Principles and Applications of Photochemical Reactions MCQ Quiz in मराठी - Objective Question with Answer for Principles and Applications of Photochemical Reactions - मोफत PDF डाउनलोड करा

The correct answer is Mixture richer is cis isomer 

Concept:-

• Cis-Trans Isomerization: 
  Cis-trans isomerization involves the conversion of cis- and trans-isomers of alkenes into each other in the presence of UV light.
• Energetics of Isomerization:
  The isomerization process is typically reversible. The relative stability of cis and trans isomers depends on the specific alkene.
• Reaction Mechanism:
  During photolysis, the UV light provides energy to break the π bond in the alkene. The subsequent rearrangement can lead to the formation of both cis and trans isomers.

Explanation:-

Photochemical Reaction of Alkenes:

When alkenes undergo a photochemical reaction, such as photolysis in the presence of ultraviolet (UV) light, a process known as cis-trans isomerization can occur.

The answer "Mixture richer in cis isomer" suggests that, during the photochemical reaction, more cis isomers are generated compared to trans isomers.
The specific conditions and characteristics of the alkene determine the extent of isomerization and the relative stability of cis and trans isomers.

Cis-Trans isomerization of alkenes by direct irradiation
Because of the large energy absorptions, direct irradiation of the simplest alkenes (whose λmax is below 200 nm) is difficult to achieve.  the energy of a π → π* excited species is a function of the angle twist around the carbon-carbon (sigma) bond, and trans-to-cis-isomerization is thought to be affected by distortion of the trans-excited state initially produced, to an excited state common to both cis- and trans isomers.

A phantom state is the name given to this aroused state.In terms of the sample model, the cis-trans isomerization is triggered by the excitation of
an electron to a planar excited state, which then relaxes to the twisted state. The figure shows a simplified representation.

Twisting the planar trans singlet to the twisted singlet state requires a low activation energy of 2 kcal/mole. This twisted form can decompose into either cis or trans stilbene. However, as previously stated, the trans form absorbs more light at the exciting wavelength (εmax trans = 16300) than the cis form (max cis = 2880), and as a result, the trans isomer is almost fully transformed to the cis isomer.

Conclusion:-

The photolysis of alkenes leads to the generation of a mixture richer in the cis isomer.

The Chemical reaction involved in the above transformation can be illustrated as

Explanation:-

The reaction pathway is shown below:

Conclusion:-

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

The correct solution is option b

Concept:-

The dienone-phenol rearrangement is a type of organic reaction that involves the conversion of a dienone to a phenol or a phenolate by migration of a hydride ion (H⁻) or of an alkyl/aryl group.

This rearrangement reaction is usually facilitated under either basic or acidic conditions and can lead to the formation of sterically and electronically stabilized phenolic products from dienone precursors.

Here's a generalized sequence of steps for the dienone-phenol rearrangement:

• Under basic conditions, deprotonation would happen at the α-carbon of the carbonyl group, which is actually the β-position of the dienone system. The deprotonated species then abstracts a proton from solvent, leading to a rearrangement with migration of a hydride from an α'-carbon (the carbon next to α-carbon).
• If the reaction undergoes under acidic conditions, one of the carbonyl groups in dienone will be protonated first, followed by a rearrangement including a 1,2-alkyl or aryl shift.
• In both cases, tautomers of phenol or phenolate will be generated.

The dienone-phenol rearrangement is synthetically very useful as it allows for the rearrangement of carbon skeletons and the formation of new carbon-oxygen bonds. This is particularly useful for synthesizing complex molecules and introduces a hydroxyl functional group that can undergo further reactions.

Explanation:- 

It is a type of dienone-phenol rearrangement.

step 1:Protonation of the carbonyl group 

step 2:The carbocation intermediate then undergoes a 1,2-alkyl

Step 3:Deprotonation of this intermediate

Conclusion:-

So, the  correct option is 2

Concept: 

The mechanism of the reaction follows Norrish Type-I and Type-II reaction.

Norrish Type-I reaction involves photochemical cleavage of \(\alpha,\beta\)-unsaturated ketone/aldehyde group where as Norrish Type-II proceeds via photochemical abstraction of \(\gamma\)-hydrogen.

Example:

Norrish Type-I:

Norrish Type-II

Explanation:-

• The reaction pathway for the formation of molecules A is as follows:

• Thus, the mechanisms for the formation of molecules A involves oxa-di-π-methane rearrangement.
• The reaction pathway for the formation of molecule B is as follows:

• As, the \(\gamma\)-H present in the compound is far away from the carbonyl group, a Norrish type-II cleavge is not possible.
• Thus, the mechanisms for the formation of molecule B involve Norrish type I cleavage.

Conclusion:-

Hence, the mechanisms for the formation of molecules A and B involve, respectively oxa-di-π-methane rearrangement and Norrish type I cleavage

Concept:-

Diels–Alder reaction:

• Diels–Alder reactions occur between a conjugated diene and an alkene, usually called the dienophile.
• This reaction goes in a single step simply on heating.
• Here are some examples: first an open-chain diene with a simple unsaturated aldehyde as the dienophile

Diene:

• The diene component in the Diels–Alder reaction can be open-chain or cyclic and can have many different substituents.
• There is only one limitation: The diene must have the s-cis conformation.
• Butadiene typically prefers the s-trans conformation with the two double bonds as far away from each other as possible for steric reasons. The barrier to rotation about the central σ bond is small (about 30 kJ.mol−1 at room temperature) and rotation to the less favorable but reactive s-cis conformation is rapid.

• Cyclic dienes that are permanently in the s-cis conformation are exceptionally good at Diels–Alder reactions—cyclopentadiene is a classic example—but cyclic dienes that are permanently in the s-trans conformation and cannot adopt the s-cis conformation will not do the
  Diels–Alder reaction at all.
• Dienes permanently in the s-cis conformation are excellent for Diels–Alder reactions.

Explanation:-

• The diene that undergoes Diels - Alder reaction with maleic anhydride must have an s-cis confirmation as follows:

• Only the diene can undergo Diels - Alder reaction with maleic anhydride by attaining s-cis conformation. 

• The reaction pathway is shown below:

• The remaining dienes are locked in an s-trans conformation and hence do not react with maleic anhydride to give Diels - Alder reaction

Conclusion:-

• Hence, he diene that undergoes Diels - Alder reaction with maleic anhydride is:

Concept:-

​The Alder ‘ene’ reaction:

• The Diels–Alder reaction was originally called the ‘diene reaction’ so when half of the famous team (Kurt Alder) discovered an analogous reaction that requires only one alkene, it was called the Alder ene reaction and the name has stuck. Compare here the Diels–Alder and the
  Alder ene reactions.
• The below diagram explains the difference between a Diels–Alder reaction and an Alder ene reaction.

​

• The simplest way to look at the ene reaction is to picture it as a Diels–Alder reaction in which one of the double bonds in the diene has been replaced by a C–H bond.
• The reaction does not form a new ring, the product has only one new C–C bond, and a hydrogen atom is transferred across space. Otherwise, the two reactions are remarkably similar.

Explanation:

Mechanism:-

• The reaction pathway is shown below:

• Selective formation of the cis-disubstituted cyclopentane via intramolecular ene reaction is due to the fact that the transition state will be of lower energy if the hydrogen atoms are on the same side of the folded bicyclic structure. This constraint is similar in a way that a five-membered ring fused to a six-membered ring is lower in energy if it is cis-fused.

Conclusion:-

• Hence, the given reaction, the major product is:

Concept:-

Diels-Alder reaction:

• Diels-Alder reaction is a type of pericyclic reaction between an alkene (called dienophile) and a diene. 
• The reaction proceeds through concerted mechanism.
• It is a syn cycloaddition reaction and thus 'locked 'cis isomer favors the reactivity with dienophile.

Trapping reactive intermediates by cycloadditions:

• The trapping of a reactive intermediates can be done in a Diels– Alder reaction. One way of generating benzyne for this purpose is the diazotization of anthranilic acid (2-aminobenzoic acid).

• Benzyne may not look like a good dienophile but it is an unstable electrophilic molecule so it must have a low-energy LUMO (π* of the triple bond). If benzyne is generated in the presence of a diene, effi cient Diels–Alder reactions take place. Anthracene gives a specially interesting
  product with a symmetrical cage structure.

Explanation:-

Mechanism:

• The reaction pathway is shown below:

• Diazotization of anthranilic acid gives a diazonium salt, which on treatment with a mild base undergoes elimination of CO2 and N2 to give benzyne intermediate (I). It immediately undergoes Diels-Alder reaction to give II.

​Conclusion:-

• Hence, the intermediate I and the major product II in the following conversion involves:

Benzyne and 

Concept:-

Cycloaddition Reaction:

• The cycloaddition reaction is a chemical reaction in which two or more unsaturated molecules combine with each other to form a cyclic compound, with a reduction of bond multiplicity.
• In other words, cycloaddition reaction indicates the addition of two or more π-bonds to form a cyclic adduct with the formation of bonds at the end of the pi bonds.

Explanation:-

• The given reaction is

• The reaction pathway is shown below:

• The 8\(\pi\) electrons of the cyclic system react with the 2\(\pi\) electrons of ethene-1,1,2,2-tetracarbonitrile, 

Conclusion:-

• Hence, the following reaction is an example of

[8 + 2] cycloaddition