CHEM 323
CHEM 323 Week 8
Bromination of (E)-Cinnamic Acid (Monday)
Fischer Esterification of Carboxylic Acids by Microwave(Wednesday)
Monday Lab (Oct 19): Bromination of (E)-Cinnamic Acid
Text Assignment:
Chapter 10.6 (p. 372-385)
Detailed procedure for Monday: p. 379
Discussion:
This is our first synthesis experiment. You will be performing a reaction to make a new compound. You will then isolate the product, purify it, and analyze it.
The reaction is a simple bromination of an alkene (addition of Br—Br across C=C). This reaction is generally very fast and reliable.
Bromine (Br2) is very toxic, corrosive, and hazardous. It is a liquid at room temperature with a significant vapor pressure. That means that it readily forms a vapor in the air at room temperature; and that means that you could inhale it if it was used outside of the hood. Toxic inhalation hazards are much more dangerous than other toxics which are only dangerous if ingested or injected.
In this procedure, we avoid the direct handling of bromine by making it in situ. (That means that we will combine two reagents in the reaction flask that will generate bromine. It will then react with the alkene as it is made.) The bromine source is pyridinium tribromide. This safer solid reagent will be added to the reaction flask containing acetic acid and the alkene. The acetic acid will break down the pyridinium tribromide to make free bromine in situ. Even though this procedure does not involve the handling of bromine, the procedure still must be run in a hood because bromine is in the reaction mixture.
The dibromocinnamic acid should start forming quickly. As it forms, it will precipitate as a solid. The reaction should be heated for about 45 min. to ensure complete reaction. It would be good to see if you can follow the reaction by TLC. In other words, you could take a TLC before starting the reaction and then take others periodically through the reaction time to see if you can follow the appearance of product and/or disappearance of starting material. Since the cinnamic acidsare aromatic compounds, they should be easy to see under UV light. TLC results should parallel your observations of the increasing amount of solids in the reaction mixture.
After the reaction is finished and cool, the product may be filtered away from the reaction mixture. It is then washed with sodium bisulfite to destroy any remaining bromine (which is the cause of the color in the product). The filtered product should then be recrystallized from 50% ethanol/water.
You can compare the quality of your product before and after recrystallization by looking at the material before and after recrystallization be either TLC or melting point, or both.
You may analyze your product by IR if you have time.
When we run a reaction, we need to calculate the theoretical yield based upon the amounts of reactants used. Then, we can determine our percentage yield as the actual yield divided by the theoretical yield x 100%. The reagents used are typically arranged in a table in the lab notebook for easy access and calculation along with the reaction scheme (see example below).
| (E)-Cinnamic Acid | Pyridimium tribromide | Acetic acid | Product |
| MW = | MW = | MW = | MW = |
| g to be used | g to be used | mL to be used | g expected (theoretical yield) |
| (milli)moles | (milli)moles | (milli)moles | |
| Additional data of interest (i.e. hazards, mp’s, etc.) |
Notebook:
- Title (Bromination of (E)-Cinnamic Acid)
- Purpose/Introduction (Synthesize a pure sample of 2,3-dibromo-3-phenylpropanoic acid)
- SDS data for substances used (see chemical list below)
- Brief summary of your experimental plan (see p. 379).
- Data
- Table of reactants and products (see above)
- Calculation of theoretical yield based upon amounts used and MW’s
- Weightsactually used
- Brief procedure
- Timed observations, including any TLC analyses of reaction
- Weight of product recovered
- mp of final product (and any other mp’s taken)
- IR or other analyses (optional)
- Your conclusion; that is, do you believe you achieved your objective, and with what evidence.
Pre-Lab questions (assigned in OWL via Canvas) are due for everyone at 6:00pm Wednesday (Oct. 21).
Lab notebook pages (uploaded to Canvas) and Post-Lab questions (uploadedto Canvas) for these labs are due 6:00pm Wednesday (Oct. 28). In-person and remote students are required to complete and submit these pages.
Remote students are required to take a Canvas Quiz at 6pm on Monday (approx. 10 min.). Streaming video links will be provided for each team to participate remotely after the quiz is completed.
Post-Lab Questions
- 382: 24, 26
Extra question: Examine the IR spectra of the reactant and product (10.38 and 10.40, p. 384-385) and explain the most significant difference above 1500 cm–1 in the two spectra. It is subtle.
Complete on paper or .doc file and upload to Canvas (not in OWL)
Chemicals List:
(E)-Cinnamic acid
Pyridinium tribromide
Acetic acid (glacial)
Sodium bisulfite
Ethanol
Wednesday Lab (Oct 21): Fischer Esterification of Carboxylic Acids (Microwave)
Text Assignment:
Chapter 20.2 (p. 759-770)
Detailed procedure for Wednesday: p. 765-766
Discussion:
There are alternatives to the “traditional” reaction apparatus of a flask in a mantle with a reflux condenser. One of these is a microwave synthesizer. A microwave synthesizer is essentially a microwave oven suitable for chemicals. The heat normally applied by the mantle is instead provided as microwave energy which heats materials (such as water) that absorb it.
One advantage of microwave synthesis is the speed with which reaction temperatures can be achieved. Microwave synthesis is best suited to small scale reactions. In fact, several similar reactions may be placed in the synthesizer simultaneously, and they can all be processed in a single operation.
Microwave synthesizers typically use thick-walled tubes which have strongly clamped caps. This allows the reactions to be heated without venting (under pressure), which keeps solvent sin the liquid state at higher temperatures. For instance, microwave reactions in water may be heated well above 100°C under these conditions, and the water remains in the liquid state.
The synthesis of esters from carboxylic acids and alcohols is very general, achieved by heating the two together and removing the water by-product. In this laboratory, each team will be given a different carboxylic acid and alcohol, and all four prepped reaction tubes will be microwaved together. Then, each team will work up their own product.
You will not use ethanol and p-aminobenzoic acid as stated in the text. You will use different alcohols and carboxylic acids. The procedure will otherwise be similarwith these changes:
After neutralization with sodium carbonate, the product will NOT precipitate.
If it forms a clear, separable oil from the water layer, you can skim or pipet it off and dry it with anhydrous sodium sulfate.
If it is not easily separated from the water or is too small a quantity to pipet off, you can extract the water layer with petroleum ether (approx. equal volume, separatory funnel). The petroleum ether layer should be extracted 3x times with an equal volume of water to remove excess alcohol. Then, it may be dried over anhydrous sodium sulfate and concentrated to an oil.
Analyze your product by IR (and look for the presence of alcohol as an impurity). You are welcome to analyze your products by TLC also. Submit your product for analysis by GC-MS and 1H-NMR.
Your products may have notable and interesting smells. Esters commonly have fragrant odors and are common components of fruit oils and perfumes. If you can match the smell of your ester to a recognizable fragrance, that is a good observation to note in your write-up.
Notebook:
- Title (Fischer Esterification of Carboxylic Acids by Microwave)
- Purpose/Introduction (Synthesize an ester by microwave synthesis)
- SDS data for substances used (see chemcals list below)
- Brief summary of your experimental plan (see p. 765-766).
- Data
- Table of reactants and products (see example table above)
- Calculation of theoretical yield based upon amounts used and MW’s
- Weights actually used
- Brief procedure
- Observations, including any TLC analyses of reaction
- Weight of product recovered
- Smell and appearance of product
- IR or other analyses (optional)
- Your conclusion; that is, do you believe you achieved your objective, and with what evidence.
Pre-Lab questions (assigned in OWL via Canvas) are due for everyone at 6:00pm Wednesday (Oct. 21).
Lab notebook pages (uploaded to Canvas) and Post-Lab questions (uploaded to Canvas) for these labs are due 6:00pm Wednesday (Oct. 28). In-person and remote students are required to complete and submit these pages.
Remote students are required to take a Canvas Quiz at 6pm on Monday (approx. 10 min.). Streaming video links will be provided for each team to participate remotely after the quiz is completed.
Post-Lab Questions
- 767: 1 (Answer the question as to why a dry alcohol is preferable to one containing water.)
Extra question 1: How would you expect the product (ester) to elute on a TLC plate relative to the reactants (carboxylic acid and alcohol)? In other words, would you expect to rise faster or slower or about the same as the reactants?
Extra question 2: If you took an IR of the mixture of carboxylic acid and alcohol starting materials and compared it to an IR of the product after the reaction, what changes would you expect to see in the IR?
Extra question 3: If you take an IR of either the carboxylic acid or the alcohol starting materials, you would obtain a prominent broad IR signal for the OH at 3000-3400 cm–1. The ester product will not have this signal. However, if you take an IR of the reaction mixture after the reaction is over, there is still an IR signal at 3000-3400 cm–1 even though the carboxylic acid and the alcohol are both no longer present. Why?
Complete on paper or .doc file and upload to Canvas (not in OWL)
Chemicals List:
Concentrated sulfuric acid
Sodium carbonate
Petroleum ether
