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We have developed a general Grignard procedure for the synthesis of a variety of aliphatic tertiary alcohols, which presents students with an added challenge of product identification. Many organic syntheses in the undergraduate teaching laboratory can be infused with an extra element of discovery simply by not revealing the specific starting materials. Substitutions of isomers or homologues for the prescribed organic reagents create multiple possibilities for unknowns. We have found inclusion of this type of puzzle helpful in combating the tendency of students to approach lab work in a mechanical or cookbook fashion (1, 2).
Five articles on Grignard reactions have appeared in this Journal in the last 10 years (2-6). All organic laboratory manuals include at least one Grignard procedure. Phenylmagnesium bromide is the most commonly described Grignard reagent, used in the synthesis of triphenylmethanol (trityl alcohol) and benzoic acid (7-9). Our variation of the standard Grignard reaction incorporates anhydrous techniques, distillation, and infrared analysis to identify an unknown product.
Experimental Procedure1
CAUTION: The organic reagents and products in this procedure are flammable. Diethyl ether is highly flammable. Flame drying of glassware should be completed by all students before diethyl ether is dispensed!
Each student receives one of four alkyl halides (bromoethane, 1-bromopropane, 1-bromobutane, or 1-bromopentane)2 and one of four ketones (acetone, butanone, 2pentanone, or 3-pentanone) in premeasured amounts as unknown starting materials. Students are also provided with a list of 14 possible products,3 shown in Table 1.
All reagents and glassware used in this reaction must be dry! Magnesium turnings (0.88 g, 36 mmol) are weighed in a 100-mL round-bottom flask. A magnetic stirring bar is added and the flask is fitted with a Claisen adapter, separatory funnel, and condenser, then carefully flame dried with a Bunsen burner.4
An...