Lab #4: Microscale Synthesis of Benzoin Acetate
By: Lisa Wu



Before the Lab:

- it is a good idea to draw out a flow chart

- search out the MSDS of the following chemicals: benzoin, acetic anhydride, benzoin acetate, glacial acetic acid, calcium chloride (drying tube), conc. sulfuric acid, activated silica gel, anhydrous soldium sulphate, menthylene chloride

- the entire lab consist of purifying benzoin acetate by means of reflux, extraction, and column chromatography so it is a good idea to watch the Potter Videos link posted on CHMB42 intranet relating to reflux and extraction

- the lab is done in partners so make that even if your partner is not aware of some procedures, you should be well-prepared

- the entire lab should take about 2 hours maximum to complete IF you are WELL-PREPARED for it!!

- it is always a good idea to think ahead E.g. when you wait for your flask to cool to room temperature, get started on setting up the next apparatus!!



Purpose:

- to purify benzoin acetate by means of reflux, extraction, and column chromatography

- to use polarity of molecules to purify benzoin acetate and compare the UV activated chromatogram of crude and pure samples to obtain the Rf values on TLC or paper chromatography
- to obtain % yield of pure benzoin acetate and its melting point range



Theory:
General Reaction carried out in this lab: Benzoin + acetic anhydride        <- H+ ->       Benzoin acetate + acetic acid

* TIP! Find out which reagents are more polar and which one is nonpolar. This will help you determine which one will travel faster in a TLC.



Essentially, the first part of the experiment is refluxing the reagents at a high temperature 80-90 degrees celsius because we want the reaction to go to completion. By allowing the water in the condenser tubes to run in a counter-current flow, this allows the condenser column to be very cold such that any vapours given off are cooled back to liquid, and fall back into the reaction vessel. This is very convenient for the chemist whom want the reaction to be thermically accelerated at a higher temperature without worrying about adding excess reagents (unless needed to). In this lab, a sand bath is used instead of the heating mantel since the experiment calls for high temperatures and the sand bath provides even heat with minimal stirring.



To purify the product, separation by extraction is often used by partitioning the desired product into a solvent with a better solubility to the desired product than the previous solvent OR it is used to partition the undesired products out of the solvent. This often consists of partitioning products between aqueous and organic solvents. In this lab, the organic solvent of methylene chloride is used to partition the benzoin acetate out of its original aqueous solvent in order to obtain a more purified crude product.



Lastly, the purification method used to obtain the pure benzoin acetate product in this lab is column chromatography. This theory behind this technique consists of using polarity of solutes and solvents to obtain the desired product. The apparatus consists of a column filled with solid support that has porous medium inside which "filters" out the desired or undesired product by means of polarity when the eluent is poured over the column. Here, the concept of "like-dissolves-like" is understood where polar molecules are more absorbed in polar solvent and thus they travel slower. In this lab, methylene chloride is the eluent used to pull the nonpolar benzoin acetate down as the desired product.



In addition, a paper chromatography is done to identify different components run on the chromatogram of the crude and pure products. Here, the relative positions of the chemical components can be identified when shone under UV light and by use of their polarities. The Rf value is the retention factor = rate of travel of compound relative to solvent front.

Rf = (rate of travel of compound)/(rate of travel of solvent front)

Rf = (distance travelled by compound)/(distance travelled by solvent front)



Definitions:

Chromatography- separaion technique of components in a solution mixture by running them at different rates on insoluble, porous medium via solvent

thin-layer chromatography (TLC)- "solid-liquid" chromatography that partitions components between moving liquid phase and solid support

paper chromatography- type of "solid-liquid" chromatography where the support is cellulose

column chromatography- "solid-liquid" type where solid support is silica gel/ Al2O3

support- the porous medium

eluent- the separation solvent

development- the process of separation using chromatography



A Brief Procedure:

Part I- Reflux:

à obtain a hot plate/sand heater, sand bowl, sand, buret clamp, retort stand, thermometer

- set up a hot plate or a sand heater on a retort stand on "LO" setting (before the quiz)

- fill the sand bath around half-full with sand

- clamp a thermometer using a buret clamp onto the retort stand with the tip placed into the sand but not touching the bottom

- reach a steady temperature of 80-90 degrees celsius (around 1/2 hr)

à obtain 5mL pear-shaped flask, 40mg Benzoin, 200uL acylation solution, micropipet P200, condenser column, CaCl2 drying tube, and 1 boiling chip

*NOTE: The acylation solution is kept in the fumehood at all times!

- 40mg of weighed dry benzoin, 200uL of acylation solution, and 1 boiling chip are added in a 5mL pear-shaped flask

- set up the vertical water cooler condenser for the reflux process (drying tube on top of the condenser with the water-out tube over the water-in tube; attached to the top of the pear shaped flask)

*NOTE: Clamp the flask, not the condenser!

- stop relfux after 15 min

- turn heat off and lower the heating mantle or sand heater to allow solution to reach RT of 25 ̊C (~5-10min)

*NOTE: Make sure all equipments are cooled before putting them away, 80-90 ̊C can cause severe burns!

à obtain pasteur pipette, 10mL graduated cylinder or smaller, ~1.0mL of water, ice bath

- pipet ~0.5mL of water into the 5mL pear flask

*NOTE: Mark where the 0.5mL is on the pasteur pipette using tape or elastic band for easy reference of volume for the next part of the lab.

- cool the solution in the 5mL pear flask in an ice bath kept in the fumehood (~5-10min)

- put away all equipments (COOLED) at this point except the retort stand with the 5mL flask clamped onto it

*NOTE: While the solution cools, set up the apparatus for the next part!





Part II- Making the Solid Support aka "Column":

- obtain a glass pipette and fill it up in this order:

1.  a little bit of glass wool to form a plug

2.  ~ 50mg or 1-2mm of sand

3.  0.7g or 3/4 of the pipette full of activated silica gel powder

4.  0.4mg of Na2SO4

- tap the glass pipette slightly up and down for the solid support to "pack" evenly

*NOTE: Don't tap the glass pipette too much or else your solid support becomes too packed and the wait-time for the product to be "pulled out" of TLC will increase

*NOTE: Silica gel is POLAR, thus it will attract polar molecules.

*NOTE: Do everything inside the fumehood!

- clamp the column on a retort stand if not in use



Part III- Extraction:

- obtain 5mL of methylene chloride (only 1.5mL used in Part III), stoppered 10mL graduated cylinder, the marked "organic" pasteur pipette, disposable pipet, the cooled 5mL pear flask, stopper, small stoppered flask or vial marked "organic layer"

- make sure that the 5mL pear flask is well clamped onto the retort stand

- extract the solution in the 5mL pear flask with 3 portions of 0.5mL of methylene chloride using the marked pasteur pipette

- after each addition, stopper flask; invert and vent 10-20 times; then use a disposable pipet to pipet the bottom organic layer to a separate flask or vial labelled "organic layer" and stopper it

- after all three organic layers are collected in the "organic layer" flask, it is ready for transfer



Part IV- Column Chromatography:

- obtain a new marked "solvent" pasteur pipet, ~5mL of methylene chloride in the stoppered 10mL graduated cylinder from Part III, "organic layer" flask from Part III, and 10mL Erlenmeyer flask, the column (glass pipet) from Part II

- clamp the column onto a retort stand if not already done so

- place the 10mL E. flask below the column and label it "pure product"

- record the MASS of the 10mL E. flask + the label

- add a few drops of the methylene chloride to wet the walls of the column until solvent is flowing at the bottom

- add the "organic layer" SLOWLY drop-wise using the "organic" pasteur pipette down the column until drops of benzoin acetate are collected into the 10mL E. flask

*NOTE: Leave 1-2 drops of crude product from the stoppered "organic layer" flask for further analysis in Part V- TLC!!!

- finally, add 1.5 mL of methylene chloride to efluent any remaining sample



Part V- TLC:

- obtain a TLC chromatogram slide, a lidded chamber, a filter paper, 0.5cm or 1mL methylene chloride, 2 capillaries labelled "crude" and "pure", stoppered "organic layer" flask and "pure product" in the 10mL E. flask from Part IV, UV lamp, pencil

- place ~0.5cm of methylene chloride and a filter paper adhering to the walls of the chamber INTO the chamber

- close the lid and allow chamber to equilibrate for 15 min

- on the TLC slide, draw a horizontal line using pencil ~0.8-1cm above one edge; make 2 dots and label them "C" for crude and "P" for pure

*NOTE: Do not touch the NON-laminated side of the TLC slide since it is used for qualitative analysis!!

- dip "crude" capillary into the stoppered "organic layer" flask from Part IV and make a dot with diameter of 1mm onto the "C" pencil dot on the TLC slide

- dip "pure" capillary into the "pure product" in 10mL E. flask from Part IV and make a dot with diameter on 1mm onto the "P" pencil dot on the TLC slide

*NOTE: Do not make a dot more than diameter of 1mm because this can cause streaking on the TLC!

- once the chamber has equilibrated, placed the slide with the spot side down into the chamber and replace the lid

- once solvent front is ~0.5cm below the TLC slide, remove TLC slide and mark solvent front

- allow TLC slide to dry

- examine spots under UV lamp

*NOTE: Always wear protection goggles when viewing UV light!!

- circle any spots with pencil

- take measurements and calculate Rf values



Part VI- Mass and MP:

- turn on the sand heater and place the open 10mL E. flask with the "pure product" inside for the vapours to evaporate

- once the solid is obtained, take the mass of the flask + solid + label and calculate the pure benzoin acetate's mass

- the solid product is removed from the flask into a plastic bag and labelled with colour, dryness, solidity, etc.

- the melting point of the solid product is obtained and recorded



*FINAL NOTE: Since you are working in partners, find laspes in time where one partner can do one part and another partner can do another part and integrate results for maximum use of time and efficiency!! Eg. Part V and VI can be done this way. :)