Pictures of the Day 2-1-08

 
The important feature of organometallic reagents like organolithium and Grignard reagents is that the carbon atom attached to the Li or Mg atom has a partial negative charge. The partial negative charge can be seen as the red color on the carbon atom attached to the Li or Mg atoms on the above organolithium and Grignard reagents, respectively. The partial negative charge means that the carbon atom acts as a nucleophile with electrophiles such as protons in –OH groups or, more importantly, Grignard reagents react with electrophilic carbon atoms such as the epoxide as shown below. Note that relief of the ring strain within the three-membered epoxide ring adds a driving force (motive) to the latter reaction. 
Above is the entire reaction between and Grignard reagent and an epoxide. The nucleophilic carbon of the Grignard reagent attacks the electrophilic carbon of the epoxide. Note that the less-hindered epoxide carbon is attacked as expected for any SN2-like (C-C bond-making and C-O bond-breaking occur simultaneously) process. The magnesium salt of an alkoxide is obtained. The reaction is completed with the chemist opens the flask and adds mild aqueous acid to protonate and create the product alcohol. A key feature of this process is that a carbon-carbon bond is created!!!! Note how the stereochemistry at the lone chiral center is retained throughout the reaction, leading to production of a single stereoisomer (of the S configuration) product.