EXP 55 - Photoreduction of Benzophenone and Rearrangement of Benzpinacol to Benzopinacolone PDF

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Summary

Experiment 55’s purpose is to photoreduce benzophenone and to synthesize ß-benzopinacolone from the acid
catalyzed rearrangement of benzpinacol. The two-part procedure is to first observe if two test tubes containing
0.50 g benzophenone, 2 mL isopropyl alcohol, and one drop glacial aceti...

Description

Organic(Chemistry(Laboratory(Report(Summary(Sheet( Name: Toan Ngo EXP #55 – Photoreduction of Benzophenone and Rearrangement of Benzpinacol to Benzopinacolone Key Mechanism:

Reaction mechanism of benzopinacolone from benzpinacol

Reaction mechanism of benzpinacol from benzophenone

( Data(table( !

Part A Mass benzophenone

0.5 g

Mass experimental benzpinacol

0.388 g 178-

Melting point benzpinacol

179°!C

Theoretical yield benzpinacol =

!.#$%$&'()*+,'(*('$ -

-$.*/$&'()*+,'(*('

-$.*/$&'()+2(34*/

!!´! -01.11$%$&'()*+,'(*(' !´! 1$.*/$&'()*+,'(*(' !´!

Percent yield benzpinacol =

!.500$%$'8+'92.'(:3/$&'()+2(34*/ !.#!1;$%$:,'*9':243/$&'()+2(34*/

566.7$%$$&'()+2(34*/ -$.*/$&'()+2(34*/

!´!100

0.5027 g 77.2 %

Part B Mass experimental ß-benzopinacolone

0.1803 g

Melting point ß-benzopinacolone

184°!C

Theoretical yield ß-benzopinacolone =

!.1#$%$&'()*+2(34*/$ -

-$.*/$&'()*+2(34*/

!!´! 566.7$%$&'()*+2(34*/ !´!

Percent yield ß-benzopinacolone = ! ! ! !

(

-$.*/$ß− &'()*+2(34*/*(' -$.*/$&'()*+2(34*/

!´!

570.7$%$$ß− &'()*+2(34*/*(' -$.*/$ß−&'()*+2(34*/*('

!.-0!5$%$'8+'92.'(:3/$&'()*+2(34*/*(' !.15;;$%$:,'*9':243/$&'()*+2(34*/*('

!´!100

0.2377 g 75.8 %

Abstract( Experiment 55’s purpose is to photoreduce benzophenone and to synthesize ß-benzopinacolone from the acid catalyzed rearrangement of benzpinacol. The two-part procedure is to first observe if two test tubes containing 0.50 g benzophenone, 2 mL isopropyl alcohol, and one drop glacial acetic acid (one control test tube contain an additional 0.05 g naphthalene) crystallizes from one-week ultraviolet light exposure. The resultant benzpinacol crystals are recrystallized by boiling and cooling with 5 mL of 0.015 M iodine solution dissolved in glacial acetic acid, Hirsch funnel filtered, and rinsed with cold glacial acetic acid. Part A produced 0.388 g benzpinacol at 77.2 % yield, with a 178-179°!C melting point. Part B produced 0.1803 g ß-benzopinacolone at 75.8 % yield, with a 184°!C melting point that indicate purity. Its IR show a conjugated ketone at 1676 cm-1 and aromatic overtones at ~2000 cm-1. Comparing benzpinacol’s IR and melting point values to literature shows slight impurity, due to perhaps incomplete dryness. Drying product in Hirsch funnel overnight instead of on watch glass may have caused slight impurity and a swap in technique may be considered for future experiments.

IR(analysis(table! Part A - benzpinacol

Wavenumber (cm-1)

Assignment

3428 (s, br)

O–H stretch

1641 (m, sh)

C=C stretch

Part B - ß-benzopinacolone

( ( ( (

Wavenumber (cm-1)

Assignment

3054 (m, sh)

sp2 CH stretch

2986 (m, sh)

sp3 CH stretch

2000 (w, s)

aromatic overtones

1676 (m, sh)

C=O stretch

Questions(-(#1-3,(pg(470( 1. Can you think of a way to produce the benzophenone n-π* triplet T1 without having benzophenone pass through its first singlet state? Explain. The benzophenone n-π* triplet T1 without having benzophenone pass through its first singlet state cannot be produced since the triplet state can only be generated from a singlet excited state. It must first go through the first singlet state from the ground state. The first singlet state can then go through the intersystem crossing to produce the triplet state. However, and alternatively according to Klein, page 447, very high temperature can also initiate radical initiation.

2. A reaction similar to the one described here occurs when benzophenone is treated with the metal magnesium (pinacol reduction).

Compare the mechanism of this reaction with the photoreduction mechanism. What are the differences? A photoreduction reaction requires light of certain wavelength to excite benzophenone whereas a pinacol reduction reaction requires a reducing agent like magnesium metal. The first step in photoreduction forms a benzophenone radical whereas a pinacol reduction first forms a ketyl radical. A diketyl radical is formed in a pinacol reduction whereas a diphenylhydroxymethyl radical is formed in benzophenone photoreduction. A five membered cyclic ring containing two oxygen atoms binded by magnesium metal ion is formed in a pinacol reduction but not in benzophenone photoreduction. Aprotic solvents are used in pinacol reduction whereas protic solvents are used in a photoreduction. 3. Which of the following molecules do you expect would be useful in quenching benzophenone photoreduction? Explain. 4.

Oxygen 9,10-Diphenylanthracene trans -1,3-Pentadiene Naphthalene Biphenyl Toluene Benzene

(S1 (T1 (T1 (T1 (T1 (T1 (T1

= = = = = = =

22 kcal/mol) 42 kcal/mol) 59 kcal/mol) 61 kcal/mol) 66 kcal/mol) 83 kcal/mol) 84 kcal/mol)

Naphthalene will be useful in quenching benzophenone photoreduction because the reaction proceeds via the triplet state T1 of benzophenone. The triplet excitation energy of benzophenone (69 kcal/mol) is transferred to naphthalene (T1 = 61kcal/mol) in an exothermic collision. Other molecules lower in triplet excitation energy of benzophenone can also quench benzophenone photoreduction such as oxygen, 9,10-diphenylandthracene, trans -1,3-pentdiene and biphenyl....