Nonclassical Carbocations Real or Are Our Chemical Clocks Too Slow

C-C -type intermediate) could form a bridge with the electron-deficient carbon in carbocations. This was not too difficult to accept since a lone pair of electrons on bromine can be shared with the electron-deficient carbon to form a three-membered ring. The same stereochemical results could also be explained by assuming that bromine rapidly shifts back and forth, to and from its neighbor (like a windshield wiper), with the three-membered, bridged ion characterizing the transition state for this 1,2-shift. 

In the late 1940s and throughout the fifties, a related problem would emerge that would roil the waters of organic chemistry for three decades. It is important to remember that carbocations generated as intermediates in chemical reactions are present only in miniscule amounts since they disappear as rapidly as they are created (the steady-state assumption described in chapter 2). The lure of chemical research includes the quest to deduce the structures of such transient intermediates thousands of times too small to be viewed in the best optical microscopes, moving at velocities of hundreds of miles per hour, and having lifetimes of perhaps a millionth of a second. 

The exo stereoisomer reacts hundreds of times faster than the endo. This is explained by the exo forming a stabilized nonclassical carbocation in the rate-determining step. A nonclassical ion (note the 5-coordinate carbon) is a potential-energy minimum (lower left) rather than a transition state between two rapidly interconverting classical ions (lower right). 

Nonclassical Structure or Rapidly Exchanging Classical Ions  

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Nonclassical Carbocations Real or Are Our Chemical Clocks Too Slow ... 

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