Question

What is the theoretical yield (in grams) and percent yield of your triphenylmethanol product in this experiment? (Weight of triphenylmethanol: 0.060g)

BACKGROUND AND THEORY The Grignard reaction was one of the first organometallic reactions discovered and is still one of the most useful synthetically. By reacting an organohalide (usually a bromide) with magnesium in ethereal solvent, carbon becomes a nucleophile. Grignard reagents are the starting points for the syntheses of many alkanes, primary, secondary, and tertiary alcohols, alkenes, and carboxylic acids. Grignard reagents are extremely sensitive to moisture, therefore it is imperative that all apparatus and glassware used for their preparation be as dry as possible. Phenyl magnesium bromide is one of the easier Grignard reagents to prepare. As bromobenzene is relatively inexpensive, phenyl mag- nesium bromide may be used economically in excess. Also, a competing coupling reaction to form biphenyl is not a major concern. Triphenylmethanol is synthesized by reacting phenyl magnesium bromide with an ester of benzoic acid. The particular ester (methyl or ethyl esters being the most common ones) does not affect the final product, as the alcohol group is lost during the reaction. Since esters consume two equivalents of the Grignard reagent, the stoichiometry of the reaction would be: MgBr 2 Mg 2 | 1) PhCO,CH3, Ph.COH + CH3OH Ether 2) HCI Br Figure 10.1. The stoichiometry of a Grignard reaction. The initial reaction, between the magnesium and the alkyl halide to form the Grignard reagent, takes place via a radical mechanism. The presence of free radicals leads to the generation of biphe- nyl as a byproduct (Figure 10.3). The Grignard reagent reacts with the remaining unreacted alkyl halide to give the dimer, biphenyl. Byproduct formation is increased by an increase in concentration of the starting alkyl halide solution. The second step in the Grignard reaction is much simpler mechanistically (Figure 10.2). The elec- tropositive magnesium adjacent to the carbon causes the carbon to behave as a nucleophile. The Grignard nucleophile attacks the ester carbonyl to form intermediate (I). Loss of the methoxide ion (II) generates intermediate ketone (benzophenone), (III) that is generally not isolable. Instead, a second Grignard nucleophile attacks the newly formed ketone carbonyl yielding the final alkoxide, IV). The free alcohol is generated after the alkoxide, (IV) is protonated in the acidic workup, to give the final product, triphenylmethanol, (V). Recrystallization of the triphenylmethanol is not necessary to remove the byproduct biphenyl that forms during the reaction. Instead, biphenyl is removed by washing the crude products with hex- anes, a nonpolar solvent. There is a saying in chemistry: “LIKE DISSOLVES LIKE." Therefore, biphenyl (a nonpolar compound) would be totally dissolved in hexanes while the triphenylmethanol (a polar compound) has extremely low solubility in hexanes and crystallizes out of solution.

Answer 1

s moles of Mg = am not мму оt Boмobeuze : 2. <um volume of methyl bevizoate 0.125 ml weight of methyl benzowe density x volume =loo.gg/ml x 0.125 m2 weight : 0:13 625 gm moles of methyl benzoute= 0.13625.. 136.15 1x10? moles: moles of methyl benzoate immol 1 Scanned with CamScanner

- Br MA TS. ☺ + Mg - 2.5 mmol 2.2mmol is here limiting reagent is "mg": it. moles of mgßr after reaction = 2.2 mmol. - Mg Br OH 2 ☺ + ph to me. Hot 1. Ph. 2: 2 мл | мм 1 - here limiting reagent is Methyl benzoate a i moles of ph-c-ph after reaction = Lamol a Ph SE C 10-? mor). OH who. of ... produit obtained = 0.060g (experimentally). .wt. of product obtained (theo sitially) : 103x Molo wit ..(103 x 260.33 )gu Scanned with . . CamScanner .: . 0.26 gui .

:% yield - Cuti obtained theositatut x 100 % 0.060 100 x 100 0: 260 Fyyelda 23.07% 1 |cs Scanned with CamScanner :