Neopentyl cation

Clearly, both spectra are of the tertiary 2-methylbutyl cation and the neopentyl cation never saw the light of day. The reaction is the same rearrangement that you saw in the substitution reaction of neopentyl iodide, but here the rate of rearrangement can be measured and it is extremely fast. Ncopentyl tosylate reacts to form a cation under these conditions about 104 times as fast as ethyl tosylate, even though both tosylates are primary. This massive rate difference shows that if migration of an alkyl group can allow rearrangement to a more stable carbocation, it will happen, and happen rapidly. 

Rearrangement of the neopentyl cation labeled with deuterium in the 1 position (11) gave only tert-pentyl products with the label in the 3 position (derived from 13), though if 12 were an intermediate, the cyclopropane ring could just as well cleave the other way to give lert-pentyl derivatives labeled in the 4 position (derived from 14). ° Another experiment that led to the same conclusion was the generation, in several ways, of Me3C CH2. In this case, the only tert-pentyl products isolated were labeled in C-3, that... 

Problem 16.2 Because of the great tendency of the neopentyl cation to rearrange, neopentyl chloride cannot be prepared from the alcohol. How might neopentyl chloride be prepared ... 

Clearly, both spectra are of the tertiary 2-methylbutyl cation and the neopentyl cation never saw the light of day. The reaction is the same rearrangement that you saw in the substitution reaction of neopentyl iodide, but here the rate of rearrangement can be measured and it is extremely fast. 

The effect of substituents on the equilibrium between a carbonium ion and an isomeric n complex can easily be predicted. In the carbonium ion, one atom carries a full unit of formal charge while the others are neutral. In the n complex, the charge is shared between the apical carbon atom in the acceptor and the two basal carbon atoms in the olefin moiety. The apical carbon atom and one basal carbon atom are therefore more positive in the 71 complex than in the carbonium ion, while the other basal atom is much more positive in the carbonium ion. If then we replace one of both hydrogen atoms at a basal position in (49) by — / groups, e.g., alkyl, we will stabilize one of the classical ions very strongly relative to the n complex. There is little doubt that in the rearrangement of neopentyl cation [equation (5.195)], the intermediate n complex immediately rearranges to the more stable r-amyl... 

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