Preparation of a Grignard Reagant, and Subsequent Conversion to a Tertiary Alcohol via Grignard Reaction: Phenylmagnesium bromide to Triphenylmethanol

Brett Wilkey

Lab Partner: Rachel Polombo

TA: Meng Wang

Laboratory Section: Tuesdays, 11:30pm

Abstract:

This experiment’s objective was to first prepare the Grignard reagent Phenylmagnesium bromide via a reaction of bromobenzene and metal magnesium with an ether solvent; and then, through the mechanism of a Grignard reaction, use benzophenone on the Phenylmagnesium bromide to produce triphenylmethanol, a tertiary alcohol. (Again, with an ether solvent.) By adding hydrochloric acid to the product mixture, extraction of the organic layer was achieved through the use of a seperatory funnel (hydrolysis). Recrystallization was then performed with methanol. Upon drying, the mixture was then weighed for a percent yield calculation, and subsequently tested for melting point and IR spectra data. The mass of the final product proved to be .102 grams, generating a percent yield of 37.78%. The melting point determined was 161°C, with a range of 156-166°C shown on the melting point instrument used. This value, while slightly low, coincides fairly complimentarily with the literary value for triphenylmethanol, 162°C. The IR spectra obtained, while containing ‘background noise’ (which will be discussed later), showed aromatic carbon to carbon bonds at 1494.83cm-1, alcohol groups at 3422.3cm-1, carbon to hydrogen aromatic stretches at 3100cm-1, and other common peaks for aromatic (benzyl) rings (also to be discussed later). The IR spectrum and melting point data indicate that triphenylmethanol was present, although in a somewhat contaminated final product.
Introduction:
The main purpose of this experiment will be to form triphenylmethanol through the use of a prepared Grignard reagent. The prepared Grignard reagent will react with benzophenone to form the final, tertiary alcohol product. Again, the reaction to be demonstrated and put to use by this experiment is a typical Grignard reaction. A vital tool for the formation of carbon-to-carbon bonds, Grignard reagents tend to be vinyl, aryl, or alkyl-magnesium halides. As well as forming carbon-carbon bonds, Grignard reagents also display the ability to convert ketones to alcohols. It should also be noted that Grignard’s themselves are formed by a reaction between an organic halide and magnesium.

The mechanism behind Grignard reactions can be summarized as follows: electrophilic carbons (such as those on a carbonyl group) are attacked by the nucleophilic Grignard reagent. As previously stated, this leads to the formation of an alcohol from a ketoneThe reaction and mechanism of the formation of Gringards themselves should also be noted, seeing as to how the first step of this experiment will require this synthesis. This is accomplished by the reaction between an alkyl or aryl halide with metal magnesium, typically in an ether solvent, as this experiment will demonstrate.

The mechanism itself is best described/labeled as a single-electron transfer.

Several synthetic techniques are to be used throughout the completion of this lab. The first is the following apparatus, used for the preparation of the Grignard reagent. The main use of this apparatus will be to react the bromobenzene with metal magnesium (until all of the metal has visibly dissolved), whilst in an ethereal solvent. It should be noted that the bromobenzene is first dissolved in anhydrous ether, and then this solution is added to the metal magnesium in the apparatus via the syringe. The following synthetic techniques are to also be implemented in this lab: addition of benzophenone, hydrolysis, and separation and drying. The addition of benzophenone in an ethereal solution to the prepared Grignard reagent will lead to the formation of the sp3 carbon adduct. Hydrolysis will consist of the addition of 6M hydrochloric acid, which will convert the adduct into