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Multistep Synthesis of Benzilic Acid from Benzaldehyde

Jackson Wyers

March 17, 2013

Texas A&M at Galveston, Galveston, Texas

INTRODUCTION

Benzilic acid is known as an alpha-hydroxy acid or (AHA). Typically, benzilic acid is an expensive material; however, a three part synthesis reaction can be conducted in order to convert the rather inexpensive benzaldehyde. In the overall synthesis, three different reactions will occur. The first reaction is recognized as the benzoin condensation reaction and is demonstrated in Equation 1. The condensation occurs by adding heat to a mixture of benzaldehyde, thiamine hydrochloride, and sodium hydroxide in an ethanol solution.

Equation 1: Benzoin condensation

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Once the condensation has been executed, the benzoin must undergo an oxidation reaction in order to yield benzil. This reaction is exhibited in Equation 2.

Equation 2: Oxidation of benzoin

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The final reaction to transpire in the synthesis is the rearrangement of benzil to give benzilic acid. Equation 3 displays this rearrangement.

Equation 3: Benzilic acid rearrangement

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In the synthesis of benzilic acid, cyanide ion is the norm for the reaction catalyst. Cyanide ion, however, is highly toxic and therefore Vitamin B1 has been chosen instead. The vitamin is chemically known as thiamine hydrochloride but will often be notated as thiamine in this experiment. The purpose of this experiment is then – not only to conduct a synthesis to yield benzilic acid – but to test the effectiveness of Vitamin B1 when it serves as a catalyst in place of cyanide ion.

Measuring the melting point of the achieved products between each step is the ideal way to measure the purity of the product. To determine purity, the measured melting point is simply compared to the literature values that are given for the specific compound. Besides checking the compounds purity, it is of good practice to run an infrared spectrum on a small sample of the product to see if the functional groups shown in the IR report match up to the functional groups of the ideal product. This practice is helpful since this experiment deals with many compounds that have either (O-H) bonds, (C=O) bonds, or both.

EXPERIMENTAL

A flowchart – Figure 1 – is attached to summarize and outline the procedures and reagents.

Benzaldehyde to Benzoin. 0.5 grams of thiamine hydrochloride was dissolved in 0.8mL of water. 5.0mL of 95% ethanol was added and the solution was then cooled in an ice bath. 1.0mL of 3M sodium hydroxide was added dropwise while stirring. The pH was tested to assure it was strongly alkaline. 15.0 mmol of benzaldehyde was then added to this solution. Parafilm was then used to seal the flask containing the mixture and was heated on a hot plate for approximately two hours. Once finished heating, the flask was placed in a dark drawer and was left undisturbed for one week.

After one week, the mixture was retrieved for separation and analysis. The inside of the flask was scratched with a glass stir rod to induce crystallization. Once there was a presence of crystals, the mixture was cooled in an ice bath and then the benzoin was collected by vacuum filtration and was washed with both cold water and cold methanol. Figure 2 can be referred to for visualization on the vacuum filtration setup. The product was then dried in an oven for several minutes. Both, IR spectrum and melting point were measured for the benzoin.

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Figure 2: Vacuum filtration equipment and setup

Benzoin to Benzil. To determine the amount of ammonium nitrate to use in the reaction, the mass of dry benzoin was multiplied by 0.5. That amount of ammonium nitrate was then added to 3.5mL of 80% acetic acid in a round bottom flask. The benzoin was then added followed by 0.015 grams of copper (II) acetate. A stir bar was added and the flask was attached to a condenser which was attached to a ring stand. A hot plate was placed underneath the flask and the mixture was heated to start the reaction. When the reaction began, there was an evolution of a gas. Once the gas evolution had settled, the mixture was heated to boiling point and then heated under reflux for one hour. After reflux, the solution was left to cool and while manually stirring, was added to a small beaker containing crushed ice using a portion of water to assist the transfer. The mixture was left until the ice melted and the benzil was then collected by vacuum filtration and washed several times with cold water. Purification was the next step of this step. The benzil was recrystallized using 95% ethanol. The yellow crystals were then washed with cold aqueous 50% ethanol. Once washed, the solution was then dried and the melting point and IR spectrum were taken.

Benzil to Benzilic Acid. To determine the amount of 6M potassium hydroxide to add, the mass of the dry benzil was multiplied by 2.5. For the amount of 95% ethanol to use, the mass of dry benzil was multiplied by 3. Benzil was then combined with these two liquids in a round bottom flask. A stir bar was added and the flask was then attached to a condenser. Once assembled, the solution was heated under a gentle reflux for 15 minutes. The mixture was then transferred to a small beaker containing 10mL of water and continued to slowly heat on a low temperature (50ºC). Working under the hood, 1.5mL of concentrated hydrochloric acid was added to a 250mL beaker with a small portion of crushed ice. 1.5 mL of the potassium benzilate solution was added to the cold hydrochloric acid mixture and the sides of the beaker were scratched to assist the formation of crystals. At this point, the rest of the solution was then added with continuous stirring and the pH was tested to assure that it was near 2. The solution was then cooled in and ice bath and the benzilic acid was collected via vacuum filtration while washing it with cold water. The benzilic acid was then recrystallized from boiling water. The product was then dried and weighed. An IR spectrum and the products melting point were measured. Percent yield calculations were then done for each step of the synthesis and then for the overall reaction.

RESULTS

The results of the synthesis are split into the three different steps of the overall reaction.

Benzaldehyde to Benzoin

IR spectrum 1 shows the IR reading for benzaldehyde.

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IR spectrum 2 shows the IR reading for benzoin.

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An error occurred in this step of the synthesis. Instead of having solid benzoin form after letting the previously heated benzaldehyde, the flask simply had a dark orange liquid with no solid. Therefore, 0.51 grams of benzoin was partitioned from the instructor to continue the synthesis.

Table 1: The table shows data retrieved for benzoin

|Melting Point |Lit. Value |Theoretical |Actual Yield |Percent Yield |

| | |Yield | | |

|132.27ºC |137ºC |1.6 g |0.51 g |31.9% |

Percent Yield = (Actual Yield / Theoretical Yield) x 100

Percent Yield of Benzoin = (1.6g / 0.51g) x 100 = 31.9%

Benzoin to Benzil

IR spectrum 3 shows the IR reading for benzil.

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Initial mass of benzoin – 0.51 grams

Mass of benzil recovered – 0.173 grams

Mass of NH4NO3 used – (0.51 grams x 0.5) = 0.255 grams

Table 2: The table shows data retrieved for benzil.

|Melting Point |Lit. Value |Theoretical |Actual Yield |Percent Yield |

| | |Yield | | |

|93.14ºC |95-96ºC |0.505 g |0.173 g |34.26% |

0.51g L.R. (benzoin) x (1mol/212.2g benzoin) x (210.2g benzil/1mol) = .505g = theoretical yield

Percent Yield of Benzil = (0.173g / 0.505g) x 100 = 34.26%

Benzil to Benzilic Acid

IR spectrum 4 shows the IR reading for benzilic acid.

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Table 3: The table shows data retrieved for benzilic acid

|Melting Point |Lit. Value |Theoretical |Actual Yield |Percent Yield |

| | |Yield | | |

|129.17ºC |150ºC |0.1878 g |0.116 g |61.78% |

0.173g of L.R. benzil x (1mol/210.2g benzil) x (228.2g benzilic acid / 1mol) = 0.1878g = theoretical yield

Percent yield of benzilic acid = (0.116g / 0.1878g) x 100 = 61.78%

DISCUSSION

The two major purposes of the experiment were to get benzilic acid from benzaldehyde and to determine the effectiveness of Vitamin B1 serving as a catalyst for the reaction. Benzilic acid was successfully retrieved from the multistep synthesis however, the yields were much lower than expected. These low yields could imply that the thiamine hydrochloride is a poor catalyst for the synthesis; yet, thiamine has been shown by professionals to serve as an effective and efficient catalyst. Therefore, the low yields are likely due to procedural error. Had this been a test on a previously untested catalyst, it would be a good idea to further test and question the effectiveness of the catalyst since it could be the reason for the low yields. Aside from the low yields, the overall success of the experiment was above par and the ideal product was reached.

Judging by the results, the percent yields were an obvious problem. Out of the three yields, two were in the thirtieth percentile. However, the final yield of benzilic acid was a good yield at 61.78%. The percent yield of benzoin was 31.9% which was lower than intended. The similar problem occurred with the percent yield of benzil which had a yield of 34.26%. Both of these values fall into the thirtieth percentile range, which is considerably low when an ideal yield should have been up into the seventieth percentile or above. It is noteworthy that the actual yield of 0.116 grams of benzilic acid did not come from the conducted experiment. In the experiment conducted, there was an error when the benzilic acid was to be filtered, there was not any solid recovered. One cause of this error could have been due to the filter paper that was used. However, this would have only affected a small portion of the solid so there obviously were other problems. Therefore, data on amount of benzilic acid had to come from other peer data. This is the likely explanation as to why the yield is higher and does not correlate with the yields of benzoin and benzil. It was previously stated that due to an error in the first step of the experiment, a set amount of pure benzoin was given to begin the second step. The portions that were given out varied, and therefore the percent yield of benzoin was directly determined by how much benzoin the instructor happened to measure out. The numerous problems were solved as sufficiently as possible and should not have affected the data any more than the previously discussed possibilities.

When assessing the IR spectrums, they all appear to correlate with the structures of the compounds. Benzaldehyde exhibits its (C=O) stretching at 1702.39 which is in the literature range of 1740-1685. Benzoin has aromatic peak at 3003.73 which is in the literature range of 3100-3000. It also shows it (C=O) stretch at 1751.11, again in range. However, the only problem with the benzoin IR is the depth of the (O-H) stretch. You should expect to see a large bulge at the 3600-3200. It does show a stretch at 3413.31, yet it is very shallow. The likely cause is that not enough of the benzoin sample was placed on the salt disc when the IR spectrum was run. This would explain why the (O-H) bulge is shallow and the peaks shallow as well. The spectrum for benzil looks as it should. There is (C=O) stretching at 1713.04 which falls well within literature values. It also shows its aromatic peak at 3003.90, which is again ideal. Benzilic acid appears to have the same problem as seen in benzoin. The peaks are at accurate frequencies, but the intensities are weak. It shows the (O-H) stretch at 3379.15 which is ideal, and there is (C=O) stretching at 1678.41 which is in the literature range of 1725-1665. Therefore, all four IR spectrum readings confirm the presence of the ideal product.

REFERENCES

1. Lehman, J. W. (2004). Microscale operational organic chemistry. (3rd ed.). New Jersey: Pearson/Prentice Hall.

2. Figure2. abdocs/modules/vacfilt/pic/trap.gif

Figure 1: Flowchart summary of the overall procedural process.

0.015mol Benzaldehyde

NaOH

Thiamine Hydrochloride (Vit. B1)

Reflux; cool; dark (1 week)

Vacuum filter

Rinse with chilled D.I. H2O

Rinse twice with 1mL chilled methanol

(Solid) (Filtrate)

Benzoin – dry in oven 5 mins. Benzaldehyde

In 10mL RBF, NaOH

+1/2 mass NH4NO3 Vit. B1

+3.5mL 80% CH3COOH H2O

+Benzoin (Waste, down sink)

+0.015g Cu(CH3COO)2

Heat until gas evolves

Boil, reflux (1Hour)

Air cool. Pour on 8Ml ice.

Vacuum filter

Rinse with chilled H2O

(Solid) (Filtrate)

Benzil Benzoin

Recrystallize w/ ethanol NH4NO3

Air cool. Chill on ice. Cu(CH3COO)2

Vacuum filter (Waste, down sink)

Rinse twice w/ 50% ethanol

Save solid. Waste down sink.

Benzil

Weigh. IR. Mp.

In 10mL RBF,

Benzil

+2.5x 6M KOH

+3x Ethanol

Reflux 15 min. after boil

Air cool, add to beaker

Add 10mL DI H2O, heat to dissolve

Pour over 10mL ice/1.5mL HCl(aq) mixture – chill 5 min

Vacuum filter, rinse w/ chilled H2O

(Solid) Benzilic Acid Potassium Benzilate

Oven dry 5 min. weigh. Mp. H2O, Ethanol, KOH

Calculate Theoretical Yields Dispose of waste

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