Synthesis of Grignard Reagents

Topics: Diethyl ether, Magnesium, Nucleophile Pages: 5 (1757 words) Published: March 29, 2012

The purpose of this lab is to prepare phenylmagnesium bromide, a Grignard reagent, and react it with benzophenone to give triphenylmethanol. Once made, the Grignard reagent will do a nucleophilic attack on the carbonyl carbon of the ketone, benzophenone. The result is an alkoxide that is then protonated to give the alcohol, triphenylmethanol. The purity of the final product will then be considered by melting point and IR spectroscopy. Final purified triphenylmethanol weighed 8.02 grams and melted at 158.5-162 degrees Celsius.

Grignard reagents are the most useful way to form primary, secondary or tertiary alcohols from a carbonyl group. The Grignard reaction involves the nucleophilic addition of an organomagenesium complex to a carbonyl carbon followed by acid work-up. This reaction involves a single electron transfer between the carbonyl group and the grignard reagent. Once the Grignard reagent is synthesized it can work as a base by abstracting a proton or it can work as a good nucleophile. When it works as a base it can be added to water, alcohols, amines, acids, terminal alkynes and etc. When it works as a nucleophile, its nucleophilicity reacts with the electrophilic carbon in a carbonyl group forming a new carbon-carbon bond. Grignard reagents when synthesized are very reactive and thus must be made in an environment free of water or any other potential proton donor. Thus in order to synthesize the grignard reagent it is run in ether which is unreactive with the grignard reagent. The grignard reagent works as a nucleophile in this experiment and when added to the ketone, benzophenone it reacts with the carbonyl and forms the triphenylmethanol.

Then using experimental techniques the triphenylmethanol is purified, and melting point and IR spectrum are recorded.
Melting points are very specific and usually melt in a range of temperatures. An impure compound will have a much lower and larger range of melting points versus a pure compound. Melting points can therefore determine a compounds purity. An IR spectrum is recorded by passing a beam of infrared light through a sample. When the frequency of the IR is the same as the vibrational frequency of a bond, absorption occurs.(3) Using the IR obtained, the peaks can help determine the molecular structure of a compound. An IR absorption table helps determine this structure based on the absorption strengths and bond types. Carbon NMR as well as hydrogen NMR were also given. Both NMR can be used to obtain chemical, physical and structural information about molecules due to their chemical shift. This is usually used with IR to again help determine the molecular structure. Based on the peaks shown in the NMR, a NMR chemical shift table can be used to help. Reagent Table:

Name| Structure| Amount Used:| Formula| Formula Weight(g/mol)| Density(g/cm3)| BP (degrees Celsius)| MP (degrees Celsius)| Safety| Bromobenzene| | 5.3 mL| C6H5Br| 157.02| 1.5| 156| -31| Toxic| Diethyl Ether| | 60 ml| (C2H5)O| 74.12 | .7134| 34.6| -116| FlammableHarmful| Sulfuric Acid| | 4.5 ml| H2SO4| 98.07| 1.84| 337| 10| High Concentration- Hazardous| Magnesium| | 1.5 g| Mg| 24.3| 1.74| 1090| 648| Explosive| Benzophenone| | 9.1 g| C13H10O| 182.21| n/a| 305| 48| Hazardous| Iodine| | Small crystal| I| 253.8| n/a| 184| 113| Irritant|

Placed 1.5 grams of Magnesium metal into a dry 250 mL round bottom flask with a magnetic stirring bar under a stir plate. Prepared a drying tube with loose plug of cotton and anhydrous calcium chloride. Added a small iodine crystal into the flask. Assembled reflux apparatus and added 40mL of anhydrous ethyl ether to the round bottom flask via the separatory funnel. Added 5.3 mL of bromobenzene and 15mL of anhydrous ethyl ether into the separatory funnel. Shaked the funnel gently and mixed often. Added half of the bromobenzene solution at one time with...
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