Carbonium ions resonance

The protonation of the ester makes the carbonyl carbon more electrophilic (note the carbonium ion resonance form) and hence more susceptible to nucleophilic attack by water. 

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. 

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

Those sets for which the resonance effect is predominant are the sets which are most likely to give rise to the free carbonium ion 5, as the substituents in these sets (sets 15-14 and 15-17 and possibly 15-18) are all donors by resonance, as is shown by their Or values. Those sets for which the localized effect is predominant may be accounted for in terms of intermediates 3 or 4. Sets 15-5, 15-7B2, and 15-12 gave significant values of jS. It is difficult to account for this fact in terms of intermediate 4. The results can be accounted for in terms of intermediate 3, however, if this species resembles other three-membered rings, such as cyclopropane, in its behavior. Sets 15-6, 15-8, 15-9, 15-12, and 15-15 include both donor and acceptor substituents. The successful correlation of... 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

The vinyl cation analog of an allylic carbonium ion is an allenyl cation 242, where the empty p orbital on the unsaturated carbon overlaps with the perpendicular n bond of the allenyl system. Allenyl cation 242 is of course a resonance form of the well known alkynylcarbonium ion,... 

TTie solvolysis of propargylic substrates (199) and formation of alkynylcarbonium ions (200) has been extensively investigated. Particularly good evidence for the formation of alkynylcarbonium ions comes from the nuclear magnetic resonance spectra of alkynyl alcohols in strong acid media (200, 201). The downfield shifts of 4ppm for the proton of HC=C— and 1 ppm for CH3C=C- relative to their neutral precursors is indicative of carbonium-ion formation and shows the importance of the allenyl resonance contribution. 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

For (XX), L py, it is likely that the major reaction path involves initial skeletal isomerization to give (XXI) followed by rapid solvolysis of this isomer. The solvolysis of this isomer is strongly metal-assisted since the intermediate carbonium ion is stabilised by the metal-alkene resonance form as shown in the Scheme. The product is the 1-D2 isomer. Now, the skeletal isomerization of (XX) is expected to be retarded by free pyridine and cannot occur when L2 = 2,2 -bipyridyl C7). Hence under these conditions the reaction must occur by solvolysis of (XX) giving largely the 3-D2 isomer. However, the product formed under these conditions is still about 30% of the 1-D2 isomer (Table I). 

An explanation not easily distinguishable from the one involving resonance with a carbonium ion structure in the transition state is that the reactive species is an ion pair in equilibrium with the covalent molecule. This is quite likely in a solvent insufficiently polar to cause dissociation of the ion pairs. Examples of second order nucleophilic displacements accelerated by the sort of structural change that would stabilize a carbonium ion are of fairly frequent occurrence. Allyl chloride reacts with potassium iodide in acetone at 50° seventy-nine times as fast as does -butyl chloride.209 Another example is the reaction of 3,4-epoxy-1 -butene with methoxide ion.210... 

The stability of a carbanion (or ion pair) is increased by certain substituents and decreased by others. It is possible to rank the various structures in an order of increasing stability of the carbanion just as was done for carbonium ions. It will be recalled that our information about carbonium ions does not suffice for a prediction of the effect of temperature changes on the relative stabilities, and that it is unknown to what degree an increase in stability actually reflects a decrease in potential energy. The situation is similar in the case of carbanions the precise relationship of the stabilities is an unknown function of the temperature. It is also likely that the effects of structural changes are somewhat dependent on the solvent. Nevertheless it is possible to make valuable qualitative comparisionsof the various structures and to interpret them in terms of resonance and other potential energy quantities. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

Hydrolysis of optically pure bromide 100 a in dioxane/water gives the optically pure alcohol. This is consistent with the transannular p-xylylene ring participating in carbonium ion formation only through tz-g charge delocalization (iz-c resonance) of the type 104 rather than by direct participation in replacement of bromide via a transannularly bridged ion such as 102a. In the latter case, racemization would be expected to take place ... 

Perdeuteriated isopropyl, t-butyl, and t-pentyl fluorides were prepared from the corresponding deuteriated alkyl chlorides (bromides) by halogen exchange. The perdeuteriated alkyl fluorides were then used in the formation of the deuteriated carbonium ions under similar conditions as those for the protium complexes. The resonance spectra were obtained at 9 2 Mc/s and the data are summarized in Table 7... 

The C -resonance investigations also provide a further possibility in the investigation of the carbonium ion complexes through evaluation of... 

Prior to 1967 acetal hydrolysis had been found to be a specific-acid catalysed reaction with the accepted mechanism [equation (46)] involving fast pre-equilibrium protonation of the acetal by hydronium ion, followed by unimolecular rate-determining decomposition of the protonated intermediate to an alcohol and a resonance stabilized carbonium ion (Cordes, 1967). An A-1 mechanism was supported by an extremely large body of evidence, but it appeared unlikely that such a mechanism could expledn the... 

Five years ago a brief review focused on the applications of nuclear magnetic resonance (nmr) as a method for determining charge density in carbonium ions and pointed out some of the precautions required (Fraenkel and Famum, 1968). Since then, proton nmr (pnmr), which was emphasized in that review, has continued to attract primary attention as a probe into the structure and charge density of organic cations and anions (Olah and Schleyer, 1968,1970, 1972, 1973 Oth ef al., 1972 Takahashi et a/., 1973 van... 

The downfield shift of the central carbon resonance in trimethyl-carbonium ion (330 p.p.m.) as compared with dimethylcarbonium... 

This agrees with the theory that resonance-nonstabilized primary carban-ions are more stable than the corresponding secondary or tertiary carb-anions, whereas tertiary carbonium ions or radicals are more stable than the corresponding secondary and primary species. If radical intermediates were involved in a chain catalytic reaction of toluene with propylene, n-butylbenzene would be the product. 

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