Carbon-sodium bond, transition between

Rate Constants of the Transitions Between the Three Forms of the Carbon-Sodium-Bond... 

In this and in other experiments using NaBH4 with aldehydes and ketones, there are two steps (1) acyl addition of the hydride and (2) aqueous acid workup. This result is entirely consistent with the acyl addition reactions discussed in Chapter 18. There has never been evidence for an intermediate, so the alkoxide product is formed directly. With knowledge that no intermediate is formed when 9 is transformed to 11, this experiment must be explained by examining a transition state. The requisite transition state brings a B-H unit of sodium borohydride into close proximity to the polarized C=0 unit of 2-butanone due to electrostatic attraction between the 6- H and the 6-1- C of the carbonyl. The hydride of the borohydride is transferred to the 6-1- carbon of the carbonyl, breaking the ji-bond with transfer of those electrons to the boron. Structure 10 represents the requisite four-centered transition state. 

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