Ethyl carbocation

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

Ethyl carbocation, electrostatic potential map of, 196 molecular orbital of, 196... 

The determination of large values of the rate constant ratio ks/kpfrom the low yields of alkene product that forms by partitioning of carbocations in nucleophilic solvents. These rate constant ratios may then be combined with absolute rate constants for the overall decay of the carbocation to give absolute values of kp (s ).14 16 For example, the reaction of the l-(4-methylphenyl)ethyl carbocation in 50/50 (v/v) trifluoroethanol/water gives mainly the solvent adducts and a 0.07% yield of 4-methylstyrene from proton transfer to solvent, which corresponds to kjkp = 1400. This can be combined with ks = 6 x 109 s V4 to give kp = 4.2 x 106 s l (Table 1). 

The changes in the values of ks/kp observed for partitioning of the carbocations R-[14+] (Table 3) requires that the addition of a-ester, a-amide and a-thioamide substituents result in different changes in ks and kp for partitioning of the parent l-(4-methoxyphenyl)ethyl carbocation.33,41 42 The explanation for the changes in these ratios is complex, and is most easily understood by separate considerations of the effects of these substituents on ks and kp (Scheme 11). 

There is also substantial stabilization of [4+] by electron delocalization from the cyclic a-vinyl group. This is shown by a comparison of the thermodynamic driving force (p Tr lies between —7.8 and —8.5) and absolute rate constant (ks = 1 -6 x 107 s 1) for the reaction of [4+] in 25% acetonitrile in water with the corresponding parameters for reaction of the resonance-stabilized l-(4-methoxyphenyl)ethyl carbocation in water (p Tr = — 9.4and s= 1 x 108 s Table 5). 

A comparison of rate and equilibrium constants for partitioning of the cyclic carbocation [18+] with those for the l-(4-methylphenyl)ethyl carbocation Me-[6+] (Table 5) shows that placement of the cationic benzylic carbon in a five-membered ring results in the following complex changes in the reactivity of the carbocation towards deprotonation and nucleophilic addition of solvent (Scheme 15). 

Thus a tertiary carbocation like the above will give nine resonating structures while a primary will give only two hyperconjugative forms. This explains why tertiary carbocations are more stable than secondary which in turn is more stable than primary. This also explains why ethyl carbocation (CH3CIlf) is more stable than methyl carbocation (CH )-... 

The effects of a-Mc2NC(0) and a-Mc2NC(S) on the rate constants for partitioning of a-substitutcd l-(4-methoxyphenyl)ethyl carbocations between nucleophilic addition of 50 50 (v/v) MeOH-H20 (ks, s ) and deprotonation by this solvent (ke, s 1) have been examined.128 These substituents lead to 80-fold and > 30 000-fold decreases, respectively, in ks, but to much smaller changes in ke. Ab initio calculations suggest that the partitioning is strongly controlled by the relative thermodynamic stabilities of the neutral products of the reactions. 

The partitioning of a-substituted l-(4-methoxyphenyl)ethyl carbocations (42) between nucleophilic capture (ks) and deprotonation (fe) in 50 50 (v/v) MeOH-H2O has been studied (Scheme 6).20 The effect of a-(jV,A -dimcthylcarbamoyl) and a-... 

In so far as values of pATn2o for the hydration of alkenes are known or can be estimated,47 values of pATR can be derived by combining rate constants for protonation of alkenes with the reverse deprotonation reactions of the carbocations. The protonation reactions seem much less likely to be concerted with attack of water on the alkene than the corresponding substitutions. Indeed arguments have been presented that even protonation of ethylene in strongly acidic media involves the intermediacy of the ethyl carbocation.97,98... 

Notice that if the original ionization of the t-butyl ethyl ether formed a t-butoxide ion and an ethyl carbocation, this would be a less stable arrangement. (Remember, the order of stability of carbocations is 3° > 2° > 1°.)... 

The Hammett reaction constants p = 2.7138 and 3.0157 were determined for addition of water to meta-substituted l-(4-methoxyphenyl)ethyl carbocations (80-X, Scheme 44A) and 1-4-methoxybenzyl carbocations (p-Me-1 +-X), respectively. The value of p = 2.7 for addition of water to 80-X is 36% that of the value of = 7.6, the slope of Hammett-type plots equilibrium constants for addition of water to 80-X to form the alcohol.138 This shows that the addition of water proceeds through a transition state where partial bond formation to the nucleophile results in a 36% change in the interaction between the m-substituent and the cationic benzylic carbon, compared with the complete loss of this interaction at the water adduct. Scheme 44A therefore proceed through a transition state in which there is a 36% change in the interaction between the m-substituent and positive charge at the benzylic carbon due to partial bond formation to the water nucleophile. 

These data could also be rationalized if there were a change in the ratedetermining step for a stepwise reaction, from k s for acid-catalyzed addition of solvent to o-l to k for the reaction of 81. However, it is unlikely that k will be rate determining for the stepwise addition to 81. This would require k s > k for Scheme 47. A value of k i > 1010 s 1 is expected for the strongly favorable deprotonation of H-81+ by water,162 and there is evidence for a significant barrier to k s for addition of water to -SR-substituted benzylic carbocations. For example, a value of k s = 5 x I07s 1 has been determined for addition of an aqueous solvent to the l-4-(thiomethylphenyl)ethyl carbocation. The concerted mechanism is favored because it avoids formation of the unstable acidic intermediate H-81+. It is possible, but not proven, that the concerted mechanism is enforced by the absence of a vibrational barrier to the step k i for deprotonation of this very strongly acid reaction intermediate by water.163,164... 

Ethyl carbocation iso-Propyl carbocation tert-Butyl carbocation... 

The low Sn2 reactivity of 1°-alkyl bromide, 2,2-dimethyl-1-bromopropane (neopentyl bromide, 2.5), is explained by steric hindrance to the required 180° alignment of reacting orbitals. However, under Sn 1 conditions, neopentyl bromide (2.5) reacts at roughly the same rate as other 1°-alkyl halides such as ethyl bromide. Ionization of alkyl halides to carbocation in SnI is the rate-determining step. Although the product from ethyl bromide is ethanol as expected, neopentyl bromide (2.5) yields 2-methyl-2-butanol (2.6) instead of the expected 2,2-dimethyl-1-propanol (neopentyl alcohol) (2.7). This is because once formed the ethyl carbocation can only be transformed by a substitution or elimination process. In the case of the neopentyl carbocation, however, the initially formed l°-carbocation may be converted... 

The acid-catalyzed hydrolysis of -methylstyrene oxide (38a) in azide solutions has also been studied, but the yields of azido alcohols are only very slightly greater than the yields predicted if all of the azido alcohol is formed from the bimolecular (fcN) pathway.41 For example, in water containing 0.025 M azide ion, pH 5.75, the observed yield of azido alcohol from 38a is 22%. The calculated yield from the bimolecular reaction is 19%. This yield is very close, perhaps within experimental error, to that expected from only the bimolecular pathway (kN). If it is assumed that 3% of azido alcohol is actually from trapping of the intermediate carbocation 39a with azide ion, then a lower limit of 3 x 109 s 1 can be estimated for ks. This value is fortuitously close the value of ks estimated for reaction of l-(p-methylphenyl)ethyl carbocation with 50 50 TFE/H20 (4 x 109 s 1), and is most likely an underestimate. 

The ethyl carbocation is unstable because the carbon has only three bonds (six outer electrons) around it. To achieve an octet of electrons about this carbon, the carbocation donates an H to a proton acceptor, such as HS04, and forms the double bond of the alkene product ... 

Seeing the synthons above may help us to reason that we could, in theory, synthesize a molecule of 1-butyne by combining an ethyl cation with an ethynide anion. We know, however, that bottles of carbocations and carbanions are not to be found on our laboratory shelves and that even as a reaction intermediate, it is not reasonable to consider an ethyl carbocation. What we need are the synthetic equivalents of these synthons. The synthetic equivalent of an ethynide ion is sodium ethynide, because sodium ethynide contains an ethynide ion (and a sodium cation). The synthetic equivalent of an ethyl cation is ethyl bromide. To understand how this is true, we reason as follows if ethyl bromide were to react by an Sf,jl reaction, it would produce an ethyl cation and a bromide ion. However, we know that, being a primary halide, ethyl bromide is unlikely to react by an Sf l reaction. Ethyl bromide, however, will react readily with a strong nucleophile such as sodium ethynide by an Siyf2 reaction, and when it reacts, the product that is obtained is the same as the product that would have been obtained from the reaction of an ethyl cation with sodium ethynide. Thus, ethyl bromide, in this reaction, functions as the synthetic equivalent of an ethyl cation. 

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