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Bryanna Tanase David and Mark Grignard Synthesis of Triphenylmethanol Christopher Cain 11-20-17

Introduction The Grignard Reaction was discovered in 1900 by Victor Grignard as part of his Ph.D. dissertation2. It proceeds with the help of a Grignard reagent. A Grignard reagent is an organometallic (combination of metal and organic molecule4) reagent that is made by heating alkyl halides with magnesium in an ether solvent2, and it has the general formula R-MgBr3. The reaction mechanism for the formation of a Grignard reagent is shown below in Figure 1.

Figure 1:Grignard Reagent Formation4

These reagents can be thought of as ionic compounds that have a magnesium cation and 2 different anions, one being a carbon species and the other being bromine3. They are also strong bases that react violently with water and alcohols.2 Because of this, any reaction [performed with a Grignard reagent must involve dry reactants (no acid or water) and a dry reaction vessel1. Grignard reagents can react with carbonyl compounds (contain C=O bond) alkyl halide, and other organometallic compounds, but for this experiment the focus will be placed on their reactions with carbonyls2. The result of a Grignard reaction is a new carbon-carbon bond2. If a Grignard reagent is reacted with an aldehyde, a secondary alcohol is produced, and if it is reacted with a ketone or ester, a tertiary alcohol is produced4. The first step in an any Grignard reaction is the formation of the Grignard reagent (reaction shown above). Once the Grignard reagent is formed, the carbon-magnesium bond with in it acts as a nucleophile (electron rich compound) and attacks the positive carbon of the carbonyl compound4. This forms an alkoxide (a carbon compound that has an oxygen with a negative charge), and that alkoxide is protonated by water to form a secondary or tertiary alcohol4. The general mechanism for a Grignard reaction is shown below in Figure 2.

Figure 2: General Grignard Mechanism4

The objective for this experiment was to use the Grignard reagent phenyl magnesium bromide (made by reacting bromobenzene with magnesium and diethyl ether) and react it with benzophenone (a ketone) to produce triphenylmethanol, a tertiary alcohol. The mechanism for both the formation of the Grignard reagent and Grignard reaction is shown below in Figure 3.

Figure 3: Formation of Grignard Reagent and Grignard Reaction

As pictured above, bromobenzene reacts with magnesium in an ethanol solution to form the Grignard reagent phenyl magnesium bromide. Then the carbon bonding the phenyl group and magnesium bromide together attacks the carbon of benzophenone (ketone), causing the carbonmagnesium bromide bond to break and the phenyl group to be attached to benzophenone. As this

happens, a pair of electrons from the double bonded oxygen in benzophenone moves up to become a lone pair, forming an alkoxide. Then, the electronegative oxygen on the alkoxide, attacks the hydrogen from hydrochloric acid, breaking the H-Cl bond and forming triphenylmethanol. Magnesium bromide is left as a side product. There are many different side reactions that could take place once the Grignard reagent is formed. Phenyl magnesium bromide could react with water to form a benzene ring, bromobenzene to form diphenyl methane, half a molecule of oxygen to form magnesium bromide phenolate, which is then protonated to form phenol, or carbon dioxide to form benzoate which is protonated to form benzoic acid

Figure 4: Side Reactions

Procedure The first step in the procedure was to prepare the Grignard reagent, phenyl magnesium bromide. To do this, 1.5 mL of bromobenzene and 5 mL of anhydrous diethyl ether were added to a 10-mL conical vial and swirled to dissolve. Once the bromobenzene was dissolved in the ether, 0.5 g of magnesium metal tunings were added to the vial and the solution was crushed with a glass stirring rod for 20 minutes or until all or of it was crushed to make sure it was thoroughly incorporated into the reaction solution. The crushing initiated the reaction between the

bromobenzene and metal, and a cloudy solution was observed within a few minutes of beginning the process. As the reaction between the halide and metal continued, the solution turned a brownish gray color and began to bubble. After the crushing was complete, the solution was brownish gray and cloudy as expected, and there were some small magnesium fragments still floating in it. Despite the presence of leftover magnesium, the solution was decanted quickly into a dry 3-4 mL test tube to avoid exposing the Grignard reagent to air and prevent it from oxidizing. If the Grignard reagent becomes oxidized, it will act as a base instead of a nucleophile and ruin the reaction. If for some reason crushing did not start the reaction, a few solid iodine crystals were to be added to the conical vial and the observations for the reaction were recorded. If nothing occurred following the addition of iodine, a prepared Grignard reagent was obtained from the stockroom. Following synthesis of the Grignard reagent phenyl magnesium bromide, the Grignard reaction to obtain triphenylmethanol was performed. The test tube containing the Grignard reagent was capped with a Claisen head to minimize the exposure of the reaction air and prevent oxidation. A Claisen head is pictured in Figure 5.

Figure 5: Claisen Head8

The Claisen head for this experiment was filled with a drying tube containing calcium chloride, which acted as a trying agent and soaked up any excess moisture that may have entered the reaction vessel, as well as a rubber septum. Then, 0.54 g of benzophenone and 1 mL of anhydrous diethyl ether were swirled together in a 3-mL conical vial and capped with a rubber septum. The benzophenone solution was added to the Grignard reagent using a rubber syringe and once all the solution was added, the entire reaction mixture was cooled to room temperature. The cooling process caused the reaction mixture to turn a pink-red color and a precipitate began to form in the vial. At this point, the syringe was removed and the reaction mixture was swirled for five minutes to ensure that the contents had enough time to react. Then the benzophenone vial was rinsed with 2 mL diethyl ether, and this solution was added to the red mixture. The vial was capped and swirled occasionally for a 20-minute period, which allowed the triphenylmethanol product to fully form. The reaction mixture was then neutralized using 2 mL of 6 M hydrochloric acid. The addition of the acid caused the reaction mixture to separate into two layers, the organic layer, which contained the product triphenylmethanol, and the aqueous layer which contained inorganic products. 1 ml of diethyl ether and more hydrochloric acid (if needed) was added to the vial to help redissolve the solid. Once the reagents were added, the vial was capped and shaken with venting until the solid was dissolved. Then the aqueous layer was drawn off using a pipet and extracted with 1 mL of diethyl ether. The inorganic and organic layers were combined in a 50 mL Erlenmeyer flask, the organic layer was dried using anhydrous sodium sulfate and decanted into another 50 mL Erlenmeyer flask. Once the organic layer was in the flask, the ether solvent was evaporated using an aspirator.

After evaporation, the organic layer became a mushy brown colored solid containing an oil. This mixture contained triphenylmethanol and an impurity. 2 mL of petroleum ether was added to the flask and it was heated on a steam bath while swirling, and then cooled to room temperature. Once the mixture was cool, the solid triphenylmethanol was collected by vacuum filtration and rinsed with a small amount of petroleum ether. The crude solid was dried by being pressed between sheets of filter paper, and then it was weighed and assessed for melting point and percent yield. The crude solid was then purified by recrystallization hot isopropanol. The crude triphenylmethanol was dissolved in the minimum amount of hot isopropanol. If impurities remained, the solution was heated until clear, cooled to room temperature, and moved to an ice bath. Following this, the crystals were collected by filtration, dried, weighed, and assessed for melting point and percent yield. The melting point of the obtained crystals was compared with the literature value. Table of Chemicals

Chemical Bromobenzene

Molar Mass 157.02

Boiling Point °C 156.2

Melting Point °C -30.6

Diethyl ether

74.12

34.6

-116.3

Magnesium

24.31

110

651

Phenyl magnesium bromide Benzophenone

151.31

78.8

NA

182.22

308.4

49

110.99

1670

772

Calcium chloride

Toxicity/Hazards Do not inhale, ingest, or put in contact with skin or eyes. Eye irritant. Toxic to skin, kidneys, liver. Wear PPE. Do not inhale, ingest, or put in contact with skin or eyes. May be toxic to skin, nervous system. Wear PPE. Do not inhale, ingest, or put in contact with skin or eyes. Wear PPE. Flammable. Do not inhale, ingest, or put in contact with skin or eyes. Causes severe burns and eye damage. Do not inhale, ingest or put in contact with skin or eyes. Wear PPE. Do not inhale, ingest, or put in contact with skin or eyes. May be toxic to heart, cardiovascular system. Wear PPE.

Table 1: Physical and Chemical Properties

Results Chemical Crude Triphenylmethanol Pure Triphenylmethanol

Appearance/ Color White, solid crystals White, solid crystals

Mass 0.3 g

Experimental Melting Point °C 160-163

Literature Boling Point °C 164

Percent Yield -

0.05 g

164-166

-

6.49%

Table 2: Grignard Reaction Results

Calculations Mass of phenyl magnesium bromide 1.5 mL phenyl magnesium bromide x magnesium bromide Determining Limiting Reagent

1.14 g phenyl magnesiumbromide = 1.71 g phenyl 1 mL

1mol benzophenone 1 mol triphenylmethanol x x 182.22 g benzophenone 1 mol benzophenone 260.33 g triphenylmethanol = 0.771 g triphenylmethanol from benzophenone 1 mol triphenylmethanol 1mol phenyl magnesium bromide 1.71 g phenyl magnesium bromide x x 151.31 g phenyl magnesium bromide 1 mol triphenylmethanol 260.33 g triphenylmethanol x =2.942 g 1 mol phenyl magensiumbromide 1 mol triphenylmethanol triphenylmethanol 0.54 g benzophenone x

Benzophenone is the limiting reagent because it produced less grams of triphenylmethanol Theoretical Yield - Shown above in calculating grams of benzophenone Percent Yield actual yield trip h enylmet h anol t h eoretical yield trip h enylmet h anol

x 100=

0.05 g trip h enylmet h anol x 100= 6.49% 0.771 g trip h enylmet h anol

Discussion There was one particular error that occurred during the experiment which most likely impacted the results. When the flask containing benzophenone with was rinsed with diethyl ether during the synthesis of triphenylmethanol, 2 mL of diethyl ether were added instead of the required 0.2 mL. Thus, when the benzophenone/ether mixture was added to conical vial containing the red mixture, there was not enough room left to add the required 2 mL of hydrochloric acid. However, the group thought there would be just enough room left and decided to add it in, but the contents of the vial spilled out over the edge, resulting in a loss of some of the HCl and possibly a small amount of the reaction mixture. This resulted in the incomplete neutralization of the triphenylmethanol reaction mixture, which affected the mass of pure product collected and the percent yield obtained. According to Table 2, the melting point ranges of the crude and pure triphenylmethanol were 160-163°C and 164-166°C, respectively, while the literature melting point value of

triphenylmethanol was 164°C1. In comparing both the crude and pure melting points, it is evident that both were within range of the literature value, but were either a degree or two less (crude) or a degree or two more (pure). This indicates that both samples contained a slight amount of impurity because if the samples were pure the range, they would only be about 1-2°C wide instead of three, and not higher than the literature value5. Of course, for the crude sample this broadened range makes sense because it was not yet purified, but the pure sample must have contained some impurity from the reaction mixture that was unable to be eliminated via the recrystallization process. The impurity most likely resulted from the spilling over of the HCl, which as discussed previously, led to the incomplete neutralization of triphenylmethanol. Reasoning as to why the pure triphenylmethanol range ended at a higher temperature than the literature value could be due to the improper neutralization of the compound that occurred during the experiment, or that the sample was heated too quickly in an effort to save time and finish the lab within the class period5. The percent yield obtained for triphenylmethanol, as shown in Table 2, was 6.49%. This is a very low yield, given that most successful reactions would have a yield of 80% or higher. The interpretation of this yield would be that only 6% of the reactants were converted to products, while the other 94% remained behind as impurities. This low yield was observed despite constant efforts to perform the Grignard reaction in a closed and dry environment, the reaction mixture was most likely exposed to air at some point, causing the triphenylmethanol to become oxidized. This resulted in a loss of some of the product, which explains why there was such a low yield following recrystallization. Other possible explanations could be that the product was not dried completely following filtration, leaving behind impurities that affected the yield of the pure produce when it was recrystallized, or that the measurements of chemicals for

the reaction were not as exact as they were recorded to be.6 Another factor that should be considered is that the crushing of the magnesium tunings was ended before the recommended time, which result in a lower amount of Grignard regent, thereby affecting the percent yield. The solvent in a Grignard reaction should be aprotic because if a Grignard reagent reacts with a protic solvent such as water or an alcohol, it will act as a base instead of a nucleophile, and an acid-base reaction will occur instead of a Grignard reaction, or no reaction will occur at all3. The reaction was not efficient as evidenced by the low percent yield obtained, percent yield reveals how much of the reactants were converted to products during a chemical reaction. The percent yield of pure triphenylmethanol was only 6.49% as shown in Table 2. As discussed previously this means that only 6% of the reactants were converted into product, which is a poor efficiency. The efficiency of the Grignard reaction was most likely compromised as a result of the reaction mixture being exposed to air, which caused other unnecessary byproducts to be formed and lowered the amount of triphenylmethanol produced. The other factors discussed before such as improper measurement, and incomplete neutralization, could have also played a role in reaction efficiency. Conclusion The objective for this experiment was to synthesize triphenylmethanol through a Grignard reaction with phenyl magnesium bromide and benzophenone. The experiment accomplished what it set out to do because triphenylmethanol was obtained, although the reaction was not as efficient as it should have been given the low percent yield of 6.49%.The theory of Grignard reactions were applied to this experiment because the Grignard reagent (phenyl magnesium bromide) was formed using magnesium and an ether solvent to prevent it from becoming a base,

and the Grignard regent reacted with a ketone to produce the tertiary alcohol, triphenylmethanol. The experimental data revealed that keeping the Grignard reaction in a closed environment, measuring out materials properly, and making sure that enough Grignard reagent is formed are important to achieve a high yield of product. If these steps are not performed correctly, the yield as well as purity of the product will be affected. The melting point data revealed that any small mistake throughout the procedure will affect the purity of the product. The techniques of Grignard Chemistry can be applied to other situations in that Grignard regents can be used to analyze oxygen balance and stability in mineral oils7.

References [1] Weldegirma, S. Experimental Organic Chemistry Laboratory Manual, 7th ed.; Procopy Inc: Tampa, Florida [2] Grignard Reaction: Synthesis of Triphenylmethanol, 2009. [3] Jasperse. Grignard Synthesis of Triphenylmethanol. [4] Kilway, K. V.; Robert Clevenger. Grignard Reaction, 2007. [5] Widner. MELTING POINT TIPS AND GUIDELINES http://www.csi.edu/ip/physci/faculty/rex/MPTips.htm (accessed Nov 22, 2017). [6] Grobsky. Percent Yield and Limiting Reactants [7] Larsen, R. G. Industrial & Engineering Chemistry Analytical Edition 1938, 10 (4), 195–198. [8] Claisen Head; Wikimedia Commons, 2012...