Grignard Synthesis: Synthesis of Benzoic Acid and Triphenylmethanol
By: Erin H.
Phenylmagnesium bromide was used as the Grignard organometallic for two reactions. The first was a synthesis of triphenylmethanol from benzophenone. This was done using a gentle reflux followed by several Separatory washes and extractions to purify and separate product. The second reaction was the synthesis of benzoic acid from phenylmagnesium bromide using dry ice and a variety of washes. The benzoic acid reaction was straightforward and a 54.4% yield was recovered. The triphenylmethanol reaction was a bit more tricky and only 24.7% was recovered, though what was recovered was seemingly very pure.
Many reactions done so far have consisted of combining compounds and the switching of substituent groups. This lab is centered around the addition of carbons to a compound, an extremely useful ability in chemistry. To add carbons to a molecule with a carbonyl carbon, it is possible to use an organometallic, in this case a Grignard reagent. The alcohols produced from aldehydes and ketones are also important because they can be easily converted into other compounds – this makes them extremely useful in the laboratory.
The first reaction is to synthesize triphenylmethanol from benzophenone and an organometallic. Next, benzoic acid will be synthesized from carbon dioxide and phenylmagnesium bromide (the same organometallic used in the first reaction). Both substances will also be purified and analyzed. An important note to these reactions is that Grignard reagents use a very basic carbanion intermediate and therefore no protic acid should be allowed to permeate the reaction – it will destroy the reagent. Acid is only used after the synthesis of desired compounds has been completed.
Synthesis of Triphenylmethanol
2.021g of benzophenone and 15mL of anhydrous diethyl ether were added into a 100mL round bottom flask.
The benzophenone was dissolved by adding a clamshell stir bar and placing the round bottom flask over a stirring hot plate (no heating).
After it was dissolved, a Claisen adapter and a condenser column were added.
A Separatory funnel containing 15mL of phenylmagnesium bromide was placed into the side of arm of the Claisen adapter and the Grignard reagent was added drop-wise while stirring.
The solution was refluxed gently for twenty minutes while solid accumulated in the round bottom flask.
When the reflux was finished, the contents of the flask were transferred to a 125mL Erlenmeyer flask. The flask was stored overnight to allow for evaporation of diethyl ether and tetrahydrofuran.
Purification of Triphenylmethanol
26.0mL of 1.0M sulfuric acid was added to the 125mL Erlenmeyer flask. All of the contents were added to a clean Separatory funnel.
19.5mL of diethyl ether and 6.9mL of the sulfuric acid were used to rinse Erlenmeyer flask of remaining crystals, and the wash liquid was added to the Separatory funnel.
The Separatory funnel was shaken for several minutes and then the two phases were allowed to settle.
The lower aqueous phase was extracted and discarded.
A second wash/extraction of 1.0M sulfuric acid was performed.
A third wash/extraction of saturated NaCl was performed to enhance removal of water from ether phase.
The ether layer was then decanted from the Separatory funnel into a 125mL Erlenmeyer flask.
2.1g of anhydrous sodium sulfate was added to bind water. The crystals clumped on the bottom of the flask and refused to form a free-flowing slush, so more Na2SO4 was added, up to 6.85g.
After 15 minutes, the sodium sulfate solids were removed via gravity filtration of large folded filter paper.
15mL of ligroin was added, after which large particles of a white precipitate formed.
The solution was boiled for 15 minutes, but remained the same in appearance.
After cooling to room temperature, the solution was placed in an ice bath for 12 minutes.
A vacuum filtration was performed to collect the triphenylmethanol crystals (the filter paper was wetted with ligroin).
The product was placed in an oven overnight for later analysis.
Synthesis of Benzoic Acid
5-10g of dry ice was added to a 250mL beaker, followed by 15mL of the Grignard reagent (phenylmagnesium bromide).
The beaker was left alone to sublime and when finished had formed a viscous, brownish, glassy mass.
10mL of 6.0M HCl was added slowly while stirring, until the pH fell to 1.
The sample was left overnight to dry.
Purification of Benzoic Acid
9.4mL of diethyl ether was added to the dry benzoic acid sample.
When dissolved, all sample was transferred to a Separatory funnel.
The container was rinsed with a small amount of diethyl ether.
10mL of diethyl ether and 10mL of de-ionized (DI) water were added to the Separatory funnel, which was then shaken.
After the two phases separated, the lower aqueous phase was extracted.
Next, 10.3mL of 1.0M NaOH was added to form benzoate ion, the Separatory funnel was shaken and the bottom was layer was extracted with the aqueous ion.
A second NaOH wash/extraction was performed to ensure ionization and collection of all benzoic acid.
To isolate benzoic acid, HCl was added until the pH fell to one.
The acidic mixture was cooled and then the crystals collected via vacuum filtration. (DI water was used to wash container and filter paper, but not ice cold water).
The crystals were stored to dry overnight for analysis.
As noted in the methods section, the triphenylmethanol reaction behaved strangely during the purification process. During the addition of sodium sulfate, the crystals clumped to the bottom and refused to form a free flowing mixture. Later when the ligroin was added, the white particles appeared nearly immediately and there was no other change to the solution through the boiling. After drying of the purified product, only 24.7% yield was achieved. What was recovered though, had a melt temp of 161° C, (the textbook value is 163° C) and the crystals were much whiter than other teams and had a fine sugary consistency, not solid and clumpy.
The benzoic acid synthesis and purification went very smoothly. The recovered crystals were shard-like and close to pure white. The melt temp recorded for the sample was 122.45° C, nearly exact to the table value of 122.4° C. Only 54.4% of the theoretical sample was recovered, potentially due to the use of room temperature DI water in the vacuum filtration at the end of purification (instead of ice cold water, as specified in the original protocol).
This lab specifically helped me understand the concept and practicality of organometallic reactions. When going over reaction mechanisms in class, I generally get the gist of it and see the commonality, but I often forget things like which ones need to take place in ether, which ones can’t be in acid, and etc. Coming to the laboratory helps to reinforce these details through manually running the reaction.
This lab also illustrates the variety of uses of organometallic compounds for synthesis. Grignard reagents can be used to combine whole groups of carbons, such as the benzene ring attaching to form triphenylmethanol. They can also be used to attach a carbon containing functional group – for example the formation of benzoic acid using a benzene organometallic and attachment of a carboxylic group to it.
CRC Handbook of Chemistry and Physics, 44th ed.
Zubrick, James. Lab Guide: The Organic Chem Lab Survival Guide, 8th ed.