Ethyl isopropyl stabilization

All alkyl groups not just methyl are activating substituents and ortho para direc tors This IS because any alkyl group be it methyl ethyl isopropyl tert butyl or any other stabilizes a carbocation site to which it is directly attached When R = alkyl... 

However, experiments in the gas phase gave different results. In reactions of OH with alkyltrimethylsilanes, it is possible for either R or Me to cleave. Since the R or Me comes off as a carbanion or incipient carbanion, the product ratio RH/MeH can be used to establish the relative stabilities of various R groups. From these experiments, a stability order of neopentyl > cyclopropyl > rcrt-butyl > n-pro-pyl > methyl > isopropyl > ethyl was found. On the other hand, in a different kind of gas-phase experiment, Graul and Squires were able to observe CHi ions, but not the ethyl, isopropyl, or (ert-butyl ions. 

The cumulative effects of multiple substituents have been studied at length in search of particularly stable radicals. It is generally found that the repetitive addition of identical substituents leads to a stepwise decrease in RSE values. This is well illustrated by the comparison of the methyl, ethyl, isopropyl, and ferf-butyl radicals with RSE values of 0.0, - 13.8, - 23.3, and - 28.3 kj/mol. Thus, while the stability of the alkyl radicals clearly increases with the number of alkyl substituents attached to the radical center, the substituent ef-... 

This can be tested by drawing on extensive information on carbocation stabilities in the gas phase. Heats of formation of ethyl, isopropyl, sec-butyl and /-butyl cations2 are shown below. From these values it is evident that the /-butyl cation is more stable than the sec-butyl cation by 13kcalmol 1. This corresponds to the direct comparison of (isomeric) ion stabilities noted above by Arnett and Mayr. 

With structurally similar formate esters, the ratio is ethyl/isopropyl/l-butyl formate = 1/55/4570 at 650 °K. If charge polarization was in the usual direction, C-1 would be a 5 -charge center. Alkyl substitution at that center would therefore stabilize the transition states and significantly accelerate reaction rates. Since it is possible to account for almost all of the observed rate increase per methyl substituent at C-1 in terms of gauche destabilizations of the alcohol ground states (see appendix), it would appear that charge polarization in the transition states of these reactions is almost negligible. 

It is also apparent that the degree of stabilization generally follows the order isopropyl < ethyl < methyl, except for electropositive substituents such as Li and BH2 where the order is reversed. These trends, noted by others , will be explained in due course. It is worth noting that calculated and experimental isopropyl stabilizations... 

Stabilization energy model X Methyl Ethyl Isopropyl ... 

The secondary -deuterium kinetic isotope effects in reactions of -perdeuterated ethyl-, isopropyl- and rm-butylmagnesium halides with four different ketones were described [24] and were found to be small (within 5%). There, too, hyperconjugative stabilization was supposed to play a role, but this effect was opposed by the steric effects. The role of hyperconjugation in reactions of Grignard reagents, therefore, seems complicated and requires further studies. 

The data in Table 5.4 indicate that the AH for heterol)dic dissociation of alkanes in the gas phase varies with the alkyl group as follows methyl > ethyl > isopropyl > t-butyl, which is consistent with the generalization that the ease of formation of carbocations is 3° > 2° > 1° > methyl. What is the source of this increase in stability We can explain some, but not all, of the results by saying that an sp hybrid orbital on carbon has a Pauling electronegativity of 2.5, while an sp hybrid orbital on carbon is about 0.25 imits more... 

Figure 11.20 Stabilization quotient Q of nicotinic acid esters (ethyl, isopropyl, butyl, benzyl and hexyl) as a function of percentage binding of ester, (a) to three non-ionic surfactants based on PEG lOCX) O PEG l(X)0-lauryl ether, X PEG l(XX)-myristyl ether, PEG 1000 palmityl ether, and (b) to three stearyl surfactant derivatives, O PEG 900 stearyl ether, X PEG 1400 stearyl ether and PEG 2000 stearyl ether. In both (a) and (b) ionic strength is 0.083, temperature 20.0 0.1° C and borate buffer pH 10.(X) 0.02 used. Redrawn from Lippold et al [137], by permission.

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.)...

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

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