Organic Chemistry Lab II
March 19, 2012
The purpose of this experiment was to synthesize the Grignard reagent, phenyl magnesium bromide, and then use the manufactured Grignard reagent to synthesize the alcohol, triphenylmethanol, by reacting with benzophenone and protonation by H3O+. The triphenylmethanol was purified by recrystallization. The melting point, Infrared Spectroscopy, 13C NMR, and 1H NMR were used to characterize and confirm the recrystallized substance was triphenylmethanol. Introduction
A Grignard reagent is a type of organometallic, which consists of a bond between a metal and a carbon. There are three types of carbon-metal bonds: ionic, polar covalent, and covalent. The ionic bonded compounds (example: RNa) have a weak bond between the carbon and the metal, and are therefore not useful because they are so volatile, and they will react with nearly anything. The covalent bonded compounds (example: R2Pb) are toxic. The compounds that are polar covalent bonded are Grignard reagents and are useful in making carbon-carbon bonds and reducing carbonyls. Grignard reagents are any of the numerous organic derivatives of magnesium (Mg), commonly represented by the general formula RMgX (in which R is a hydrocarbon radical: CH3, C2H5, C6H5, etc.; and X is a halogen atom, usually chlorine, bromine, or iodine). They are called Grignard reagents after their discoverer, French chemist Victor Grignard, who was a corecipient of the 1912 Nobel Prize for Chemistry for this work (1). Grignard reagents commonly are prepared by reaction of an alkyl halide (RX) with magnesium in a nitrogen atmosphere because the reagent is very reactive toward oxygen and moisture, which would cause the reagent to react with the water instead of any carbon atoms (2). Grignard reagents react with water to produce alkanes. This is the reason that everything has to be very dry during the preparation.
Alkyl halides vary greatly in their rates of reaction with magnesium. For example, alkyl iodides generally react very rapidly, whereas most aryl chlorides react very slowly, if at all. Their chemical behavior resembles that of carbanion species that contain a negatively charged carbon (:CH3-). Grignard reagents are strong bases and strong nucleophiles. Thus, the Grignard reagent methylmagnesium bromide (CH3MgBr) behaves as if it were equivalent to the methide ion (:CH3-). Grignard reagents are made through single electron transfers with magnesium and an alkyl halide. Grignard reagents are manufactured through the process of a radical reaction as shown below.
Grignard reagents react with molecules to extend carbon-carbon chains through the attraction of a nucleophilic carbon to an electrophilic carbon (nucleophilic addition). The Grignard reagent can serve as a nucleophile because of the attraction between the slight negativeness of the carbon atom in the Grignard reagent and the positiveness of the carbon in the carbonyl compound. The Grignard reagent can oxidize a carbonyl functional group into a hydroxyl group. The metal is less electronegative than the carbon, so the carbon bears a partial negative charge. This partial negative charge attacks the carbonyl at the partially positive carbon, forms a new carbon-carbon bond, and pushes an electron pair out of the double bond into the lone pair position. The metal then attaches itself at the now negatively charged oxygen. This compound is then treated with an aqueous acid to protonate the oxygen and forms the hydroxyl group.
Except for hydrocarbons, ethers, and tertiary amines, almost all organic compounds react with Grignard reagents. Many of these reactions are used for synthetic purposes, notably those with carbonyl compounds (e.g., aldehydes, ketones, esters, and acyl chlorides), with epoxides, and with halogen compounds of certain metals (e.g., zinc, cadmium, lead, mercury) to form the alkyl derivatives of those metals. Grignard...