6309 Grignard Reaction: Preparation of 2/19/2014 Benzoic AcidAMDG

Fig- ure 1. Reaction Mechanism showing the preparation Benzoic Acid with a Grignard Reagent.

Abstract

Synthesis of Benzoic Acid using a Grignard reagent and dry ice with an acidic

workup was the focus of this experiment. Anhydrous reaction conditions were used to

favor the product and to eliminate the possibility of quenching the Grignard reagent. The

product was purified and isolated by extraction and vacuum filtration. Melting point and

13C NMR analysis was done to confirm the identity and purity of the product and it was

concluded that Benzoic acid was synthesized.

Results and Discussion

The reaction mechanism shown in Figure 1 depicts the the magnesium inserting

itself into the bromobenzene to form the Grignard reagent. In lab the bromobenzene

and ether solution was added in small aliquots to the magnesium turning. This pre-

vented the reaction mixture from getting too hot. Due to the exothermic nature of the re-

action, the solution was added slowly to maintain control of the heat of the reaction. An

ice bath was also kept just in case the reaction go too hot.

Table 1. Melting Point and Percent Yield of the product

After the sample was allowed to dry for 24 hours, the melting point was taken

and 13C NMR was taken. Given that the actual melting point of benzoic acid is 120˚C

the melting point observed is conclusive with that of benzoic acid and the synthesized

product is very pure. The percent yield was calculated from the moles of magnesium

used in the experiment. Not all of the turnings reacted and it was calculated to be the

limiting reagent, however not factoring in the amount of magnesium that did not react

could throw off the calculation of the percent yield.

The 13C NMR displayed 5 peaks which is would correlate with the structure of

Benzoic Acid. The most downfield peak at 171.11 ppm is indicative of the carbon bear-

ing the carboxyl group. The peak at 133.56 ppm is indicative of the carbon on the aro-

matic ring that is furthest from the carboxyl group. The peaks at 129.94ppm and

128.22ppm are indicative of the 2 sets of equivalent carbons on either side of the ring

and the peak at 128.98ppm is indicative of the carbon adjacent to the carboxyl group

carbon. All the peaks together give the predicted peaks for benzoic acid and give con-

clusive data to the identity of the product. If 1H NMR were done to further confirm the

identity and structure of the product, there would be 4 peaks. One broad singlet (1H)

downfield around 10ppm for the H on the carboxyl group. There would be complex split-

ting between 7.5 and 8.5ppm to represent the peaks associated with the hydrogens on

the aromatic ring to display a total of four signals.

Conclusion

The Grignard reagent was successfully prepared slowly by adding bromoben-

zene to magnesium turnings in an anhydrous ether to maintain control over the heat of

the reaction. The preparation of the Grignard to be done in an anhydrous solvent to pre-

vent protonation of the reagent to yield benzene, so a solvent like ethanol would not be

ideal for this experiment.

Figure 2. Reaction Mechanism depicting the formation of the biphenyl impurity formed as a result of double addition of the Grignard reagent.

Grignard reagents are very reactive and will add to electrophilic sites twice if pos-

sible. The mechanism in Figure 2 shows the formation of the possible biphenyl impurity

that is removed in extraction. Upon obtaining the crude product an acid-base reaction

was done with the use of sodium bicarbonate to cause the product to be soluble in wa-

ter and thus a part of the aqueous layer when dichloromethane was added. Sodium bi-

carbonate is not a strong enough base to deprotonate the alcohol on the biphenyl impu-

rity so it does not become soluble in the aqueous layer. This technique allowed for a

successful extraction of the biphenyl impurities and undesirable organic product that

may have been in the reaction mixture. In order to improve the accuracy of the percent

yield calculation it would have been ideal to weigh the amount of the magnesium turn-

ings that did not react to subtract them from the mass of the magnesium used to calcu-

late the yield. After completing the experiment and allowing the compound to dry, the

melting point and 13C NMR data gave conclusive data to the identity of the compound

as benzoic acid.