2-methyl-2-propyl cation

Display and examine electrostatic potential maps for ethyl cation, 2-propyl cation and 2-methyl-2-propyl cation. Which cation shows the greatest localization of positive charge If you find that the methyl groups delocalize the positive charge, where does the charge go Write resonance contributors for the three cations to rationalize your conclusion. (Note You may need to draw resonance contributors that contain a CC double bond and are missing a CH bond see also Chapter 7, Problem 8.)... 

Calculate energies for reaction of 2-methyl-2-propyl chloride (to 2-methyl-2-propyl cation), 2-phenyl-2-propyl chloride (to 2-phenyl-2-propyl cation) mdphenyl chloride (to phenyl cation). (The energy of chloride is given at right.) Assuming that reaction (1) is normal , what is the effect of a benzene ring Does it facilitate or hinder loss of chloride ... 

One possible explanation is that adamantyl cation, an intermediate in the reaction, is particularly unstable because it cannot accomodate a planar carbocation center (see Chapter 1, Problem 9). Examine the geometry of adamantyl cation. Does it incorporate a planar carbocation center Compare electrostatic potential maps of adamantyl cation and 2-methyl-2-propyl cation. Which cation better delocalizes the positive charge Assuming that the more delocalized cation is also the more stable cation, would you expect adamantyl tosylate to react slower or faster than tcrf-butyl tosylate Calculate the energy of the reaction. 

Another possible explanation is that 2-methyl-2-propyl cation allows better access to solvent than adamantyl cation. Examine hydrates of 2-methyl-2-propyl and adamantyl cations. How many water molecules does each accomodate Calculate hydration energies for the two cations. (The energy of water is provided at left.)... 

Resonance theory suggests that the positive charge in 2-methyl-2-propyl cation is dispersed onto the hydrogens. 

Electrostatic potential map for 2-methyl-2-propyl cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

Experimental reactivity patterns are based on solution behavior which are influenced by interactions between solvent and reacting molecules (especially ions). Compare electrostatic potential maps of 2-methyl-2-propyl cation and dimethylhydroxy cation. Identify sites that might form strong hydrogen bonds with water. Which ion will be better stabilized by its interaction with water ... 

As expected for C2 symmetry, the 2-propyl cation 6 is chiral and racemizes by methyl group rotation, passing through a Cs transition state with a calculated barrier of ca. 0.7 kcal/mol. This process is rapid on the NMR time scale, even at temperatures well below 77 K. 

The modern view of HX addition is that H+ is transferred from HX to the alkene to give a carbocation. The major product is the one derived from the more stable carbocation. Compare the energies of 1-propyl and 2-propyl cations (protonated propene), 2-methyl-1-propyl and 2-methyl-2-propyl cations (protonated 2-methylpropene), and 2-methyl-2-butyl and 3-methyl-2-butyl cations (protonated 2-methyl-2-butene). Identify the more stable cation in each pair. Is the product derived from this cation the same product predicted by Markovnikov s rule Is the more stable carbocation also the one for which the positive charge is more delocalized Compare atomic charges and electrostatic potential maps for one or more pairs of carbocations. 

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