Lab Report on Protection of 1-Phenylethanol by 3,4-Dihydro-2H-pyran and Purification of 2-(1-Phenylethoxy)-tetrahydro-2H-pyran by Flash Column Chromatography

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The lab report below was submitted as part of the coursework for CM2121 Organic Chemistry. Please do not plagiarise from it as plagiarism might land you into trouble with your university. Do note that my report is well-circulated online and many of my juniors have received soft copies of it. Hence, please exercise prudence while referring to it and, if necessary, cite this webpage.

Aim:
The aim of this experiment is to protect the hydroxyl moiety of 1-phenylethanol with 3,4-dihydro-2H-pyran and to purify the product by flash column chromatography.

List of instruments and reagents used:
Instruments:
Reaction
Work-up
Purification
magnetic stirrer w clamp
50 ml round bottom flask
2 cm magnetic stirring bar
metal spatula
weighing paper
500 ml crystallizing dish
calcium chloride guard tube
100 ml separatory funnel
50 ml conical flask (3)
filter funnel
100 ml round bottom flask
50 ml measuring cylinder
3 cm diameter column
250 ml beaker
glass rod
50 ml & 100 ml conical flask
100 ml measuring cylinder
250 ml round bottom flask
filter funnel
Reagents:

1-phenylethanol
3,4-dihydro-2H-pyran
Diethyl ether
p-toluenesulfonic acid monohydrate
Saturated sodium bicarbonate
Saturated sodium chloride
Hexane:ethyl acetate (4:1)
Silica gel
Sand

Experimental procedures:
0.976g (8.0mmol, 1 equiv.) of 1-phenylethanol was added into a 100mL-round bottom flask (rbf) with a dropper. After putting in a magnetic stirring bar, calcium chloride guard tube was attached to the rbf. 0.8mL (8.8mmol, 1.1 equiv.) of 3,4-dihydro-2H-pyran  was then added into the rbf, followed by 20mL anhydrous ether. The calcium chloride guard tube was then replaced immediately. 0.0760g of p-toluenesulfonic acid monohydrate (0.04mmol, 0.005 equiv.) was weighted and added into the mixture swiftly. The reaction mixture was monitored every 30 minutes with a hexane:ethyl acetate (4:1) solvent system.

After the reaction was completed, the reaction mixture was poured into a 100mL separatory funnel, and the rbf was rinsed with technical ether (2 x 5 mL). The organic layer was then washed with 30mL saturated sodium bicarbonate solution followed by an equivalent volume of saturated aqueous sodium chloride. The organic layer was then transferred into a clean 100mL of conical flask and anhydrous sodium sulfate was added in small portions until a fine suspension was obtained on swirling. The mixture was left to stand for 10 minutes. The organic extract was then filtered into a 100mL rbf and the conical flask was rinsed with technical ether (3 x 3mL). The solvent was then removed using rotary evaporator. The crude product was weighed and recorded.

Flash column chromatography was performed to isolate the wanted pure product. The purified product was weighed and recorded.
Result and Calculations:
Mass of 1-phenylethanol used                            = 0.976g
Mass of p-toulenesulfonic acid monohydrate = 0.0076g
Mass of 3,4-dihydro-2H-pyran                            = 0.730g


Mass of crude product in 100mL-rbf with cock ring
= 99.358 g
Mass of 100mL- rbf with a cock ring
= 98.360 g
Mass of crude product
= 99.358 – 98.360 = 0.998g

Mass of purified product in 100mL-rbf with cock ring
= 99.148 g
Mass of                100mL-rbf with a stopper and a cock ring
= 98.360 g
Mass of purified product
= 99.148 – 98.360 = 0.788g

No. of moles of 1-phenylethanol =  ≈ 7.989 x 10-3mol

Since the mole ratio of 1-phenylethanol to 2-(1-Phenylethoxy)-tetrahydro-2H-pyran is 1:1,
No. of moles of 2-(1-Phenylethoxy)-tetrahydro-2H-pyran formed                                 ≈ 7.989 x 10-3mol

Theoretical yield of 2-(1-phenylethoxy)-tetrahydro-2H-pyran = 7.989 x 10-3 mol x 206.280g/mol = 1.65g

Percentage yield of purified 2-(1-phenylethoxy)-tetrahydro-2H-pyran = 47.8% (2 d.p.)

1.0cm
 
Solvent Front
 
Baseline
 
0
 
Thin Layer Chromatography (TLC) results
At different time intervals:

 




















After purification by flash column chromatography:

 













From the TLC results at different time intervals,
Rf values of 1-phenylethanol = 1.0 / 3.9 and 1.5/ 3.9 = 0.26 and 0.38   (spots present at 0 min)
Rf value of 2-(1-phenylethoxy)-tetrahydro-2H-pyran = 2.1 / 3.9 = 0.54 (spot that appear at 60 mins)

From the TLC result of test tube 7 after purification by flash column chromatography,
Rf value of 2-(1-phenylethoxy)-tetrahydro-2H-pyran = 2.1 / 4.0 = 0.53

The Rf values of 2-(1-phenylethoxy)-tetrahydro-2H-pyran, before and after purification by flash column chromatography, are close. This suggests that the purification process is thorough.


Discussion:

Reaction Mechanism
In this experiment, the OH group was protected by forming 2-(1-phenylethoxy)-tetrahydro-2H-pyran. The OH group was masked, thereby preventing the compound from taking part in reactions that might involve it.

Overall reaction:
As this protection of OH group is reversible, after the compound has reacted in the desired manner, the protection group can be removed.

The mechanism of this protection reaction is shown below:




Thin Layer Chromatography (TLC)
The progress of the reaction was monitored through TLC. Part of the reaction mixture was drawn out with a dropper every 30 minutes and analysed by TLC with a hexane:ethyl acetate (4:1) solvent system.

It was observed that the spots corresponding to the reactant, 1-phenylethanol, became smaller with the progress of the reaction. On the other hand, the spot corresponding to the product, 2-(1-phenylethoxy)-tetrahydro-2H-pyran, became more visible.

Since silica is a polar adsorbent, the more polar reactant will interact more strongly with it and moves slower up the plate. Hence, 1-phenylethanol has a lower Rf value. 2-(1-phenylethoxy)-tetrahydro-2H-pyran, however, is less polar and will travel faster up the TLC plate; therefore, it has a higher Rf value.

The choice of solvent is critical. Generally, the more polar a solvent is, the more effective it is at eluting both polar and non-polar compounds. This is because a polar solvent more effectively competes with the compounds for adsorption on the relatively polar surface of the adsorbents. However, the solvent must not be too polar such that movement becomes too rapid and all the components end up at the top of the plate without effective separation. Hence, a solvent system of hexane: ethyl acetate (4:1) was used.

Once the spot corresponding to the product no longer increases in size, the mixture was separated with a separatory funnel, followed by flash column chromatography.

Experimental set up for TLC
1)      A jar with lid was chosen for TLC. It is important for the atmosphere in the developing container to be saturated with the vapors of the eluting hexane:ethyl acetate (4:1) solvent system. This prevents the plate from drying out during the run. To aid in this saturation, the beaker was lined with a filter paper, which serves as a wick to saturate the atmosphere in the jar.
2)      The baseline and solvent front was drawn with graphite pencil. Ink was not used as it may be soluble in the solvent system, thereby affecting the accuracy of the results.
3)      It is important not to spot too much or too little sample on the TLC plate. If excess sample was used, the sample will run as a streak, rather than a spot. If too little sample was used, the spots may not be visible. For this experiment, the sample were spotted several times in one position as it was fairly dilute.
4)      The TLC plate was placed carefully in the developing container. It was ensured that the solvent did not rise above the baseline to prevent the direct dissolution of sample into the solvent. To achieve even separation, the plate was placed upright and not disturbed.  

Work-up by a separatory funnel
The reaction mixture was transferred into a a separatory funnel. The organic layer was washed with saturated sodium bicarbonate solution. Carbon dioxide will form due to an acid-base reaction. A significant pressure will build up in the funnel and care was taken to vent more often. Another reason to vent often was because the volatile ether was used as solvent.

The subsequent washing with saturated sodium chloride solution moved the bulk of water from the organic layer to the aqueous layer as there is a difference in concentration gradient. Final traces of water was removed by  addition of anhydrous sodium sulfate, a drying agent that readily takes up water to become hydrated.

Flash Column Chromatography (FLC)

After the crude product was obtained by rotary evaporation, FLC was carried out to extract the desired pure product.

Cotton and a layer of sand was placed in the bottom end of the column to provide a uniform bed for the stationary phase (silica gel). A second layer of sand was placed on top of the stationary phase to act as shock absorber. As the stationary phase used in this experiment is dense, it will take a long time to run the experiment through gravity FLC. Hence, air pressure is introduced from the top of the column to push the mobile phase more quickly.

Techniques and theoryof FLC
 Column chromatography is useful in separating and purifying the consituents in a mixture according to their polarities. In FLC, the coloumn is packed with silica gel that is polar. Any components that are polar would bind to the gel more tightly, thus taking a longer time to travel down the column. On the other hand, the non-polar components would have less interaction with the polar silica gel and thus eluted from the gel by the mobile phase.

In this experment, the product, 2-(1-phenylethoxy)-tetrahydro-2H-pyran, is less polar than the reactant, 1-phenylethanol. The polar product bound to the polar stationary phase more strongly and the less polar reactant is more soluble in the mobile phase (5% ethyl acetate in hexane). Hence, the desired product was only eluted by the test tube 7.

FLC is useful for it can separate components in the mixture for further analysis. However, care must be taken when carrying out the experiment. When packing the column, air bubbles should not be trapped and the layers should be evenly spread out. Each subsequent addition of sample and eluant had to be done meticulously to ensure that the column was not disturbed. It was also ensured that  the column was covered by the elaunt at all times during the experiment; this is because the silica gel would not be able to adsorb the compounds efficiently if it is dried.

Silica gel is toxic and care must be exercised when using it to pack the column. To prevent inhalation of the silica particles, this part of the experiment was carried out slowly in the fumehood.

Yield of 2-(1-Phenylethoxy)-tetrahydro-2H-pyran
The yield of the purified compound was 47.8%. There are several factors contributing to this low yield. The protection reaction is reversible; hence, it is not possible for the reaction to go to completion. Also, minute amounts of the sample were lost during each transfer between apparatus. This added up to a significant amount lost.

Conclusion:
The mass of crude 2-(1-phenylethoxy)-tetrahydro-2H-pyran is 0.998g.
The mass of purified 2-(1-phenylethoxy)-tetrahydro-2H-pyran is 0.788g.
The percentage yield of 2-(1-phenylethoxy)-tetrahydro-2H-pyran is 47.8%.
The Rf value of 1-phenylethanol is 0.26 and 0.38.
The Rf value of 2-(1-phenylethoxy)-tetrahydro-2H-pyran is 0.454.

Questions
1)  Name three criteria of a good protecting group.

A good protecting group should be: 
·         chemically reactive to most chemcial reactions
·         easily converted to the orginal functional group after a simple procedure
·         stable enough to be be isolated

3,4-dihydro-2H-pyran is a good protecting group as it forms a ether stable to alkali, lithium aluminium hydride, acetic anhydride, chromium trioxide, organolithium and Grignard reagents. However, it can be easily converted to the alcohol by cleaving with a dilute acid.


2)  Is the calcium chloride guard tube essential to the success of this experiment? Why or why not?

Calcium chloride guard tube is essential for this experiment’s success.

The ether solvent is volatile and may cause pressure to build up in the reaction vessel. The calcium chloride guard tube prevents this build up of pressure by providing an outlet, while excluding atmospheric moisture at the same time.

This is crucial as water could act as a nucleophile and compete with 1-phenylethanol to react with 3,4-dihydro-2H-pyran, thereby decreasing the yield of the desired product. 


3)  What is the role of p-toulenesulfonic acid monohydrate in this experiment? How does its amount affect the reaction?

p-toulenesulfonic acid monohydrate is a non-oxidising acid catalyst soluble and stable in organic solvents. It drives the reaction forward by producing H+ions that serve as catalyst in this reaction. In slight amount, this reagent acts as a catalyst.

However, when in excess, the reagent may cause the hydrolysis of the acetal product - particularly if any water is present – thereby decreasing experimental yield. Hence, it should only be added sparingly.


References:
Anonymous. The Organic Chemistry Laboratory. University of Colorado, Boulder, Department of Chemistry and Biochemistry.

Peter Taylor. 2002. The Molecular World: Mechanism and Synthesis. The Royal Society of Chemistry.

Wothers, Peter; Greeves, Nick; Warran, Stuart and Clayden, Jonathan. Nucleophilic substitution at C=O with loss of carbonyl oxygen. Organic Chemistry. Oxford University Press, 2009.

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