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Transcript of Benzoin Condensation
Purpose- to synthesize benzoin using benzaldehyde and thiamine hydrochloride as a catalyst.
Benzoin can be synthesized using cyanide ion
There were many color changes throughout the experiment that allowed for the monitoring of the reaction completion. The final product was a white crystal powder.
In the experiment, the final product weighed 2.8 g, making the percent yield 56%.
The synthesis of benzoin, an essential oil.
Some problems arose during the testing of melting point. The temperature rose to over 26OC and still did not melt.
The IR analysis from the starting material to the final product showed changes that accounted for the new side groups.
The percent yield was very low and could have been due to a number of reasons.
Changes were made to the procedure to accommodate the issues we were experiencing during the experiment.
Sources of Error
Conclusion and Analysis
This reaction procedure was chosen after consulting many online references of similar and identical reactions.
Thiamine was required as a catalyst because the other catalyst contained cyanide ions. If the pH-value of the solution was not properly controlled, HCN would be formed which is volatile and deadly.
The amount of benzaldehyde to use was calculated by taking the desired amount of 5.0 g yield and converting that to moles and then multiplying by two, to take into account the 2:1 ratio of the reaction (benzaldehyde to benzoin). This left 4.81 mL starting benzaldehyde.
The rest of the reagent amounts were taken from an online reference.
A reflux apparatus was chosen to complete the reaction between the benzaldehyde and thiamine catalyst, instead of a 24 hour wait or hours of a hot water bath.
To remedy the reflux, the experiment should be allowed to sit at room temperature for at least 24 hours to proceed, instead of vigorous heating. Or a warm water bath, not to exceed 60 C, should be used for at least three hours.
To stop benzoic acid from forming, the benzaldehyde should be taken from an unopened bottle.
The ethanol water mixture used at the end for washing should have been cold.
The low yield, at 56%, and high melting point show the product isn't pure.
The IR shows the appearance of benzoin due to the addition of a large OH peak and a more prominent C-H aromatic ring, characteristic of benzoin in comparison to benzaldehyde. This indicated that benzoin was successfully made.
Choosing to reflux might have contributed to the poor yield of the reaction.
When choosing the method, it was unknown that thiamine degrades at high temperatures. It is very possible that the thiamine catalyst was partially destroyed due to the vigorous heating of the reflux.
The boil was high, with temperatures reaching above 60 C, which no referenced reaction exceeded.
Benzaldehyde oxidizes to form benzoic acid, a white crystal substance. Even though no white crystals were on the bottle, the pipette holder had white crystals on it, which could mean that the pipette transferred crystals. There is also the possibility the benzaldehyde was exposed to air in the flask and procedure enough to produce the benzoic acid.
Sodium carbonate was added to the benzaldehyde to remove any benzoic acid. There was a problem separating sodium carbonate aqueous layer from the benzaldehyde organic layer and this method was not used.
When benzoic acid, or any acid, is introduced to the catalyst, the carbon atom that holds a negative charge (and drives the reaction) is reprotonated. This makes the thiamine revert to thiamine hydrochloride and not function as a catalyst, thereby reducing yield.
The IR spectrum that was run on the benzaldehyde provided in lab had a slight peak at 3400-3600. This could indicate the presence of an O-H group and therefore, benzoic acid.
The amount of ethanol used could have inhibited the reaction.
Ethanol is added to lower the polarity of the solution. It enables benzaldehyde to become miscible in the water to create a homogenous solution.
Without homogeny, the benzaldehyde is able to form a layer on top, which the catalyst only reacts with.
This slows the reaction down greatly and reduces yield.
In addition, the ethanol water mixture added at the end to wash the crystals of thiamine was at room temperature. This could have allowed the benzoin to solubilize and go into mixture, reducing the yield.
Vacuum filtration for retrieval of crystal product from liquid solution.
Liquid solution obtained from reflux.
Ice bath to induce crystallization