Reduction of Benzophenone to Produce Benzhydrol and Study It’s Properties

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Strong

sp C-H stretching

Table 6. Tabulated 1H NMR data for Benzophenone1

Chemical Shift

(ppm)

Multiplicity

Assignment

~7.40- 7.80

Multiplet

Aromatic protons (HA,HB,HC,HD,HE)

Singlet

Water

Singlet

Chloroform

Figure 1. Assignment of chemically equivalent protons in Benzophenone

Table 7. Tabulated 1H NMR data for Benzhydrol1

Chemical Shift (ppm)

Multiplicity

Assignment

1.55

Singlet

Water

~ 2.25

Singlet

HF

~ 5.85

Singlet

HG

~7.20- 7.45

multiplet

Aromatic protons (HA,HB,HC,HD,HE)

Figure 2. Assignment of chemically equivalent protons in Benzhydrol

Observation: When benzophenone and sodium borohydride was refluxed with 2-propanol being the solvent for 40 minutes and allowed it to cool, the solution in the round-botton flask turned a milky white color. When the solution was added to hydrochloric acid in ice, bubbled formed. The precipitate turned out to be a frothy, white liquid. The product was suction filtered, which left a dull white powdered precipitate.

Water was then added drop by drop until the reflux solution of methanol and the isolated products became cloudy. After the recrystallization of the product, it turned into shiny, white powdery crystals.

Calculations:

Percent yield for benzhydrol:

Percent yield = actual value/ Theoretical value x 100%

= 4.4509x 10-3 mol/ 5.96 x 10-3 mol x 100%

= 74.76%

Discussion:

The melting point range of Benzhydrol was 55oC – 64oC. In comparison with the literature, that stated the melting point range to be 65oC – 67oC. This difference in melting point range could be because of impurities in the product.

A high percent yield of benzhydrol was seen, the percent yield being at 74.76%.

Both benzophenone and benzhydrol contain two aromatic rings connected by a single, central carbon. In benzophenone, the central carbon is a sp2 hybridized carbonyl group C=O, making benzophenone a conjugated ketone. In benzhydrol, the central carbon is sp3 hybridizied and attached to a methyl group and a hydroxyl group. The presence of the hydroxyl group makes henzhydrol an alcohol.

In the Infra red (IR) spectroscopy, the presence or absence of the ketone or alcohol group are the distinguishing features between the spectrums. The IR spectrum of nenzhydrol can be distinguished from the IR spectrum of benzophenone due to the presence of the –OH stretch at 3275 cm-1, and the absence of the carbonyl stretch at 2200 cm-1 due to the conjugated ketone. Since the benhzhydrol sample was prepared using Nujol oil, peaks due to Nujol mull were observed in the IR spectrum.

Proton Nuclear Magnetic Resonance (1 H NMR) acts on nuclei of the hydrogen atoms making them observable for analysis. In this experiment, the 1H NMR of benzophenone (top of page 49 in lab manual1 shows a complex multiplet in the aromatic region at approximately 7.80-7.40 ppm.

The 1 H NMR spectrum of the end product, benzhdrol (bottom of the page 49 in the lab manual1) has a complex multiplet between 7.45-7.20 ppm as well as singlets approximately at 5.8ppm and 2.2 ppm.

The larger ppm range of the complex multiplet in benzophenone in contrast to the complex multiplet of benzhydrol is because the carbonyl group of benzophenone is an electron-withdrawing group. Electron-withdrawing groups de-shield protons around them and increase their chemical shift. The electron withdrawing groups that affect the carbonyl group of benzophenon has the most significant affect on the ortho and para protons, namely HD, HE, and HB of benzophenone.

Benzhydrol has more peaks than benzophenone because benzhydrol has more protons. The peak at approxoimately 5.8 ppm is because of the protons on the CH3 methyl group (Hf). These protons are far downfield due to the de-shielding affects of the neighboring aromatic rings and the hydroxyl groups the peak at 2.2 ppm is due to the hydroxyl group of the alchohol (HG).

Conclusion:

The weight of Benzophenone used was 1.093g and the percent yield of benzohydrol was 74.76%. The determining melting point of the benzohydrol sample was 65oC – 67oC.

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