Synthesis of Triphenolmethanol and the Trityl Carbocation

Introduction:

The objective of this lab is to synthesize triphenylmethanol via a Grignard reaction, and then convert the product obtained to trityl fluoborate and determine the color of the compound.

Grignard reaction is the key reaction for this experiment.  The Grignard Reaction is the addition of a (Grignard reagent) to a ketone or aldehyde, to form a tertiary or secondary alcohol. If a Grignard reagent were to react with formaldehyde a primary alcohol would form. Grignard reagents have the formula of R-Mg-X where X is a halogen, and R is an alkyl or aryl group (based on a benzene ring) and can be formed in the following way;

For this experiment the Grignard reagent (phenylmagnesium bromide) will be used to synthesize triphenolmethanol;

In the first step, the Grignard reagent is formed by combining bromobenzene with magnesium using anhydrous ether as a solvent. The reagent phenylmagnesium bromide is then added with benzophenone and a magnesium salt precipitate forms. Finally the salt is converted into triphenylmethanol by adding water, and acid is added to dissolve any basic product. Grignard Reagents are also used in the following important reactions: Grignard reagent can be added in excess to an ester or lactone to give a tertiary alcohol in which two alkyl groups are the same, and the addition of a Grignard reagent to a nitrile produces an unsymmetrical ketone via a metalloimine intermediate.


It is important that all apparatus must be dry throughout the reaction to prevent unwanted side reactions involving water and the Grignard reagent;

The inorganic product, Mg(OH)Br, is referred to as a “basic bromide”.

The product obtained from the first part of the experiment will then be reacted with an acid  to yield a carbocation called Trityl Fluoborate.

Results:

Mechanism:

Physical Properties:

mp
Triphenyl methanol 164°C
Trityl fluoborate
Synthesized Triphenyl methanol 151°C
Magnesium used (silver coloured solid) 0.539 g
Benzophenone used (white solid) 4.009g
Diethyl ether used 10 ml
Acetic Anhydride 1 ml
Magnesium sulfate used 0.156 g
Sodium Bicarbonate used 15 ml
Compound A 1 g
Mass of Product 0.2 g

Observations (Triphenyl methanol)

Iodine + Mg + bromobenzene +diethyl ether èdark brown

Benzophenone + 10ml diethyl ether è redish precipitate

è15 minutes of heat è yellow solutionè white solid precipitate

IR Spec Analysis for Triphenyl methanol

Wavenumber range (cm-1)
2850 – 3000 Sp3 C-H
3200-3600 O-H
450-1600 Fingerprint Region

 

Two initial separate layers in round bottom flask (clear layer and yellow layer)

After adding NaHCO3 èclear layer forms white precipitateè collection of white solid, Evaporation

Hexane dissolves solidèproduct collection through vacuum filtration

Observations for Separation and formation ofTrityl Fluoborate

è 1ml of acetic Anhydride forms a clear solution with white granules

è fluoboric acid & diethyl ether precipitate è brown orange

è Product = bright orange solid

Discussion

The first step in the lab is to prepare Triphenyl methanol and eventually react that product to produce a Trityl Carbocation. It is important to dry all equipment of any water residue as to prevent any unwanted side reaction from water on the glassware. What may happen is that water may protonate the carbanion producing benzene. The carbanion is supremely basic so any water present in the mixture will protonate the anion.  Another possible side product is biphenyl which is formed in competition with the Grignard reagent phenyl magnesium bromide. After the electron transfer the phenyl radical can either accept another electron or combine with another phenyl radical to make biphenyl.

Once all of the apparatus is properly dried (using a flame or Bunsen burner) the reaction can begin. First 22.0 mmol of dry bromobenzene is added to an Erlenmeyer flask and it is dissolved in 5 ml of anhydrous diethyl ether. The solution is then transferred to a seperatory funnel and stopper. It is important to have also added iodine to the bromobenzene solution. Iodine serves two purposes; iodine acts as an indicator when the magnesium begins to react with bromobenzene, and it rarely acts as an activator by sometimes being able to clean the surface of the magnesium exposing a fresh surface to redox chemistry.  Since the apparatus has been set up for addition under reflux conditions, you must have the magnesium in the reaction flask directly below the seperatory addition funnel containing the bromobenzene and iodine solution. Once the bromobenzene solution is added to the magnesium in the reaction flask the reaction begins to take place and the Grignard reagent forms (note that the Grignard reagent has the formula R-Mg-X where X is a halogen, and R is an alkyl or aryl (based on a benzene ring) group). A positive indicator for the initiation of the Grignard reaction is that the iodine colour will begin to go away, the solution will begin to get hot and there will be bubbling, at this point there was the formation of a cloudy precipitate.  This reaction is very exothermic so the reaction will boil on its own without the aid of a hot plate or open flame.


After the reaction has reached completion, the second step in forming triphenylmethanol would be to react the Grignard reagent with (Phenylmagnesium Bromide) with Benzophenone.  20.0 mmol of benzophenone is dissolved in 10 ml anhydrous diethyl ether in a dry Erlenmeyer flask and then placed in the seperatory addition funnel.  This solution is then added drop by drop into the reaction mixture flask (containing the Grignard reagent) and this mixture should be shaken throughout the process. At this stage the solution begins to turn a pinkish red. Once all of the benzophenone solution has been added, the mixture is then heated in a bath under reflux for 10 to 15 minutes.

Once the reaction mixture has cooled, 5 ml of water is added drop by drop and then 15 ml of hydrochloric acid and ethyl ether. The reaction must be stirred until all solid in the solution has been broken up and dissolved.

The final step in this part of the experiment is separation.  There was some undissolved magnesium so to get rid of the magnesium, gravity filtration is used.  After this, the reaction mixture is transferred to a seperatory funnel where it is gently shaken and the layers are mixed. The aqueous layer is then discarded and the ether layer is washed with 15 ml of sodium bicarbonate. After evaporation a solid residue remains and can be dissolved using 10 ml of hexane. The product is the vacuum filtered and collected.

The product is then recrystallized from a 2:1 mixture of hexanes, product is then weighed and the melting point was recorded to be 151 degrees Celsius. This is extremely close to the theoretical melting point of 164 degrees for triphenyl methanol. The reason why it was not exactly the same is because the product contained some impurities.


The second part of the experiment is to prepare Trityl Fluoborate from the triphenyl methanol produced in the first part of the lab. 1.0g of triphenyl methanol product is mixed with 7.0 ml of acetic anhydride in an Erlenmeyer flask (the solution is clear with white granules). 1.0 ml of fluoboric acid is added and the mixture is then cooled for 15 minutes, the solution is no longer clear and will turn a brown orange colour. This product is then collected via vacuum filtration and then is air dried.

To conclude, our results show that triphenyl methanol was formed or at least a derivate of it. The melting point of 151 °C is close to 161°C but due to impurities in the product, the recorded temperatures were off. IR spec analysis did show the presence of an alcohol and sp3 carbon as well as aromaticity but the results from the IR alone would not be enough to confirm the compound. It is a combination of both melting point and IR Spec analysis that has confirmed the product to be triphenyl methanol.