1.3- Dimethylallyl cation

An orbital overlap description of electron delocalization mil dimethylallyl cation H2C=CH—C(CH3)2 is given m Figure 10 2 Figure 10 2a shows the rr bond and the vacant p orbital as independent units Figure 10 2b shows how the units can overlap to give an extended rr orbital that encompasses all three carbons This permits the two rr electrons to be delocalized over three carbons and disperses the positive charge... 

The greater stability of 1,1-dimethylallyl cation tells us that a vinyl group (H2C=CH—) is a better electron-releasing substituent than methyl (H3C—). This greater stability can be attributed to allylic resonance. 

Product Studies The preceding resonance picture shows a sharing of positive charge between a tertiary and a primary carbon in 1,1-dimethylallyl cation. If this carbocation reacts with a nucleophile, to which carbon does the nucleophile form a bond The answer is both, as the hydrolysis of 3-chloro-3-methyl-l-butene reveals. 

Chloro-3-methyl-l-butene Chloride ion 1,1-Dimethylallyl cation... 

The positive charge in the 1,1-dimethylallyl cation is shared between two allylic carbons. 

The mechanism of this reaction is exactly the same as that shown for 3-chloro-3-methyl-1-butene in Mechanism 10.1 only the structure of the starting material has changed. The same 1,1-dimethylallyl cation forms in both reactions. 

The methyl groups attached to the terminal allyl carbons in the ( , )-1,3-dimethylallyl and ( )-l,2-dimethylallyl cations have conformations analogous to the methyl group in the... 

Comparison of the 1,3 7r-density matrix elements of different alkylated allyl cations demonstrates the competing influence of hyperconjugation with non-bonded resonance. The MMP2 as well as the MP2(full)/6-31G 1,3 a -density matrix elements, which represent the magnitude of 1,3 a -overlaps, reveal a parallel decrease in the allyl, the l-meihylallyl, and the 1,3-dimethylallyl cation series (see Table 4). 

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