A Facile Synthesis of Triphenylmethanol and Benzoic Acid by Grignard Reaction

A Lab Report


In the laboratory, there are numerous ways of synthesizing organic target molecules using different starting materials and reaction conditions. Therefore, considering factors such as cost of reagents, percentage yield, complexity of the experimentation process, and time taken to create the desired compounds, it is possible to identify the most appropriate method to use. In this experiment, Grignard reagent, phenylmagnesium bromide, was prepared by reacting bromobenzene with magnesium metal in dry diethyl ether. The reagent was later reacted with benzophenone to form triphenylmethanol. In another segment, the prepared Grignard reagent was reacted with Carbon (IV) Oxide to form benzoic acid after a series of acid hydrolysis reactions.

The process takes place via the following reactions:

Synthesis of Triphenylmethanol

  1. Formation of Grignard reagent (phenylmagnesium bromide)

C6H5Br + Mg(s)          C6H5MgBr

  • Reaction between Grignard reagent and benzophenone to give triphenylmethanol
    C6H5MgBr + C13H10O+ HClC            19H16O + MgBrCl

Synthesis of Benzoic Acid
a)         Formation of Grignard reagent (phenylmagnesium bromide)

C6H5Br + Mg(s)              C6H5MgBr
b) Reaction of the phenylmagnesium bromide with dry ice and neutralization with hydrochloric acid

C6H5MgBr + CO2(s)+ HCl            C7H6O2 + MgBrCl

Overall Balanced Chemical Equation

2C6H5MgBr + C13H10O + 2HCl + CO2(S)                          C7H6O2 + C19H16O + 2MgBrCl

Figure 1: Chemical reaction showing the Grignard synthesis of benzoic acid and tryphenylmethanol (Drawn using ChemDraw Ultra Version 12.0)


The Grignard reagent was discovered by a French chemist, Victor Grignard, who realized that haloalkanes have the capability of reacting with magnesium metal in the presence of dry diethyl ether, which formed the foundation of organometallic reagents. The reactions leading to the formation of Grignard’s reagent must be carried out in dry conditions because the presence of water destroys the targeted product. Therefore, in formation of phenyl magnesium bromide in this experiment, dry ice was used to avoid meting into water. Grignard reactions involve transforming the reagent into various organic compounds (Silverman and Rakita 557). However, these reactions are well known to produce accurate results when alkanes and arenes are used. Depending on the reaction conditions utilized and the molecules that are reacted with phenylmagnesium bromide, the end products of these Grignard reactions vary.

The experiment is built on this major foundation. Triphenylmethanol and benzoic acid are some of the possible resultant compounds of Grignard reactions and the reaction conditions can be controlled to give either of them. Since the experiment was to be carried out in a laboratory setting, the selection of reactants and catalytic conditions were considered for effectiveness and accuracy. In most cases, organometallic compounds such as the Grignard’s reagent are versatile intermediaries in the synthesis reactions for alkanes, ketones, carboxylic acids, and alcohols (Williamson and Katherine 494). The entire experiment needs to be carried out in an inert environment to avoid the reaction between the Grignard compound and oxygen or water. In this experiment, anhydrous diethyl ether was used as a protecting group for the phenylmagnesium bromide formed in the first step of the experiment.

In the synthesis of benzoic acid, the Grignard reagent shows characteristics of strong covalent bonds and is highly polarized (Williamson and Katherine 504). Therefore, the reagent reacts like a partially negative carbonyl compound. The reaction can proceed through a nucleophilic attack on the carbonyl or as a base that abstracts a proton from a hydroxylic solvent. When the compounds react with Carbon (IV) Oxide gas, they produce carboxylic acids as a resultant product of neutralization with an acidic proton through the reaction mechanism shown below. The ability of phenylmagnesium bromide to act as either a base or a nucleophile is responsible for production of two different compounds that are targeted in this experiment using different reagents and substances and reaction conditions.

Figure 2: A reaction mechanism for the Grignard reagents with Carbon (IV) Oxide and acidic neutralization to give carboxylic acids such as benzoic acid (Williamson and Katherine 491).

In preparation of the triphenylmethanol compound, the reaction proceeded in two steps. The first step involved reacting the Grignard reagent with bromobenzene. The second step involves reacting the intermediate compound of the first step with hydrochloric acid to produce the desired phenylalcohol. The synthesis reaction obeys the principle of refluxing the phenylmagnesium bromide compound and then recrystallizing to obtain a pure substance. In its mechanism, the Grignard reagents acts as a nucleophile by attacking the electrophilic carbonyl carbon of the benzophenone; hence pushes the electrons up to the oxygen (Silverman and Rakita 558). Hydrochloric acid allows the negatively charged oxygen to attack the H+ to give an alcohol. To achieve the target molecule, refluxing must be done and then the collected product can be purified using isopropyl alcohol to exclude impurities such as bisphynyl, which is formed through the reaction between phenylmagnesium bromide and the unreacted bromobenzene. The pure product is thereafter tapped via a vacuum filter.

Results and Calculations

Table 1: Amount of reagents and substances used in the experiments (Deduced from the experiment procedure that is provided)

CompoundMolecular weight (g/mol)Quantity usedmmolEquivalents
Magnesium 24.310.05821
Anhydrous diethyl ether74.12250.7mL  
Bromobenzene 157.0090.33g2.10˜ 1
3M HCl36.46   
NaOH 1mL  

Benzoic Acid

Moles of magnesium solid used         =


                                                            = 0.0023858494 moles

The mole ratio for reaction that forms phenylmagnesium bromide is 1:1

Therefore, the number of moles of the Grignard reagent used is equal to moles of magnesium metal used                                                 = 0.0023858494 moles

In the overall reaction for formation of triphenylmethanol and benzoic acid, the mole ratio for phenylmagnesium bromide to benzoic acid is 2:1

Hence, the number of moles of benzoic acid produced is equal to 0.5 × 0.0023858494moles

                                                = 0.0011929247 moles of C7H6O2

 Mass of benzoic acid produced = moles × molar mass

                                                            = 0.0011929247 × 122.12134g/mol

                                                            0.1456815628 grams

Table 2: Quantitative results (From the Experiment)

CompoundTheoretical Yield(grams)Actual Yield (grams)Percentage yield (%)
Pure triphenylmethanol0.31054216grams
Pure Benzoic acid0.1456815628g0.031g0.45%

 Percentage yield = {} ×100%

                                    = 0.031g/0.1456815628 × 100%

                                    = 0.45161284%


Moles of phenylmagnisium bromide used is = 0.0023858494 moles

From the overall reaction, the mole ratio of phenylmagnesium bromide to the triphenylmethanol produced is 2:1. Therefore, the number of moles of triphenylmethanol produced equals to 0.0011929247 moles of C19H16O

Mass of triphenylmethanol produced = number of moles × molar mass

                                                            = 0.0011929247 moles × 260.32g/mol

                                                            = 0.31054216grams

Discussion and Conclusions

In the experiment, a small percentage yield of benzoic acid is registered. The great deviation from the theoretical value is due to reaction errors and loss of the target product through impurities. Inaccuracy in measurements, heat loss to the surrounding, and failure to adhere to the cautions of the experiments are possible reasons for the deviation. For instance, in the laboratory preparation of Grignard reagent, dry glassware must be used to avoid jeopardizing the functionality of the substance. If exposed, phenylmagnesium bromide reacts with oxygen and water to form other compounds, which are ineffective in synthesis via Grignard reaction. Furthermore, the experiment for formation of benzoic acid and triphenylmethanol require different reaction conditions. Therefore, the two reactions proceed via nucleophilic principles and as a base that abstracts a proton from an acidic solution. The reaction is not concerted.

During the purification process, benzoic acid can be obtained through vacuum filtering. Other substances that are used in this process need to be excluded completely e.g. through protonation of the bisphenol. The reaction procedure followed does not cater for the preparation of triphenylmethanol. However, the procedure provides a facile method for synthesizing benzoic acid. In the formation of triphenylmethanol, either benzophenone or sodium benzoate is used to react with the Grignard reagent formed in early stages of the experiment. In the experiment carried out in the laboratory, the salt formed, sodium benzoate can be better excluded by addition of a basic solution, which is excluded by an acidic solution to crystallize the product to benzoic acid. Further recrystallization using hot water should be used to further purify benzoic acid that is produced and extracted for use. Moreover, the experiment did not take care of the benzene bi-product, which is product by reacting the Grignard reagent with acidic hydrogen. The benzene lowers the effect of phenylmagnisium bromide in the synthesis of benzoic acid and triphynylmethanol.

In conclusion, the reaction did not go to completion; hence, a small percentage of benzoic acid is formed. To improve the production of benzoic acid and triphenylmethanol, reaction conditions must be adhered to as provided in Grignard synthesis reaction manuals and instructions. For instance, the reaction must be carried out in environments that are free from protic solvents such as atmospheric water, alcohol, and amines. Structural characterization through IR and TLC are considered the most appropriate test procedures to confirm the formation of pure benzoic acid and triphenylmethanol compounds after synthesis.


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