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PHOTOREDUCTION OF BENZOPHENONE

1. AIM To observe the effectiveness of an alcohol as a reducing agent for the excited state of

benzophenone, by comparing it with the product of thermal reduction, to see if the overall reaction

is similar or different.

2. INTRODUCTION Benzophenone is an aromatic ketone which contains the carbonyl group (C=O). It is this group that

dictates most of its thermal reactivity. Do to the high electronegative nature of oxygen, the carbonyl

group is polarised giving oxygen a partial negative charge and the carbon a partial positive charge.

Moreover the bonding occurring between the carbon and oxygen is unsaturated giving rise to a

double bond. Ketones are usually attacked via a nucleophilic group which is attracted to the partial

positive charge on the carbon. Once benzophenone is attack by a nucleophile an oxygen centred

anion is produced. This picks up an abstract proton to yield diphenylmethanol.

Two common reducing agents used are sodium borohydride (NaBH4) and lithium aluminum hydride

(LiAlH4). Sodium borohydride is a weaker reducing agent and thus more selective than the latter, and

this allows it to be used in aqueous or alcoholic solvents. Sodium borohydride acts as the necessary

nucleophilic agent to attack the carbon atom (figure 1).

O

+ NaBH4

O B-

OHNa+

1/4

Benzophenone DiphenylmethanolSodium Borohydrate

FIGURE 1

Apart thermal reduction benzophenone can also undergo photoreduction. This is done via ultraviolet

irradiation, which produces the electronically excited state (triplet state) which changes the

chemistry of benzophenone. Instead of having the large polarisation of the carbonyl group present

(as in the ground state) the oxygen atom obtains an odd electron character, which makes it act just

like a radical species. This new radical character is much more susceptible to electron transfer than

the corresponding ground state. An effective reducing agent for such a species would be a hydrogen

donor rather than a hydride donor. One such example would be a secondary alcohol such as propan-

2-ol (figure 2). The radical product produced can further undergo other reactions to give the final

products.

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O

CH3CH3

OH

H

C

OH

CH3

C

CH3

OH

+ +

*

FIGURE 2

This experiment produced two separate products for the two different reductions. The melting

points of the compounds gave some idea to the identity of these compounds. However the

produced IR spectra were inadequate and little conclusions could be made from that data.

3. METHOD

3.1 CHEMICALS USED

Chemical Grade Manufacturer

Benzophenone Purum Acros

Pronpan-2-ol Analytical reagent Lab-Scan

Ethanol GPR Chemic

Dichloromethane GPR N/A

Petroleum ether GPR Ficher

Sodium borohydride GPR Scharlau

Aqueous hydrochloric acid (2M) Analar BDH

Anhydrous magnesium sulfate GPR BDH

Diethyl ether Analytical reagent Lab-Scan

3.2 APPARATUS USED

Measuring cylinders (100 mL & 10 mL), conical flasks (250 mL), Breakers (150 mL), Separating funnel,

clamp & stand, round bottomed flask (250 mL), rotary evaporator, separating funnel (250 mL), ice

bath, cork, Photochemical reactor, Buchner funnel, Buchner flask, Buchner rubber cone, spatula,

pipette, water bath, capillary tubes, melting point apparatus, weighing balance weighing boat,

vacuum pump, thermometer, test tube, IR spectrometer.

3.3 PROCEDURE

Photochemical reduction

4.003 g of benzophenone were dissolved in 35 cm3 of propan-2-ol by shaking in a 50 cm3 conical

flask and warming gently in a water-bath. One drop of glacial acetic acid was then added to

neutralise the alkaline impurities in the surface of the glass and a Pyrex test tube was then filled with

the solution. The tube was corked loosely and supported as close as possible to the Pyrex water-

cooling jacket around the medium pressure mercury arc. After making sure that the apparatus was

properly enclosed in its protective surround, the lamp was switched on and left to irradiate for 4

hours. It was made sure that all the apparatus of the photochemical reaction were properly enclosed

before switching on the UV lamp as this type of radiation is dangerous.

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After switching off the lamp, the precipitated product was filtered from solution using a Buchner

funnel under suction, and then washed with a little cold ethanol. The crude product was

recrystallized by dissolving a minimum amount of hot dichloromethane and adding petroleum ether.

Until the solution just became cloudy. The solution was allowed to cool to room temperature, and

cooled to room temperature, and then cooled in an ice bath. The product was filtered using a

vacuum pump and the crystals were sucked as dry as possible. The melting point and infrared

spectrum of the products were taken.

Thermal reduction

In a 250 mL conical flask, a solution of benzophenone was prepared by mixing 1.010 g of

benzophenone in aqueous ethanol ( 8 mL ethanol and 2 mL water). 0.253 g Sodium borohydride was

then added and the flask was swilled to assist the dissolution. After about 30-40 minutes iced water

(100 mL) was added and the product was extracted in ether, after 2 consecutive washes of 50 mL

ether. The washings were made using a separating funnel. because of pressure build up in the funnel

periodic opening of the valve while holding the funnel upside-down was done in order to allow the

release of the built up of gas. The combined ether washings were then washed with 50 mL

concentrated aqueous hydrochloric acid followed by a 50 mL washing using water. The solution was

then dried using 4 g anhydrous magnesium sulfate, which was then filtered off and the crude

reaction product isolated by distilling off the ether using a rotary evaporator. The crude product was

recrystallized from the minimum amount of hot (using a water bath as an open flame could not be

used because of the presence of ethyl ether which is highly flammable) petroleum ether and filtered

using vacuum filtration, sucking the crystals as dry as possible. The melting points and infrared

spectrum of the product were taken.

4. RESULTS

4.1 RESULTS

Photochemical reduction

Mass of product obtained: 0.235 g of a white crystalline solid.

Melting point: 184 - 185 °C

Thermal reduction

Mass of product obtained(diphenylmethanol): 1.078 g of a white crystalline solid.

Melting point: 65 - 66 °C

For the IR spectra refer to Appendix 1.

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4.2 CALCULATIONS

Percentage yield of diphenylmethanol (thermal reduction)

Mass of benzophenone used: 1.010 g (MW = 182.22)

Mass of diphenylmethanol obtained: 1.078 g (MW = 184.24)

Theoretical yield

Since benzophenone was the limiting reagent:

Percentage yield:

Percentage yield of benzopinacol (photochemical reduction)

Mass of benzophenone used: 4.003 g (MW = 182.22)

Mass of benzopinacol obtained: 0.235 g (MW = 366.45)

Theoretical yield

Since benzophenone was the limiting reagent:

Percentage yield:

5. DISCUSSION From the data of both the melting points and IR spectra it can be concluded that the product

produced from the photochemical and thermal reduction of benzophenone gave two different

products. This means that different mechanisms are in play in these two reactions. The product

which was to be synthesised from the thermal reduction was diphenylmethanol which has a melting

point literature value of 63- 65 °C 1. This falls close within the obtained melting point range of

65 - 66 °C, which goes to show that diphenylmethanol was actually produced. The melting point

obtained for the photochemical reduction had a melting point of 184 – 185 °C. The literature

suggests that the most likely product to be obtained from this photochemical reduction is

benzopinacol 2.

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Continuing from figure 2, benzopinacol is formed by a dimerization reaction between two diphenyl

ketyl radical to yield benzopinacol (figure 3).

C

OH

C

OH

+ OHOH

Benzopinacol

FIGURE 3

However this chemical has a melting point of 171 °C 2 which is considerably different from the

melting point range obtained. It is most likely that a major contaminant remained present such as a

solvent used in the recrystallization process or that benzopinacol was chemically altered in the

purification process itself. However another possibility might be that the product obtained was not

benzopinacol but another compounded altogether.

The IR spectra could have shed some light on this, but unfortunately the spectra obtained were of

low quality (refer to Appendix 1). Because of this it is difficult to assign individual peaks to different

functional group. For diphenylmethanol and benzopinacol most of the peaks observed in the 700-

600 cm-1 region are likely due to C-H stretching in the aromatic rings 3. The higher peaks up to 1000

cm-1 would be due to other stretching and bending modes of the aromatic rings such as C-C

stretching/bending 3. However the higher peaks were too low in intensity to be able to be assigned

accurately. This is likely due to the IR spectrometer being not calibrated properly Furthermore

characteristic peaks such as the -OH peak at <3000 cm-1 was not present for diphenylmethanol,

which seems to be due to errors arising from the functioning of the IR spectrometer since the

melting point obtained suggest that the proposed compound was synthesised. However the were

clear enough in producing two distinct graphs for the two products obtained showing that they were

not identical.

The obtained yields for both the photochemical and thermal reduction were very unsatisfactory.

Firstly the yield obtained of for the photochemical reduction was very low. This was to be

expected however as the photochemical reduction is a very slow process and take a lot of time. The

4 hours left for the reaction to take place were inadequate to product a high yield, but supposedly

the time was long enough to product a product which could be analysed. Also one must keep in

mind that the aim of this experiment was not to synthesis a high yield of products but to be able to

find out whether these two different forms of reductions produced the same compounds, which was

found out that they do not. Secondly the yield of 105.96 % for the thermal reduction goes to show

that a number of impurities (most probably liquids from the purification process) were still present.

This is most likely due to insufficient drying of the product, and could have been amended by

allowing the products to dry in air for a few hours. Because of time constraints in the practical this

was not possible.

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This experiment was successful in determining if two separate products are formed in these two

types of reactions, but it did not produce good data for the IR spectra and thus only the melting

points could have been used to try and determine the identity of the products. For the thermal

reduction since the melting point came close to that of the literature value one can confidently state

that diphenylmethanol was actually produced, even though the percentage yield showed that

impurities were present. The identity of the product produced in the photochemical reduction could

be speculated to be partially benzopinacol, but both the melting points and IR spectra obtained

show no proper evidence for this. In order to improve this experiment, proper accurate IR spectra

have to be taken.

6. CONCLUSIONS In this experiment the photochemical reduction and thermal reduction of benzophenone were

conducted. Limited success was achieved in the thermal reduction which produced a compound

with melting point of 65- 66 °C which agree with the literature value of diphenylmethanol which was

predicted to be formed. However the yield of 105.69 % shows that a number of impurities were in

the final product. In the photochemical reduction of benzophenone the melting point obtained did

not agree with the suspected product benzopinacol. The IR spectra for both compounds were of a

very low quality and in order to improve this experiment, IR spectra of a much higher quality must

be measured.

7. REFERENCES 1. Merck-Chemicals. [Online] 2011, http://www.merck-chemicals.com/united-

kingdom/benzhydrol/MDA_CHEM-801644/p_uuid?WT_oss=801644&WT_oss_r=1

(accessed March 25, 2012)

2. Organic Syntheses, Coll.,1943; Vol. 2, pp.71

3. Robert M. Silverstein; Francis X. Webster; David Kiemle.; Spectrometric Identification of

organic compounds, 7th ed,; Wiley: New York, 2005.

8. BIBLIOGRAPHY 1. Luzchem. [Online] 2010, www.luzchem.com/edu/docstore/2-benzophenone.pdf

(accessed March 25 2012)

2. Vollhardt; K. P. C.; Schore, N. E.; Organic chemistry : structure and function; W. H.

Freeman and Co.: New York, 2002.

3. Atkins, P.; Physical Chemistry.; 7th ed.; W.H. Freeman: New York, 2002.

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APPENDIX 1 Photochemical reduction product:

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Thermal reduction product: