Methyl carbocation, electrostatic

Methyl carbocation, electrostatic potential map of. 196 Methyl group, 83 chiral, 407... 

Figure 6.10 Electrostatic potential maps for (a) tert-butyl (3°), (b) isopropyl (2°), (c) ethyl (1°), and (d) methyl carbocations show the trend from greater to lesser delocalization (stabilization) of the positive charge. (The structures are mapped on the same scale of electrostatic potential to allow direct comparison.)...

The methyl carbocation, CH3+, provides an even more dramatic visualization of an electrostatic potential. The entire ion is blue in color, corresponding to the net positive charge. The central atom is the deepest blue, corresponding to the location of the largest fraction of the positive charge. 

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. 

Compare electrostatic potential maps for ethyl, 2-propyl, 2-methyl-2-propyl and 2-butyl cations. Does the extent to which positive charge is localized at the carbocation center parallel proton affinity Explain. 

Is the stable cation that formed as a result of protonation of the more electron-rich end of the alkene Examine electrostatic potential maps for propene, 2-methylpropene and 2-methyl-2-butene. For each, can you tell whether one end of the 7t bond is more electron rich than the other end If so, does protonation on the more electron-rich end lead to the more stable carbocation ... 

Figure 6.11 A comparison of inductive stabilization for methyl, primary, secondary, and tertiary carbocations. The more alkyl groups there are bonded to the positively charged carbon, the more electron density shifts toward the charge, making the charged carbon less electron-poor (blue in electrostatic potential maps).

The second part of lanosterol biosynthesis is catalyzed by oxidosqualene lanosterol cyclase and occurs as shown in Figure 27.14. Squalene is folded by the enzyme into a conformation that aligns the various double bonds for undergoing a cascade of successive intramolecular electrophilic additions, followed by a series of hydride and methyl migrations. Except for the initial epoxide protonation/cyclization, the process is probably stepwise and appears to involve discrete carbocation intermediates that are stabilized by electrostatic interactions with electron-rich aromatic amino acids in the enzyme. 

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. 

The electrostatic potential maps for the two carbocations (Figure 11.11) show the greater dispersal of positive charge in 1-methyl-1-phenylethyl cation compared with tert-hutyl cation. 

Begin by performing an AMI geometry optimization on methyl, ethyl, isopropyl, and ferf-butyl carbocations. These carbocations are built as described in Part C of Experiment 20A. Don t forget to specify that each one has a positive charge. Also select a density surface for each one with the electrostatic potential mapped onto the surface. 

When the calculations are completed, display all four density-electrostatic potential maps on the same screen and adjust the color values to the same range as described in Experiment 20C. What do you observe Is the positive charge as localized in the ferf-butyl carbocation as in its methyl counterpart ... 

Alkyl groups stabilize carbocations because they decrease the concentration of positive charge on the carbon. Notice that the blue area in the following electrostatic potential maps (representing positive charge) is the least intense for the most stable ferf-butyl cation (a tertiary carbocation) and the most intense for the least stable methyl cation. 

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