Grignard Reaction

1 January 2017

In this experiment we synthesize the tertiary alcohol triphenylmethanol from a Grignard reagent. The Grignard reagent was synthesized from bromobenzene and magnesium and then reacted with benzophenone to produce triphenylmethanol. The Grignard reagent is considered to be an organometallic reagent, a combination of a metal and organic molecule. The Grignard reagent is a successful reducing agent that can be used to reduce a ketone to an alcohol. Usually, the Grignard reagent is characterized by an alkyl or aryl magnesium halide.

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The nucleophilic Grignard reagent attacks an electrophilic carbon, which results in the arrangement of a carbon-carbon bond. The electrophilic carbons found within a carbonyl group are the most likely to be attacked because of their polarity. The Grignard reagent is created by the reactions between an alkyl or aryl halide and magnesium and proceeds by radical electron transfer. In this reaction we yielded 0. 423g of triphenylmethanol and expected a theoretical yield of 1. 562 g. The melting point range for the product was determined to be 156-160°C, and the literature melting point is 162°C.

The product contains some impurities; one of the impurities contributing to the difference of melting point from the literature melting point might have been biphenyl. This by-product is formed as a result of radical pairing of the Grignard reagent with unreacted bromobenzene. To decrease the formation of the biphenyl, the bromobenzene was added slowly to maintain a low concentration. Before starting this experiment we were explicably told to have all glassware was dried to insure no water.

However water could have still gotten into the reaction and this would react with the Grignard reagent, causing a reduction of the phenyl magnesium bromide to the hydrocarbon benzene. This would contaminate our product, consequently lowering our melting point. To avoid this contaminate, the glassware used was dried and the apparatus was closed with calcium chloride and a cotton ball.

During the experiment we also had to avoid some of the Grignard reagent reacting with carbon dioxide which may have gotten into the reaction apparatus and interfered with the purity of our product. Another possible reason for low product was several transfers of product to different flasks, and it’s likely that some of the product remained in the flask even after proper rinsing of the sides.

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