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Pericyclic reactions ,Photochemistry, Free radicals mcq M.Sc. SY Organic Chemistry

Photochemistry, Free radicals and Pericyclic reactions

Multiple Choice questions

1. Stilbene on photochemical reaction gives cis stilbene and trans stilbene what will be the percentage
a)  50:50
b) 93:7
c)  67:33
d) 60:40
Ans. (b)

2. Rearrangement of 1,4-Diene by photochemical process this rearrangement is called as
a)  di-pi methane rearrangement
b) Zimmerman rearrangement
c)  Both a and b
d) None of these
Ans. (c)

3. Irradiation of benzene gives
a)  Benzvalene
b) Fulvene
c)  Dewar benzene
d) All of the above
Ans. (d)

4. Ortho xylene on irradiation with photon of light gives
a)  Only meta xylene
b) Only para xylene
c)  Mixture of o,m,p-xylene
d) None of these
Ans. (c)

5. Ketene formation takes place in
a)  Norrish Type-1
b) Norrish Type-2
c)  Both a and b
d) None of these
Ans. (a)

6. β bond cleavage observed in which of the Photochemical process
a)  Norrish Type-1
b) Norrish Type-2
c)  Both a and b
d) None of these
Ans. (b)

7. α- bond cleavage observed in which of the Photochemical process
a)  Norrish Type-1
b) Norrish Type-2
c)  Both a and b
d) None of these
Ans. (a)

8. Carbonyl group reacts with alkene in photochemical way gives

a) Thioetane

b) Oxetane

c) Oxepane

d) All of these

Ans. (b)

9. In Paterno Buchi reaction which species is formed?

a) Thioetane

b) Oxetane

c) Oxepane

d) All of these

Ans. (b)

10. Norbornene on Photochemical reaction in solvent Acetone gives

a) Oxetane type product

b) Dimer product

c) Both a and b

d) None of the above

Ans. (b)

11. Photo fries rearrangement is

a) Intermolecular reaction

b) Intramolecular reaction

c) Both inter and intermolecular rearrangement

d) None of these

Ans. (b)

12. In Diels Alder reaction bonds are formed

a) 2 sigma bond 1 pi bond

b) 1 sigma bond 1 pi bond

c) 1 sigma bond 2 pi bond

d) 2 sigma bond 2 pi bond

Ans. (a)

13. Name the reaction between 1,4 Butadiene and quinine in presence of heat

a) Electrocyclic reaction

b) Sigmatropic reaction

c) Cycloaddition reaction

d) All of these

Ans. (c)

14. Trans diene undergoes cycloaddition reaction

a) True

b) False

c) Sometime correct

d) Depend on temperature

Ans. (b)

15. When both Diene and dienophiles are cyclic then which adduct formed abundantly

a) Endo adduct

b) Exo adduct

c) Internal adduct

d) External adduct

Ans. (a)

16. Which of the following group acts as withdrawing group in Diels Alder Reaction?

a) CHO

b) COOH

c) Both a and b

d) CH3

Ans. (c)

17. Cycloaddition reactions are

a) Supra facial

b) Antra facial

c) Superfacial

d) Downfacial

Ans. (a)

18. First sigmatropic reaction discovered as

a) Schemidt rearrangement

b) Cope rearrangement

c) Claisen rearrangement

d) None of these

Ans. (c)

19. Thermal electrocyclic reactions involving (4n + 2) pi electrons are

a) Disrotatory

b) Conrotatory

c) Antrafacial

d) Suprafacial

Ans. (a)

20. Thermal electrocyclic reactions involving (4n) pi electrons are

a) Disrotatory

b) Conrotatory

c) Antrafacial

d) Suprafacial

Ans. (b)

21. For Mc-Murry reaction the correct reaction condition is

a) MnCl3/THF 3oK

b) TiCl3/THF 3oK

c) CrCl3/THF 5oK

d) None of these

Ans. (b)

22. Benzene undergoes Birch reduction to give product

a) 1,3-cyclohexadienes

b) 1,5-cyclohexadienes

c) 1,2-cyclohexadienes

d) 1,4-cyclohexadienes

Ans. (d)

23. Benzoic acid undergoes Birch reduction to give…….

a) 2,6-cyclohexadienecarboxylic acid

b) 2,3-cyclohexadienecarboxylic acid

c) 2,5-cyclohexadienecarboxylic acid

d) 2,4-cyclohexadienecarboxylic acid

Ans. (c)

24. Given reduction is called

a) Birch reduction

b) Bouveault-Blanc reduction

c) McMurry Reduction

d) None of these

Ans. (b)

25. Name of given reaction is

a) Birch reaction

b) Bouveault-Blanc reaction

c) McMurry Reaction

d) None of these

Ans. (c)

26. Given reduction is called

a) Birch reduction

b) Bouveault-Blanc reduction

c) McMurry Reduction

d) None of these

Ans. (a)

27. Given reaction is called

a) Sandmeyer Reaction

b) Birch reaction

c) Bouveault-Blanc reaction

d) McMurry Reaction

Ans. (a)

Symmetry properties of Molecular Orbital’s

 

Symmetry properties of Molecular Orbital’s:

·       Molecular orbital’s of Ethene

            In alkenes molecular orbital’s of ethene the carbon atoms are sp2 hybridized. The double bond between two carbon atoms is comprised of a σ-bond and π-bond. The former (C-C σ-bond) is formed by overlap of two sp2 orbital’s. The overlapping result’s in two π-molecular orbitals. In this case bonding π-orbital’s is formed by the overlap of in phase (same phase) of ‘p’ orbital and other π* anti bonding orbital’s arises from the interference between two p-orbital’s of opposite phases. These are designated by ψ1 and ψ2 respectively. 

Molecular orbital diagram of ethene

·        Molecular orbital’s of 1,3 butadiene:      

    In butadiene there are four p-orbital’s (ignore the σ-skeleton) and their combination gives four orbital’s having wave function ψ1, ψ2,ψ3,ψ4 with different energies of these ψ1 and ψ2 π-molecular orbital is bonding and ψ3 and ψ4 are antibonding molecular orbital’s. Molecular orbital is bonding if the number of bonding interactions is greater than number of nodes between nuclei and molecular orbital’s is antibonding, if the number of bonding interaction is less than the number of nodes between the nuclei.

 Molecular orbital diagram of 1,3 butadiene 

    In case of butadiene out of the four molecular orbitals is ground state two vic. Ψ1 & ψ2 are lowest energies ψ2 is the highest occupied molecular orbital (HOMO) and ψ3* is the lowest unoccupied molecular orbital  (LUMO). Both HOMO and LUMO are referred to as Frontiers molecular orbital and are used in analysis of pericyclic reactions. On absorption of a photon of proper wavelength (in the UV range) an electron is promoted from ψ2 to ψ3* which then becomes a new HOMO.

Molecular Orbital’s of 1,3,5-hexatriene

            The six p-electron of 1,3,5-hexatriene are accommodated in the first three molecular π-orbital’s  ψ1,ψ2,ψ3 which are bonding orbital’s and the remaining three higher energy molecular π-orbital’s (ψ4,ψ5,ψ6) are antibonding orbital’s which are remain unoccupied in the ground state. That six π-electrons are accommodated in ψ1,ψ2 and ψ3 the bonding orbital’s.

Molecular Orbital digram of 1,3,5-hexatriene

The bonding or antibonding interactions of Frontier Molecular Orbital’s (FMO) determine whether reactions are thermally or photo chemically allowed or forbidden.

Classification of Pericyclic Reactions

 

Classification of Pericyclic Reactions

Introduction

            Most of the organic reactions proceeds in step wise manner.  However there are certain reactions, which proceed in a single concerted step via formation of a cyclic transition state involving π or σ electrons.

OR

Electron moves round a circle and there are no positive or negative charges on any intermediates-indeed, there are no intermediates at all, such type of reaction is called Pericyclic reactions.

eg. Diels-Alder reaction.

The energy for Pericyclic reaction is supplied by heat in thermally induced reaction or by ultraviolet in a photo-induced reaction. Pericyclic reactions are highly stereospecific. Thermal and photo-chemical process gives different products but specific stereochemistry.

            Pericyclic reactions do not involve ionic or free radical intermediates and so solvents and reagents have no effect on the source of reaction.

Types of Pericyclic reactions:

The three most common types of Pericyclic reactions are

1)      Cycloaddition reaction

2)      Electro cyclic reaction

3)      Sigma tropic reaction

  1)    Cycloaddition Reactions

In these reactions two or more π electron system combines to form a ring. The reaction involve conversion of two π bonds into two sigma bonds (or reveres).

An example of cycloaddition reaction is well known as Diels-Alder reaction.

 

2)2)Electrocyclic  Reactions

These are reversible reaction in which a compound with conjugation double bond undergoes cyclization. In this process two π electrons are used to form a σ-bond.

eg. 1,3,5 hexatriene on heating gives 1,3 cycloheaxadiene

 

 In the above illustration, an electrocyclic ring closer takes place. The reverse of this process is an electrocyclic ring opening.

 3)    Sigmatropic Rearrangements

            These are concerted intermolecular rearrangements. In these rearrangements an atoms (or group of atoms) shift from one position to another. The process involves breaking of bond and forming of a new bond.

(But number of π bonds remains same)

 eg.  3-methyl 1,5 hexadiene on heating gives 1,5 heptadiene.

Common example of sigmatropic reaction is Claisen rearrangement.

 

            Types of Pericyclic reaction

 Group Transfer Reaction: - a reaction in which one or more groups or atom transfer from one molecule. In this reaction both molecule are joined together by σ-bond.

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