Coenzyme Synthesis of Benzoin: Q's about the experiment

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What is a catalyst? How does it do its job? what factors does it deal with?

A catalyst is a compound that increases the rate of the reaction by lowering the activation energy for the reaction without itself being consumed. A catalyst does not change the equilibrium constant itself, it just allows the system to equilibrate faster.

In the Coenzyme Synthesis of Benzoin, what acts as the catalyst?

In this reaction, the thiamine that is generated in-situ is the catalyst

Why is it important to use fresh benzaldehyde in this reaction? What will happen if you used old benzaldehyde?

Most aldehydes slowly oxidize in air forming carboxylic acids (which is visible in the form of white crystals on the top of the bottle for benzaldehyde). If an acid is introduced to the catalyst (thiamine), the carbon atom with the negative charge will be protonated. The catalyst is destroyed and the condensation reaction does not occur anymore.

What is the lock and key model?

A model for enzyme-substrate interaction suggesting that the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another. Like a key into a lock, only the correct size and shape of the substrate (the key) would fit into the active site (the key hole) of the enzyme (the lock).

How is benzaldehyde purified from benzoic acid?

The purification cannot be done by simple distillation in this case because benzoic acid sublimes at higher temperatures (b.p. benzaldehyde = 178 ˚C, b.p. benzoic acid = 249 ˚C but starts to sublime significantly around 100˚C already!). The best method would be to dissolve the mixture in an organic solvent (i.e. diethyl ether), extract the solution with sodium bicarbonate solution, and then dry the organic layer with MgSO4, before removing the solvent by distillation. If required, the crude benzaldehyde could be further purified by vacuum distillation.

simple distillation

In simple distillation, all the hot vapors produced are immediately channeled into a condenser that cools and condenses the vapors. Therefore, the distillate will not be pure - its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law. As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C), or to separate liquids from involatile solids. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be neglected due to the insignificant contribution of the less volatile component. In this case, the distillate may be sufficiently pure for its intended purpose.

Dissolve thiamine in water and then add 95% ethanol and swirl till the mixture is homogeneous. Then add aq NaOH to the solution and it will change yellow. then add fresh benzaldehyde to the mixture and swirled again until it is homogenous. Why is it important that the reaction mixture is homogenous? What is another thing you can do to this sequence of steps to get the same results of this mistake?

If the mixture is not properly mixed or 95% ethanol was not added, the benzaldehyde will form a second layer on the top. The catalyst only reacts with the benzaldehyde on the interface, which slows down the reaction dramatically. In addition, the oxygen in air will oxidize benzaldehyde relatively quickly to form benzoic acid.

What general method can a scientist use to induce crystallization in solution if you have access to the product? Why does scratching on the inner walls of the flask help to initiate the crystallization? How could one seed a solution if the compound was not available?

Many organic solutions tend to supersaturate. One way to induce crystallization would be to add a small seed crystal of the material (from the pure or crude product). Scratching the inner walls i.e. bottom of the flask with a glass rod might induce crystal growth. The scratching provides a little bit of glass powder that can serve as a seed as well. For the last Q, a glass rod is placed in the solution. Upon removal, a small amount of the solution remains on the tip and is allowed to dry. The solids formed on the tip are placed in the mixture and serve as seed crystals.

What does supersaturate mean? Which type of solutions tend to do this?

which means that they contain more of the solute in the solution than they should have based on the maximum solubility of the compound at a given temperature.


the liquid that has been condensed from vapour during distillation; normally a purified form or a fraction of an original liquid

What is an ice-bath?

An ice-bath consist of a mixture of water and ice in a container like a beaker. The amount of water should be significantly greater than the amount of the ice. Ice alone is less effective because of the low contact area (i.e., not great for heat transfer). Also, ice has only about half of the heat capacity of water.

heat capacity

Heat capacity is the measurable physical quantity that characterizes the amount of heat required to change a body's temperature by a given amount.

What happens if you use a larger filter paper than can fit in the funnel when filtering?

You run the risk of breaking the filter paper and losing your product.

Which peaks are important to look for in the IR spectrum of benzoin?

The most important change is the conversion of a carbonyl group into an alcohol function. the corresponding OH stretching mode can be found as a broad peak around 3200 to 3400 1/cm in the IR spectrum. The peak is relatively broad as a result of the strong inter- and intramolecular hydrogen bonds found in benzoin.

Why is it impossible to have a yield above 100% in this reaction?

A chemical reaction cannot have a yield over 100% because there is no additional mass generated in the reaction. A yield slightly over 100% usually indicates that the compound probably was still a little wet when determining its weight. If the catalyst was used as limiting reagent, the yield would be significant above 100% because only 1-10 mole% are used as catalyst in many reactions.

What is the purpose of this experiment? What are we using and what do we want? What kind of reaction is this?

In this experiment, thiamine will be used to promote an enzymatic condensation of benzaldehyde to benzoin (acyloin condensation)

On Fig 1a, what compound do you see? Which of the hydrogens is the most acidic in the thiazole ring and why? Why is this hydrogen important for this reaction?

It is thiamine hydrochloride (enzyme in the reaction). The most acidic proton on the ring on the right (thiazole ring) is the one attached to the C double bonded to the positively charged N. The first step in this reaction is to deprotonate the thiazolium salt with the hydroxide ion, thus creating the catalyst which emits a yellowish color.

Using the molecules in Fig 1, draw out the synthesis of Benzoin starting from benzaldehyde and thiamine.

Figure 2

Describe what is going on in Figure 2.

The positive charge on the nitrogen atom makes the hydrogen atoms on the carbon atom more acidic. In the reaction, the first step is the deprotonation of the thiazolium salt using OH minus. The resulting thiamine is yellow in color and constitutes the actual catalyst. The carbon that carries the negative charge then attacks the electrophilic carbon of the benzaldehyde to form an intermediate. After a second addition of a benzaldehyde molecule, the intermediate disproportionates to form benzoin and the catalyst. The product precipitates from the relative polar solvent mixture (ethanol: water).

What is the name of the molecule in Fig 1 b.


What is the name of the molecule in Fig 1 c


What crystals do you get from this reaction and why do they form?

The crystals are the benzoin forming and they are insoluble in the water: ethanol mixture.

Why is water used in this experiment?

to solubilize the thiamine

The same reaction can be done using cyanide ions as a catalyst instead of thiamine. Why do we not use it?

If the pH-value of the solution was not properly controlled, HCN would be formed which is volatile and deadly.

What are coenzymes?

small organic molecules that are associated with enzymes

What are cofactors?

usually metal ions associated with enzymes

What is the purpose of ethanol in the Benzoin synthesis experiment? (You add it after dissolving the thiamine in the water and before the addition of benzaldehyde).

The ethanol is used to lower the polarity of the solution (water is more polar) and allows benzaldehyde to become miscible in the solution so that it can become homogenous.

When you come back for the second time to collect the crystals, before you remove it from the mixture, one has to put it in an ice bath before isolating the product. Why?

The purpose of the ice bath is two fold: You want to cool down the mixture to decrease the solubility of the benzoin which will increase the amount of crystals made and, when your trying to isolate it, it will not solubilize so easily (i.e., you will not lose so much product).

What happens when you add excess NaOH into the mixture of Water, Thiamine, Ethanol, and Benzaldehyde.

The NaOH will react with the glyceraldehyde in a cinazarro rection


describes two or more liquids that are able to dissolve into each other in various proportions


pouring liquid off the top when sediment has settled to the bottom

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