Carbonium ions destabilized

The formation of any vinyl products in electrophilic additions to RCH=C=CH2 and RCH=C=CHR is surprising, since central protonation should yield a secondary carbonium ion compared to terminal protonation and formation of a vinyl cation. Perhaps a secondary carbonium ion destabilized by... 

The acyl residue controls the formation and stability of the carbonium ion. If the carbonium ion is destabilized (by electron withdrawing groups), then cyclization to the phenanthridine nucleus will be sluggish. The slower the rate of cyclization, the greater the chance of side reactions with the cyclization reagent. Therefore, the yield of the phenanthridine will depend on the relative rates of cyclization and side reactions, which is controlled by the stability of the carbonium ion. 

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). 

In the reverse direction, protonation of the phosphate of glucose-1-phosphate destabilizes the glycosidic bond and promotes formation of a glucosyl oxocarbonium ion-phosphate anion pair. In the subsequent step, the phosphate anion becomes essential for promotion of the nucleophilie attack of a terminal glucosyl residue on the carbonium ion. This sequence of reactions brings about a-1,4-glycosidic bond formation and primer elongation. 

No kinetic data are available for the hydrolysis of phenyldiazomethane, but some measurements have been done with p-nitrophenyldiazomethane [210]. A solvent isotope effect of kH/kD = 2.5 in 60 % dioxane—water with 0.014 M HC104 at 20 °C supplies evidence for rate-determining proton transfer in the mechanism of hydrolysis. Since the electron-withdrawing p-N02 group must destabilize the carbonium ion and increase the acidity of the diazonium ion, it may be expected that ku must be higher and must be lower for the unsubstituted compound. It follows that fen/fe i must be higher in the hydrolysis of phenyldiazomethane in comparison to p-nitrophenyldiazomethane. Therefore, it is very likely that rate-determining proton transfer occurs also in the hydrolysis of phenyldiazomethane. 

The --NO2 group, on the other hand, has an electron-withdrawing inductive effect (III) this tends to intensify. the positive charge, destabilizes the carbonium ion, and thus causes a slower reaction. 

Through its inductive effect halogen tends to withdraw electrons and thus to destabilize the intermediate carbonium ion. This effect is felt for attack at all positions, but particularly for attack at the positions ortho and para to the halogen. 

We have encountered this kind of kinetics before in SnI reactions and know, in a general way, what it must mean in the rate-determining step, the substrate is reacting unimolecularly to form an intermediate, which then reacts rapidly with solvent or other nucleophile. But what is this intermediate It can hardly be the carbonium ion. A primary cation is highly unstable and hard to form, so that primary alkyl chlorides ordinarily react by Sn2 reactions instead and here we have electron-withdrawing sulfur further to destabilize a carbonium ion. 

A potentially anti-homoaromatic species (79, see Table 16) has been generated by protonation of l,6-methano-[10]annulene at the 2-position 224>. As expected, 1,3 overlap is not significant and the compound can be formulated as a classical carbonium ion. In fact the abnormally large H(2a) to H(2a) coupling constant of —24.9 Hz indicates that the 1,3 distance has been maximized in order to avoid the expected destabilization, and thus there is no evidence for a paramagnetic ring current in this system. 

Eaborn, Murrell and coworkers56 calculated AE for the reaction 13 to be -6.2 kcal mol"1, and Hopkinson57, using calculations at the double-C level, obtained —14.6 kcal mol"1 and —14.3 kcal mol"1 when polarization functions were included. The most recent high-level calculations58 used a variety of basis sets and confirm that a-silicon destabilizes carbonium ions relative to CH3 at the MP3/6-31G level AE for equation 13 was —16.2 kcal mol". +... 

In such reactions, any steric effect of a neighboring hydroxyl group is weak, compared to its electron-withdrawing effect which, presumably, destabilizes one of the two carbonium ion transition states more than the other, as pointed out by Lemieux. ... 

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