Grignard Synthesis Lab Report

Grignard Synthesis of Tirphenylmethanol

David Szuminsky
Organic Chemistry Lab II
Shaopeng Zhang
Monday 1PM
2/10/14 & 2/24/14

- Abstract A sample of triphenylmethanol was prepared using Grignard synthesis techniques. Reflux was used in order to speed up the reaction and the final product was purified using recrystallization methods. The percent recovery and percent yield were 80.46% and 47.526%, respectively. A melting point range of 85-87oC was obtained from the purified product.
- Introduction A Grignard reaction adds an alkyl-magnesium halide to an aldehyde or ketone carbonyl carbon. The alkyl-magnesium halide is known as the Grignard reagent. The carbon bonded to the magnesium
halide acts as a nucleophile, donating its electrons to the carbonyl carbon on either the aldehyde or ketone which is the electrophile. Aldehydes and ketones are chemically converted into alcohols. The octet rule on the carbonyl carbon is not violated because the carbon on the alkyl-magnesium halide bonded to the magnesium forms a bond with the carbonyl carbon and the magnesium halide forms an ion with a positive charge that counter balances the negative charge on the carbonyl oxygen. The Grignard reactions with an aldehyde and ketone are shown below in Equations 1 and 2.

Equation 1. Grignard Reaction with an Aldehyde

Equation 2. Grignard Reaction with a Ketone
The Grignard reaction must be performed in an aprotic solvent. A protic solvent will quench the reaction by protonolysis and oxidation. Aprotic solvents provide several advantages compared to a protic solvent. Aprotic solvents enable the reaction to have a blanketing effect, coordinating effect and is anhydrous. The blanketing effect is provided by the solvent vapors that limit side reactions with air. The coordinating effect helps to stabilize the Grignard reagents preventing them from clustering together and becoming unreactive. Being anhydrous prevents the solvent from quenching the Grignard reagents.
Before the Grignard reaction can occur the Grignard reagent needs to be synthesized. An example of a scheme to synthesize the Grignard reagent is demonstrated in Equation 3 below. Equation 3. Preparation of Grignard Reagents
Infrared Spectroscopy (IR) is used to confirm the presence of specific functional groups in compounds. H1NMR and C13NMR is then used to help determine the exact location of the functional groups and hydrogens in the compound. NMR also gives valuable information regarding the carbon skeleton of the molecule.

-Reagent Table
Name
Structure
M.W. (g/mol)
Amount Used
Concentration
Density (g/ml)
m.p. (oC)
b.p. (oC)

Safety Note
Magnesium3
Mg2+
24.31
1.53g

651
1100
Slight Irritant
Ethyl Ether3

74.12
255 mL

.71
-116
34.6
Flammable, Irritant
Bromobenzene3

157.02
5.3 mL

-30.6
156.2
Irritant, Flammable
Benzophenone3

182.22
9.1g

49
305.4
Irritant
Sulfuric Acid3
H2SO4
98.08
4.5 mL

-35
270
Corrosive, Irritant

- Experimental About 1.5g of Mg metal turnings were weighted out into a dry 250mL round bottom flask with a stirring bar and stirred for 5 minutes. A small iodine crystal was then added to the flask. The reflux apparatus was assembled according to Figure 1 below.

Figure 1. Reflux Apparatus
40 mL of anhydrous ethyl ether was added to the flask through the separatory funnel. Then 5.3 mL of bromobenzene and 15 mL anhydrous ethyl ether was added to the flask through the separatory funnel. The first half was added quickly then the second half was added at a dropwise rate while being stirred with the magnetic stirring bar. The mixture spontaneously refluxed for 10 minutes. 9.1 g of benzophenone was dissolved in 100mL of anhydrous ethyl ether then was transferred into the separatory funnel and then was added to the round bottom flask at a rapid dropwise rate while it was being stirred. A steam bath was then used to reflux the mixture for 10 minutes. The mixture was then cooled to room temperature then was cooled on ice while stirring. A 50mL beaker was filled with ice and then 4.5 mL of concentrated sulfuric acid was added to the ice. This mixture was then transferred into a 250 mL beaker and water was added to reach a volume of about 175 mL. The drying tube was removed and the sulfuric acid was added dropwise at first then was poured slowly. The mixture was extracted twice with 50 mL of ethyl ether and the organic/ether layers were combined and died over MgSO4. The solution was filtered and the crude product was evaporated on a steam bath. It was cooled to room temperature then cooled on ice. A recrystallization was performed and an IR spectrum was collected. The melting point range was determined and a proton and carbon NMR was obtained.
- Results Several color change observations were made throughout this lab.
The reaction mixture initially was a brown color which then turned a milky white color with the addition of bromobenzene and ether. The mixture turned back to a brown color with the addition of the remaining bromobenzene and ether added at a dropwise rate. The reaction vessel became warm at this point. The mixture then turned a reddish, pink color with the addition of 9.1g benzophenone in 100mL anhydrous ethyl ether. During the reflux the reaction mixture first turned a “pepto bismal” pink color then became a thick white liquid/solid. When the sulfuric acid was added the solution turned a yellow
color. The initial mass of the magnesium used was 1.53 grams. The initial mass of benzophenone was 9.11g. The melting point range collected for the purified product was 85-87oC. The significant peak from the IR spectrum are indicated in Table 1 below. The IR, proton and carbon NMR’s are attached. The percent recovery was 80.46%. The percent yield for this reaction was 47.526%.

Table 1. Significant Peak Positions from IR Spectrum
- Discussion The purpose of this lab was to synthesis triphenylmethanol using a Grignard reaction mechanism. The mechanism for this lab is highlighted below in Equation 4.

Equation 4. Abbreviated Reaction Mechanism for Synthesis of Triphenylmethanol Carbonyl carbon chemistry and Grignard synthesis was utilized in this lab to produce the desired product of triphenylmethanol. In order to properly prepare Grignard reagents as the first part of the lab, I2 was added to strip the magnesium metal of its outer non-reactive coating in order to start the reaction. Once the synthesis process was completed the identification and percent yield and recovery measurements were able to be taken in order to determine if the product was the desired one and how pure the product was. Several side reactions may occur during a Grignard reaction. If the Grignard reagent acts as a base, an enolate can be formed through deprotonating the compound. The significant peaks from the IR spectrum, as indicated in Table 1, are at 3058.86 cm-1, 1595.69cm-1, and 1488.55cm‑1.
The first peak at 3058.86cm-1 indicates an alcohol functional group present in the compound. The other two peaks correspond to the peaks of aromatic carbon-carbon double bonds. The proton NMR acquired indicates two different hydrogen’s present in the compound which correctly corresponds to triphenylmethanol. The peak at about 7.4ppm is the peak for the aromatic hydrogens on the three phenyl rings. The peak at about 3.6ppm is the hydrogen part of the hydroxyl group. The carbon NMR acquired also corroborates the conclusion that the final compound is triphenylmethanol. The peak at about 82ppm corresponds with the carbon bonded to the alcohol functional group. The other four peaks correspond respectively to the carbons of the phenyl groups as indicated on the attached carbon NMR. The purified sample provided a melting point range of 85-87oC which is rather far off of the desired theoretical value of about 160oC. This large skew on the melting point range is most likely due to impurity being present in the final product producing a melting point depression. A Grignard synthesis must be initially prepared in the absence of water in order to ensure the highest yield possible because water will quench the reaction ultimately affecting the yield. This is one of several sources of error that could have been encountered in this experiment. During the
reflux that are done in this experiment, the solution could have been overly boiled which would have bumped the solution up into the drying tube lowering the yield. Recrystallization could also have been performed too quickly allowing some impurity to form into the products lattice.

Works Cited
1. Padias, Anne B. Making the Connections: A How-To Guide for Organic Chemistry Lab Techniques. Plymouth: Hayden McNeil, 2011. Print.
2. Huston, Ericka, Ryan Weiss, Nicole Kennedy, and George Bandik. Chem 0340 Organic Chemistry II Laboratory Manual. Spring and Summer 2014 ed. Plymouth, MI: Hayden McNeil, 2014. Print.
3. "ScienceLab: Chemicals & Laboratory Equipment." ScienceLab: Chemicals & Laboratory Equipment. N.p., n.d. Web. 16 Mar. 2014.