Trigonal stereogenic centers, tetrahedral

Like reduction, addition of organometallic reagents converts an sp hybridized carbonyl carbon to a tetrahedral sp hybridized carbon. Addition of R - M always occurs from both sides of the trigonal planar carbonyl group. When a nevr stereogenic center is formed from an achiral starting material, an equal mixture of enantiomers results, as shown in Sample Problem 20.1. 

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