Advanced Organic Chemistry. Part A. Structure and Mechanisms

Francis A. Carey, Richard J. Sundberg

Chapter 12 Photochemistry - all with Video Answers

Educators

Chapter Questions

Problem 1

The bridged radical 1-A has been suggested as a possible intermediate in the photochemical decarbonylation of 3-phenylpropanal. Suggest an experiment to test this hypothesis.

Harsh Gadhiya

Numerade Educator

07:24

Problem 2

Predict the structure, including all aspects of the stereochemistry, based on orbital symmetry principles for the following photochemical electrocyclic and cycloaddition reactions.

A. Elizabeth Hildreth

Numerade Educator

01:38

Suggest reasonable mechanisms for the following observations:
a. The optically active allene, 2,3-pentadiene, is racemized under toluenesensitized photolysis.
b. Direct photolysis of diene 3-A at $254 \mathrm{~nm}$ produces a photostationary state containing about $30 \%$ 3-A and $70 \%$ 3-B. When photolysis is carried out at $300 \mathrm{~nm}$, photocyclization to 3-C occurs and little 3-B is present in the mixture.
c. Photocyclization of the acetal 3-D to 3-E is catalyzed by benzoic acid.

Raghvendra Singh

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08:02

Problem 4

Provide a mechanistic rationalization for each of the following reactions:

Nicholas Sacco

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06:16

Problem 5

Benzene-sensitized photolysis of methyl 3-cyclohexene-1-carboxylate in acetic acid leads to addition of acetic to the double bond. Only the trans adducts are formed. What factor(s) might be responsible for this stereoselectivity? Which of the two regioisomers do you expect to be the major product?

Anish Wadhwa

Numerade Educator

02:35

Problem 6

Photolysis of bicyclo[2.2.2]octan-2-one (6-A) gives 6-B in good yield. When 6-A is labeled as shown, the aldehyde group carries $48.3 \%$ of the deuterium. Write a mechanism to account for the overall transformation. Calculate the isotope effect for the hydrogen abstraction step. What mechanistic conclusion do you draw from the magnitude of the isotope effect?

Madi Sousa

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01:33

Problem 7

The photolysis of benzobarrelene 7-A has been studied in considerable detail. Direct photolysis gives 7-C (benzocyclooctatetraene), but when acetone is used as a photosensitizer, the di- $\pi$-methane rearrangement product 7-B (benzosemibullvalene) is formed. A deuterium-labeling study gave the results shown. What mechanistic conclusions do you draw from these results? Is there a feasible mechanism that would have resulted in a different isotopic label distribution in 7-B?

Grigoriy Sereda

Numerade Educator

02:21

Problem 8

Quantum yield data for three competing processes that occur on photolysis of $S$-methyl-1-phenyl-1-hexanone at $313 \mathrm{~nm}$ in benzene have been determined and are tabulated below. When the reaction is run in $t$-butanol, the racemization is entirely suppressed and the Type-II fragmentation is the major reaction. What information do these data provide about the mechanism operating under these conditions?

Anand Jangid

Numerade Educator

01:43

Problem 9

Show by means of an energy diagram the reason that the energy of light emitted from an excited electronic state by fluorescence or phosphorescence is of lower energy than the exciting radiation. Would you expect the shift in energy to be more pronounced for fluorescence or phosphorescence? Explain?

Mir Afzal

Numerade Educator

01:33

Problem 10

cis-2-Propyl-4- $t$-butylcyclohexanone undergoes cleavage to $4-t$-butylcyclohexanone on photolysis. The trans isomer does not undergo fragmentation directly, but is converted to the cis isomer, which then fragments. The trans $\rightarrow$ cis isomerization is quenched by 1,3-pentadiene, but the photofragmentation is not. Offer an explanation of this pronounced stereochemical effect.

Grigoriy Sereda

Numerade Educator

01:58

Problem 11

The quantum yield for formation of 3-methylcyclobutene from $E-1,3-$ pentadiene by $254 \mathrm{~nm}$ radiation is ten times greater than for cyclization of the $Z$-isomer. 1,3-Dimethylcyclopropene is also formed but the difference in $\Phi$ for this reactions is only a factor of two. Offer an explanation for these differences in terms of energy surfaces that are involved.

Adriano Chikande

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01:47

Problem 12

The irradiation of 2-methoxytropone (12-A) leads to methyl 4-oxo-2cyclopentenylacetate (12-D). The course of the reaction can be followed by gas chromatography and two intermediates are observed, which have structures 12-B and 12-C. Indicate a mechanism for the three successive reactions. The first two are photochemical, whereas the third is probably an acid-catalyzed reaction that does not require light.

Grigoriy Sereda

Numerade Educator

06:17

Problem 13

When an aryl substituent is placed at $\mathrm{C}(5)$ of a 4 -substituted cyclohexenone, a new product type involving formation of a cyclobutanone ring is formed.
The reaction products are the same for both direct irradiation and acetophenone sensitization. When reactant 13-B is used in enantiomerically pure form, the product 13-D is nearly racemic (6\% e.e.). Relate the formation of the cyclobutanone to the more normal products of 4-substituted cyclohexenones.

Rajesh Singh

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02:15

Problem 14

In the rearrangement of 4,4 -diphenylcyclocyclohex-2-enone to 5,6 diphenylbicyclo[3.1.0]hexan-2-one, there is a strong preference for formation of the endo phenyl stereoisomer. Offer an explanation for this stereoselectivity.

Mercedes Mazza

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00:39

Problem 15

Compound 15-A is photochromic; that is, it becomes colored on exposure to light. The process is reversible, giving back the starting material in the dark. Suggest a structure for the colored photoisomer.

Lottie Adams

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01:33

Problem 16

The photolysis of 16-A, 16-B, and 16-C has been studied. 16-A gives both 16-D and the cleavage product benzaldehyde. 16-B gives only 16-E. 16-C gives 16-B and benzaldehyde. Discuss how the presence of the $t$-butyl group and its stereochemistry can influence the reaction outcome.

Grigoriy Sereda

Numerade Educator

02:38

Problem 17

The azo compounds 17-A and 17-B were prepared and the thermal and photochemical behavior of these materials was investigated. The results are summarized in the Formulas below. It is also known that triplet photosensitization converts 17-C to 17-D and 17-E to 17-F, respectively. Discuss how these results relate to the mechanism of the di- $\pi$-methane rearrangement.

Grigoriy Sereda

Numerade Educator

05:53

Problem 18

Direct irradiation of $1,1,3,3$-tetraphenyl-5-methyl-1,4-hexadiene gives the products 18-A and 18-B shown below. When the reaction is carried out by photosensitization, only 18-A is formed. Suggest mechanisms for formation of 18-A and 18-B. A minor product 18-C is formed only in the photosensitized reaction. What other products might have been expected? Can you explain the preference for the observed products?

Zubair Abdulla

Numerade Educator

03:50

Problem 19

Suggest a mechanistic pathway for formation of each of the products formed on irradiation of the Diels-Alder adduct of 2,3-dimethylbutadiene and quinone.

Shazia Naz

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02:40

Problem 20

The direct irradiation of $\mathbf{2 0}$-A gives predominantly $\mathbf{2 0}-\mathbf{B}$, but the photosensitized reaction gives more 20-C. Explain these observations.

Nima Gharibi

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03:15

Problem 21

9,10 -Ethanoanthracenes undergo the di- $\pi$-methane rearrangement. Analyze the substituent effects that are observed in this reaction.

Zubair Abdulla

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02:57

Problem 22

Formulate a mechanism for the following reaction.

Zubair Abdulla

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02:32

Problem 23

The photochemistry of 2-benzoylcyclohexanone and its 2-alkyl derivatives diverges. The unsubstituted compound undergoes ring cleavage, whereas the substituted compounds result in migration of the benzoyl substituent to the 4-position of the ring. Suggest mechanisms for both reactions and account for the effect of the alkyl substituent.

Grigoriy Sereda

Numerade Educator

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