Primary carbonium ion

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

In contrast to the above case, addition of HCl to 1,1-dimethylallene at —78°C gives at least two thirds and possibly exclusively l-chloro-3-methyl-2-butene, 33, although these results are complicated by rearrangement of the allene to isoprene and the addition of HCl to the isoprene (65). No satisfactory explanation was offered (65) and none is readily available within the carbonium framework to account for the unusual orientation in this addition. Certainly the tertiary carbonium ion, 34, should be more stable than the primary carbonium ion, 35, since neither is stabilized by the adjacent perpendicular n center. This result is all the more surprising since tetramethylallene, 36, behaves as expected... 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

The above picture applies to dilute aqueous acids and in a more acidic medium the ionic character will shift in the direction of the primary alcohols. It is, however, doubtful that a nonresonance stabilized primary carbonium ion exists, even in the most acidic medium. 

Oxidation of alkanes such as n-hexane never gives products from a primary carbonium ion, the. vec.-carbonium ion intermediates are always formed in preference... 

Kneen showed that a Wagner-Meerwein type rearrangement occurred on reaction of AuBrs(vp) with methanol and ethanol, the substituted product having a 6-membered ring system. Ionisation, rapid rearrangement of the primary carbonium ion to a more stable secondary, benzylic carbonium ion and subsequent attack by solvent can account for this reaction. 

The stability of the intermediate carbonium ion determines whether cyclization forms five- or six-membered rings. In the case of n-butylbenzene and 2-phenylpentane, acid-catalyzed six-ring cyclization would involve very unstable primary carbonium ions C6H5-CH2-CH2-CH2-CH2+ or C6H5-CH(CH3)-CH2-CH2-CH2 +. Therefore, only five-membered rings could be formed in acid-catalyzed cyclization from these two hydrocarbons. 

Both 2- and 4-methylalkanes have an ion of low intensity at M-15, owing to loss of a methyl group. The major ion of signihcant intensity in the high-mass region of the mass spectra for both isomers is due to the M-43 ion a primary carbonium ion formed by loss of isopropyl from the 2-methylalkane and a secondary carbonium ion formed by the loss of n-propyl from the 4-methylalkane. In addition, the 4-methylalkane has a primary carbonium ion of low intensity at the M-71 M-72 due to cleavage internal to the methyl branch point. The presence of this primary ion pair establishes the structure of a 4-methylalkane. However, in mass spectra of low intensity or in which other isomers are present, the ions at M-71 M-72 may not be readily apparent. 

Again, since mixtures of 2-oxazolines are obtained from nitriles and unsymmetrical epoxides,15 it does not seem likely in this case that a purely carbonium ion mechanism is involved, for this would require the formation of primary carbonium ions. An alternate possibility is that the nitrile nucleophilically attacks the oxonium salt (27) of the epoxide to give the nitrilium salts 28 and 29, which then cyclize, thus ... 

Abundances of the Primary Carbonium Ions from the Decay of Tritiated Alkanes ... 

Some related data concerning the gas-phase reactions are shown in Table 4 [65,66], Data shown in Table 4 indicate that, in the case of primary carbonium ions (e.g., C2Hs+ cation), the enthalpy of conversion of carbenium into oxonium or ammonium ion (>220 kJ/mol) by far exceeds the strain of even the most strained rings (< 115 kJ/mol). 

What we see here is another example of that characteristic of nucleophilic substitution a shift in the molecularity of reaction, in this particular case between T and 1°. This shift is confirmed by the fact that reactivity passes through a minimum at 1° and rises again at methyl. Because of poor accommodation of the positive charge, formation of primary carbonium ions is very slow so slow in this instance that the unimolecular reaction is replaced by the relatively unhindered bimolecular attack. The bimolecular reaction is even faster for the still less hindered methanol. 

In fact, whenever the primary carbonium ion is formed from the proper substrate, ring expansion is observed this is the case in the acid-catalyzed dehydration of 2-hydroxymethylbicyclo[2.2.1]heptane (23), producing a mixture of 2.5% methylenebicyclo[2.2.1]heptane (7), 87% bicyclo[3.2.1]-2-octene (5), and 10% bicyclo[3.3.0]-2-octene (6) and in the nitrous acid deamination of 2-aminomethylbicyclo[2.2.1]-heptane to bicyclo[3.2.1]- and bicyclo[2.2.2]octanols 30). 

The mechanism of these reactions must be obtained by reversing the steps of the ring-cracking mechanism (principle of microscopic reversibility) therefore, in the cases of limonene, or vinylcyclohexene, a rather unstable primary carbonium ion should form, and react with the olefinic double bond (Fig. 16a). Another possibility is offered by the addition of a proton to the cyclic double bond (Fig. 16b). 

One is a primary carbonium ion, while the other is a secondary carbonium ion. Which of these two carbonium ions is the more stable, and why ... 

The 2-bromopropane molecule is the result, and not the 1-bromopropane isomer. We may invoke the Hammond postulate to explain this result. Due to the greater stability of the secondary carbonium ion, there is a lower activation energy barrier that needs to be overcome in order for it to be formed, in contrast to the higher activation barrier that must surmounted in order to form the less stable primary carbonium ion. Hence, the secondary carbonium ion is formed much more quickly, and once formed it reacts to give the product. 

But-2-ene will be formed in preference to but-l-ene, as the secondary carbonium ion is significantly more stable than the primary carbonium ion, i.e. the former is the thermodynamic product. Thus, in El reactions, Zaitsev elimination is preferred. However, it does not apply if the predicted product would suffer steric strain. 

This old system is also used to describe carbonium ions and radicals. Thus, primary, secondary, and tertiary carbonium ions are given the respective label, normal, secondary (abbreviated to sec- or just s-), and tertiary (abbreviated to t-). The term neo may be used to describe a primary carbonium ion that is immediately adjacent to a quaternary carbon, e.g. (CH3)3CCH2+. 

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