wittig reaction will take place in which MePPh₃ will convert the carbonyl group into alkene in presence of NaH.

Mechanism:

Conclusion:-

So, The correct sequence of reagents that will lead to the formation of the given product in the following transformation is I. active MnO2; II. MeI, NaH; III. Me3S(O)I, NaH; IV. MePPh3Br, NaH

The correct answer is DDQ, CH2Cl2

Concept -

 –The p-Methoxybenzyl (PMB or MPM) group can be protected or deprotected under the same conditions as the benzyl group.

-PMB trichloroacetimidate (PMB-O(C=NH)CCl3) provides a way to protect base sensitive compounds under acidic conditions.

-There are two dimethoxybenzyl (DMB or DMPM) groups (2,4-dimethoxy and 3,4-dimethoxy), both of which can be deprotected under milder conditions than PMB.

Explanation - 

In this reaction, it is deprotected under mildly oxidizing conditions using DDQ (dichlorodicyanobenzoquinone) or strongly acidic conditions and converted to alcohol.

Conclusion:-

So, The reagent that will effect the following selective conversion is DDQ, CH2Cl2.

Concept:

Epoxides are three-membered cyclic ethers that are highly reactive due to ring strain. They can be opened via nucleophilic attack under both basic and acidic conditions. The mechanism of epoxide ring-opening differs based on the conditions, leading to different regioselectivity.

• Basic Medium (Nucleophilic Attack on Less Substituted Carbon): Under basic conditions, the nucleophile (in this case, methoxide ion, MeO⁻) attacks the less substituted carbon of the epoxide ring. This is because the nucleophilic attack occurs via an SN2 mechanism, where steric hindrance is minimized. The result is the formation of a methoxy alcohol product.

• Acidic Medium (Nucleophilic Attack on More Substituted Carbon): In an acidic medium, protonation of the epoxide occurs first, making the epoxide more electrophilic. The nucleophile (methanol, MeOH) then attacks the more substituted carbon due to increased positive charge buildup on this carbon (since it's more stabilized by neighboring alkyl groups). This leads to the formation of a product where the nucleophile is attached to the more substituted carbon.

Explanation: 

• Step 1: In the first reaction, the epoxide is opened in the presence of methoxide ion (MeONa), which attacks the less substituted carbon, leading to the formation of compound B, where methoxy (OMe) is attached to the less substituted carbon and a hydroxyl group (OH) is attached to the other carbon.

• Step 2: In the second step, the epoxide is opened in an acidic medium (HCl and methanol, MeOH). The epoxide is first protonated, and methanol attacks the more substituted carbon, leading to the formation of compound C, where methoxy (OMe) is attached to the more substituted carbon and a hydroxyl group (OH) is attached to the less substituted carbon.

• Thus, product B results from nucleophilic attack on the less substituted carbon in the basic medium, and product C results from nucleophilic attack on the more substituted carbon in the acidic medium.

• Mechanism:

  • 
     

Conclusion:

The major products formed are B = [(OMe on less substituted carbon, OH on more substituted)] and C = [(OH on less substituted carbon, OMe on more substituted)], making Option 4 the correct answer.

The correct answer is option 2

Concept:-

Carbenes are neutral species with a divalent carbon atom and two nonbonded electrons, making them highly reactive, particularly with alkenes. The reaction generally proceeds via a concerted mechanism where the carbene inserts into the double bond, forming a cyclopropane derivative.

Explanation:-

(P): When dichlorocarbene adds to trans-but-2-ene, it forms a three-membered ring (a cyclopropane ring) where the carbene adds across one of the double bonds. Given the geometry of trans-but-2-ene, the addition of the dichlorocarbene will yield a cyclopropane derivative with both chlorines on the same carbon atom due to the linear structure of the carbene and its approach to the planar (\pi) system of the alkene.

Conclusion:-

So, the final product is option 2

The Correct Answer is Option 3.

Concept:- 

Stille Coupling Reaction- Palladium catalysed coupling reaction between aryl or alkenyl halides with aryl, alkenyl, alkynyl tin compounds which results in the C-C bond formation.

Luche's Reagent- It's a selective reduction method of α, β- unsaturated ketone to allylic alcohol where active reductant is Cerium Borohydride.

Explanation:- 

Step 1-  Iodination reaction of alkene in basic condition.

Step 2- Stille Coupling Reaction between Alkenyl iodide and alkenyl tin compund.

Step 3- Selective Reduction of  α, β- unsaturated ketone to allylic alcohol using Lucha's reagent.

The conversion mechanism is given below.

Conclusion:- The Correct answer is Option 3.

The answer is option 3.

Concept:-

Hydroxyl-Directed Reduction of B-Hydroxy Ketones
Alkoxydialkylboranes (R'OBR,) react with B-hydroxy ketones to form boron chelate
intermediates that on subsequent reduction give the 1,3-syn diols.30 ethoxydiethylborane and NaBH, are the reagents of choice for this transformation. The chelate is reduced via intermolecular hydride delivery at the face opposite from the β-R group, as shown below,

Explanation:-

In the case of NaBH4 it will attack from a less steric hindrance side, so it it will come from above the plane then the hydride will come from below the plane resulting on syn diol formation.

NaBH(OAc)3 will form chelation with Oh and C=O and it will produce anti- diol

Conclusion:-

So the correct answer will be :

option 3

Concept:

• Grubbs catalysts are a series of transition metal carbene complexes used as catalysts for olefin metathesis. Several generations of the catalyst have also been developed. 
• Grubbs catalysts tolerate many functional groups in the alkene substrates, are air-tolerant, and are compatible with a wide range of solvents.
• For these reasons, Grubbs catalysts have become popular in synthetic organic chemistry. 

Explanation:-

The given reaction is an example of a metathesis cyclisation reaction and during such reaction catalyst used is m-carbene Grubbs catalyst out of the given catalyst.

So catalyst used is Grubbs catalysts .i.e. option 1

Conclusion:-

So, catalyst used for this reaction is  Grubbs catalysts .i.e. option 1

Concept:-

• Oxidation States of Metal Complexes: Catalysts often work by transitioning between oxidation states. The ability to easily change oxidation states (like Pd(II) to Pd(0) and back) is essential to their function.
• Coordination Chemistry: This reaction showcases how organic molecules can coordinate to metals. The palladium(II) compound forms transient bonds with the alkene, showcasing one aspect of the area of chemistry known as coordination chemistry.
• Catalysis and Reaction Mechanisms: Catalysis speeds up reactions by reducing the activation energy and providing a different pathway for the reaction to follow. Reaction mechanisms, much like the mechanism shown above for Wacker oxidation, provide detailed insights as to how these catalysts operate and serve as the basis for improvement or modification of the reaction conditions. This is especially important in industrial applications, such as Wacker oxidation.

Explanation:-

The Wacker process or the Hoechst-Wacker process refers to the oxidation of ethylene to acetaldehyde in the presence of palladium(II) chloride and copper(II) chloride as the catalyst. This chemical reaction was one of the first homogeneous catalysis with organopalladium chemistry applied on an industrial scale.

[PdCl4]2 − + C₂H₄ + H2O → CH₃CHO + Pd + 2 HCl + 2 Cl⁻
This conversion is followed by reactions that regenerate the Pd(II) catalyst:

Pd + 2 CuCl₂ + 2 Cl − → [PdCl₄]₂⁻ + 2 CuCl
2 CuCl + 1/2 O₂ + 2 HCl → 2 CuCl₂ + H2O
Only the alkene and oxygen are consumed. Without copper(II) chloride as an oxidizing agent, Pd(0) metal (resulting from beta-hydride elimination of Pd(II) in the final step) would precipitate, stopping the reaction after one cycle. Air, pure oxygen, or a number of other reagents can then oxidize the resultant CuCl-chloride mixture back to CuCl₂, allowing the cycle to continue.

Conclusion:-

So, In wacker oxidation Pd(II) and Cu(II) are used