1. General

At this point in the quarter, it would be very advisable to review your IR and NMR notes from lecture. You are expected to have mastered these topics by the time the final exam comes around (in four weeks). You will need to memorize the most important numbers and trends for the exam since there will no tables provided!

Note for the teaching assistant: the students should start the reflux first and then take the quiz while the reflux stabilizes

2. Experiment

Safety note: Concentrated sulfuric acid is very corrosive. Wear nitrile gloves when you transfer it! Avoid spills on your clothes since it will ruin them!

For this week's experiment it is important that the glassware is dry. Why?

The alcohol should be used in a five-fold excess (in terms of moles) as compared to the carboxylic acid. Why?

Place a drying tube packed with CaCl₂ onto the air condenser. The proper packing of the drying tube is a loose cotton plug first, then about 4-5 cm of CaCl₂ and finally another loose cotton plug. The cotton plug is meant to keep the calcium chloride in place, but not to provide a tight seal!!

Towards the end of the reaction, the air condenser is replaced by the Hickman head to remove some of the alcohol by distillation. This will make it easier to get a phase separation when the water is added.

After the reaction, the aqueous layer and the organic layer should be separated. The aqueous layer is then extracted with two portions of diethyl ether. The organic layers are combined and then carefully extracted with sodium bicarbonate in the conical vial or centrifuge tube. In the beginning, the two layers are mixed with a spatula or a small glass rod (especially the first time!). At this point it is very important to vent the container frequently. Why?

The amount of the drying agent used should be minimized in order to minimize the loss of compound.

Before the vacuum distillation can be carried out, the diethyl ether has to be boiled off at ambient pressure. Why? This can be done in a small beaker or the conical vial if the amount of the liquid is relatively small. If you only have a small amount of crude product, pre-weigh the Hickman head before the vacuum distillation.

In order to obtain a good vacuum, the ground glass joints of the glassware have to lubricated lightly. The vacuum tubing has to fit snuggly over the drying tube. If it was too hard to push the vacuum tubing over the vacuum tube, the drying tube should be lubricated lightly as well. The better the setup is sealed, the easier and faster the vacuum distillation is going to be.

During the distillation, the setup is first evacuated without heating to remove the diethyl ether that was not removed prior to this step. Then the setup is heated gently in vacuum to remove the left-over alcohol. The Hickman head and the air condenser is not collected at this point. The alcohol is sucked into the vacuum line. If the solution stops boiling, the temperature is increased again to distill the ester into the Hickman head.

The product has to be characterized using refractive index and IR by the student. The remaining sample is submitted to the TA in a properly labeled vial (your name, the combination (i.e. Z9, your TA and section) so that the TA can prepare the NMR sample for you .Make sure to sign in your sample as well. The print-outs can be picked up in the instructor's office.


3. Things to think about

a. Why is an excess of acetic acid used and not an excess of isopentyl alcohol if you were to prepare isopentyl acetate?
b. Does the esterification go to completion? If not, how can the yield of the ester be improved using the same reagents?
c. Why are esters very important compounds in industry?
d. Why do you have to wrap a cold paper towel around the air condenser?
e. Why do you have to heat the mixture to a reflux?
f. Why is the vacuum distillation performed in the end of the purification?
g. Why does the Fischer esterification not work for tertiary alcohols?