Chemistry Research

Experiment 1

Objective: To make methyl benzoate by Fischer Esterification by using a microwave digestion system, a CEM MDS-2000. This procedure was developed for utilization in our organic chemistry laboratory course.

Procedure: The procedure was altered from that given by George Majetich and Rodger Hicks in "Synthetic Applications of Microwave Systems for Organic Synthesis: A Report for CEM Corporation". Benzoic acid (0.5 g, 4.0 mmol) and anhydrous methanol (2.5 mL) were placed in a CEM advanced composite microwave digestion vessel. To this vessel was added 0.2 mL of sulfuric acid. The vessel was heated at 25% power for 2 minutes. The vessel was left to cool for 10 minutes and then placed in an ice bath for an additional 5 minutes. The product was then worked up by diluting with 5 mL of ethyl ether and by washing with a 5 mL portion of water. The product was then washed with two 5 mL portions of saturated sodium bicarbonate and 5 mL portions of saturated brine. The organic layer was then dried using sodium sulfate. TLC Analysis was conducted to determine whether a reaction took place.

Results: TLC showed very little starting material. 0.440 g of product was recovered (79.0% yield). The boiling point of the product was 198-199 Celsius (same as the reported boiling point for methyl benzoate)

Conclusion: The procedure worked well as reported using the microwave. The percent yield was very good at 79%. It can also be seen by the TLC analysis that most of the starting material had reacted.

[This particular experiment utilizing the CEM microwave has been added to the undergraduate curriculum. It will be one of the laboratory experiments completed by the organic chemistry students in the spring of 1997.]

Experiment 2

Objective: To make a variety of esters by Fischer Esterification by using a microwave digestion system, a CEM MDS-2000. Eleven different starting compounds were used to study the effects of substituents on the benzene ring of benzoic acid.

Chemical Reaction:

Procedure: Same as in experiment 1 except that benzoic acid was replaced with 4.0 mmol of one of the following: 2-chlorobenzoic acid, 3-chlorobenzoic acid, 4-chlorobenzoic acid, 4-aminobenzoic acid, 4-bromobenzoic acid, 3-bromobenzoic acid, 4-hydroxybenzoic acid, 2-methoxybenzoic acid, 3-nitrobenzoic acid, 4-nitrobenzoic acid, and 3,5-dinitrobenzoic acid. Each ester product was identified by boiling point and IR comparison to known values. Product yields were compared to each other.

Results: The mechanism for acid-catalyzed Fisher esterification is well known (shown below):

Yields were found to follow trends based on the effect of the substituent on the aromatic carboxylic acid and its position. An example is shown below with chlorine as the substituent:

For the three chlorobenzoic acid reactants, the chlorine substituent is resonant donating with the carbonyl group when ortho (2-chloro-) or para (4-chloro-). This weakens the carbonyl double bond and helps to stabilize the protonated benzoic acid intermediate (structure 2 above), increasing ester product yield. Without resonance (3-chloro-), the chloro group is only inductively electron-withdrawing from the carbonyl, making the carbonyl double bond stronger. This destabilizes the protonated carbonyl intermediate (structure 2 above), lowering the product yield. The same trend was found in other halogens. The substituent was either electron-withdrawing or electron-donating towards the carboxylic acid functional group. Typically electron-withdrawing groups, like nitro substituents, on the aromatic ring of benzoic acid hinder esterification. Conversely electron-donating substituents make the carbonyl oxygen more electron rich, speeding up the protonation of the carbonyl oxygen by lowering the energy of activation for this step, resulting in increased product yields from the esterification reaction. In each reaction, the same excess amount of methanol was added.

Using all of the above mentioned benzoic acid derivatives with methanol, esterification reactions have been completed. Results, including melting or boiling points of the product, IR analysis, and percent yield calculation, have all been collected and compared to known data.

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Last modified: July 19, 1999. JG.