Question

Reaction conditions and product isolation…

a)What is the purpose of using Silica Gel in this Fischer esterification reaction? Explain.

b) The crude reaction product did not still contain a large excess of acetic acid (this fact should be apparent from your GC results and your answer to Question 4. How was the excess acetic acid removed? Include a complete and balanced chemical reaction equation as the basis of your answer.

c)A large excess of acetic acid was used, what is the purpose of the excess?

d)Excess of 1-butanol could be also used, but practical considerations favor the method that you followed (excess of acetic acid rather than 1-butanol). Explain the complications that would arise during the product isolation step if you were to use an excess of 1-butanol instead.

Answer 1

a) What is the purpose of using Silica Gel in this Fischer esterification reaction? Explain.

Ans:    The esterification takes place as per the below reaction:

R-(C=O)-O-H + R-O-H ⇌ R-(C=O)-O-R + H2O

The Silica Gel is used to remove the water formed during the reaction, as it has the capacity to absorb good amount of water.

b) The crude reaction product did not still contain a large excess of acetic acid (this fact should be apparent from your GC results and your answer to Question 4.

How was the excess acetic acid removed? Include a complete and balanced chemical reaction equation as the basis of your answer.

Ans:    The excess acetic acid can easily be removed by washing the reaction product with mild

Sodium carbonate solution, followed by sufficient water so that the washings bcomes neutral,

Means that the product is pure and free from any trace amount of acetic acid.

2CH3COOH + Na2CO3 (aq) → 2CH3COONa (aq) + CO2 ­ (g)

c) A large excess of acetic acid was used, what is the purpose of the excess?

Ans:    In the Fischer esterification, one of the reactant is to be used in excess to keep the reaction moving forward for product formation. In this case the acetic is used in excess to keep the ester forming in forward direction. It is easy to remove acetic acid from the reaction mixture rather than the 1-butanol. The acid can be removed by simply washings with mild base and then with water as seen in above Ans (b).

d)         Excess of 1-butanol could be also used, but practical considerations favor the method that you followed (excess of acetic acid rather than 1-butanol). Explain the complications that would arise during the product isolation step if you were to use an excess of 1-butanol instead.

Ans:    Yes, the excess butanol can also be used, if it is easy to remove the excess of this component from the reaction mixture. The solubility of 1-butanol in water is not as good as of acetic acid. It can only be separated from the reaction mixture by fractional distillation as their boiling points are also not much different. (1-butanol ~ 118 ºC and Butyl acetate ~ 126 ºC). The yield of the final ester would also be low due to the distillation efficiency and residue left after distillation in the flask.

Using the excess acetic acid and silica gel in this case is the best option to get the best yield of the ester product.