Carbocations ethyl

Butyl carbocation Methyl 2-Propyl carbocation Ethyl... 

Carbocations are classified according to their degree of substitution at the positively charged carbon The positive charge is on a primary carbon m CH3CH2" a secondary car bon m (CH3)2CH" and a tertiary carbon m (CH3)3C Ethyl cahon is a primary carbocation isopropyl cation a secondary carbocation and tert butyl cation a tertiary carbocation... 

Primary carbocations are so high m energy that their intermediacy m nucleophilic substitution reactions is unlikely When ethyl bromide undergoes hydrolysis m aqueous formic acid substitution probably takes place by an 8 2 like process m which water is the nucleophile... 

Secondary alkyl halides react by a similar mechanism involving attack on benzene by a secondary carbocation Methyl and ethyl halides do not form carbocations when treated with aluminum chloride but do alkylate benzene under Friedel-Crafts conditions The aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon-halogen bonds and these complexes are the electrophilic species that react with benzene... 

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... 

Since a carbocation can add to an alkene to form a larger cation, under acidic conditions isobutylene can dimerize to form 2,4,4-trim ethyl -1 -pen ten e [107-39-1] and 2,4,4-trimethyl-2-pentene [107-40-4J, which can then be hydrogenated in the presence of nickel to form isooctane [540-84-1]. This reaction is no longer of commercial significance. 

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

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. 

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... 

Unfortunately, it is not easy to measure acid strengths of very weak acids like the conjugate acids of simple unsubstituted carbanions. There is little doubt that these carbanions are very unstable in solution, and in contrast to the situation with carbocations, efforts to prepare solutions in which carbanions such as ethyl or isopropyl exist in a relatively free state have not yet been successful. Nor has it been possible to form these carbanions in the gas phase. Indeed, there is evidence that simple carbanions such as ethyl and isopropyl are unstable toward loss of an electron, which converts them to radicals. Nevertheless, there have been several approaches to the problem. Applequist and O Brien studied the position of equilibrium for the reaction... 

It is unlikely that free carbanions exist in solution. Like carbocations, they usually exist as either ion pairs or they are solvated. " Among experiments that demonstrated this was the treatment of PhCOCHMe with ethyl iodide, where was Li ", Na", or K" . The half-lives of the reaction were for Li, 31 x 10 Na, 0.39 X 10 and K, 0.0045 x 10 , demonstrating that the species involved were not identical. Similar results were obtained with Li, Na, and Cs triphenylmethides (PhsC M Where ion pairs are unimportant, carbanions are solvated. Cram " demonstrated solvation of carbanions in many solvents. There may be a difference in the structure of a carbanion depending on whether it is free (e.g., in the gas phase) or in solution. The negative charge may be more localized in solution in order to maximize the electrostatic attraction to the counterion. ... 

For some tertiary substrates, the rate of SnI reactions is greatly increased by the relief of B strain in the formation of the carbocation (see p. 366). Except where B strain is involved, P branching has little effect on the SnI mechanism, except that carbocations with P branching undergo rearrangements readily. Of course, isobutyl and neopentyl are primary substrates, and for this reason they react very slowly by the SnI mechanism, but not more slowly than the corresponding ethyl or propyl compounds. 

The first step of the reaction is likely to be the protonation of ethylene to produce a carbocation that undergoes the direct addition of acetic acid to produce ethyl acetate. The successive addition of ethylene to the carbocation leading to the production of alkene oligomers is a likely side reaction Formation and accumulation of these oligomers could eventually deactivate the catalyst. Detailed studies for a better understanding of the complex reaction mechanism are in progress. 

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 SnI reaction of (S)-3 -brom o-3 -m ethyl hexane proceeds with racemization because the intermediate carbocation is achiral and attacked by the nucleophile can occur from either side. 

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). 

Examples of the behavior of other substituted vinyl substrates upon exposure to the action of trifluoroacetic acid and triethylsilane are known. For example, -butyl vinyl ether, when reacted at 50° for 10 hours, gives -butyl ethyl ether in 80% yield (Eq. 65).234 In contrast, -butyl vinyl thioether gives only a 5% yield of n-butyl ethyl sulfide product after 2 hours and 15% after 20 horns of reaction.234 It is suggested that this low reactvity is the result of the formation of a very stable sulfur-bridged carbocation intermediate that resists attack by the organosilicon hydride (Eq. 66). 

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