Cations ethyl

Methyl cation Ethyl cation (primary) Isopropyl cation (secondary) tert Butyl cation (tertiary)... 

FIGURE 4 14 Electro static potential maps of methyl cation and ethyl cation The region of high est positive charge is more concentrated in CH3 and more spread out in CH3CH2 (The electrostatic potentials were mapped on the same scale in order to allow direct comparison )... 

FIGURE 4 16 Hyper conjugation in ethyl cation Ethyl cation is stabilized by delocalization of the elec trons in the C—H bonds of the methyl group into the vacant 2p orbital of the posi tively charged carbon... 

When usiag HF TaF ia a flow system for alkylation of excess ethane with ethylene (ia a 9 1 molar ratio), only / -butane was obtained isobutane was not detectable even by gas chromatography (72). Only direct O -alkylation can account for these results. If the ethyl cation alkylated ethylene, the reaction would proceed through butyl cations, inevitably lea ding also to the formation of isobutane (through /-butyl cation). 

Fig. 8. Sensitizing dyes of the cyanine class. K. = N — alkyl or chalcogens (O, S, Se, Te) R = chloro, phenyl, or additional benzene ring R = methyl, ethyl, or hydrogen n = 0, 1, 2 and RPRIME, R " = alkyl or sulfoalkyl. Solubihty in methanol for a carbocyanine dye n = 1 X = S R = Cl R = ethyl. Cationic dye (R" = R " = ethyl anion = bromide) 9.5 mmol/T. neutral dye (R" = ethyl R " = sulfopropyl) 3.6 mmol/L anionic dye (R" = R = sulfopropyl ...

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.)... 

LUMO of 1-phenyl-1-ethyl cation reveals where the cation is able to bond to water. 

Electrostatic potential map for i-phenyl-l-ethyl cation-chloride anion shows negatively-charged regions (in red) and positi vely-charged regions (in blue). 

Examine the structure of 1-phenyl-1-ethyl cation. Is it chiral Examine the LUMO. Would you expect the cation to give a racemic mixture of alcohols or the mixture that is actually obtained Explain. 

Next, examine the structure of 1-phenyl-1-ethyl cation-chloride anion, an ion pair that is initially generated. What evidence is there for cai bon-chlorine bond cleavage Examine the electrostatic potential map for the ion pair. Which face of the cation is more available for attack How could the other enantiomer form ... 

Non-classical structures are predicted to be unstable relative to classical structures (for example ethyl cation). 

Table 1. Relative energies E (kJ mol 1) of the ethyl cation dependent on calculation method used (data from 43) if not otherwise indicated)...

The equilibrium between a and b in Eq. (2) depends on the energies of both the structures. In Table 1 the relative energies of the ethyl cation in the structures a and b, calculated with different methods, are shown. 

All alkyl ions tested demonstrate a comparable behaviour independent of the sign of their charges. The decrease of the reaction enthalpies AH (11) with the change from the methyl to the ethyl cation (AAH (ll) = 165 kJ mol-1) and from the ethyl to the but-2-enyl cation (AAH°(11) = 117 kJ mol-1) corresponds to the increase of stability of these carbenium ions, which are expressed by the difference of their heats of formation (AAH f = —118 and AAHj = —42 kJ mol-1 90)) and of their hydride ion affinity (AHIA = 176 and 126 kJ mol-1 91)), respectively. 

The calculation of an activation barrier for the reactions (21) and (22) must not necessarily be considered as an error of the method. For example, the MINDO/3 calculated activation barrier for the attack of a methyl radical on ethene 137-138) which is comparable to the former reactions was confirmed by experiments 139). In contrast to a free proton (Eq. (20)) the methyl radical as well as the ethyl cation possess steric space need. For this reason, the calculation of repulsive interactions which are able to overcome the attractive forces at certain distances cannot be seen without doubt as faulty. 

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