Inductive destabilization

P-Fluonne or fluonne further removed from the cation center always inductively destabilizes carbocabons [115, 116] No simple p-fluoroalkyl cations have been observed in either the gas phase or solution, and unhke the cases of the other halogens, there is no evidence for formation of alkyl fltioronium ions (5) in solution [117, 118], although (CH3)2F is long-hved m the gas phase [119] The only P-fluonnated cations observed in solution are those that benefit from additional conjugativc stabilization, such as a-trifluoromethylbenzyl cations [112] and per-fluonnated allyl [120], cydopropenium [112], and tropylium [121] ions... 

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). 

At all levels of theory, the N-acetyl group of N-acetyl-N-arylnitrenium ions is rotated out of the plane of the aromatic ring, although to different extents." The N-acetyl group destabilizes the ion by ca. 20 kcal/mol relative to an N-methyl substituent in comparison with the neutral amide and amine precursors. This destabilization was attributed by Ford and Herman to loss of resonance in the amide precursor on going to the nitrenium ion, not to inductive destabilization of the ion by the acyl group. 

Since fluorine is the most electronegative element, it should inductively destabilize carbocations. The stability of fluoromethyl cations in the gas phase decreases in the order CFH2+ > CF2H+ > CF3+ > CH3+. The trend in solution, however, could be different, due to solvent effects, ion pairing, and so on. Indeed, fluorine has been shown to provide stabilization for carbocations. The existence of CH3CF2+, in contrast to the elusive ethyl cation CH3CH2+, is a clear evidence that replacement of H atoms by F atoms provides stabilization for carbocations.524 Furthermore, it was found that in perfluorobenzyl cation C6F5CF2+ fluorine atoms in resonance positions (ortho and para) are more deshielded than those in meta positions.536 This indicates carbocation stabilization by back-donation. 

Heteroatoms with higher electronegativity than carbon (e.g. nitrogen, oxygen, or the halogens) inductively destabilize carbocations at the /i position. Epoxides of the type shown in the last equation of Scheme 4.60 therefore react preferentially at the unsubstituted carbon atom. Only in the presence of certain Lewis acids, capable of chelate formation with simultaneous activation of the substituted carbon atom, is the alternative regiochemistry observed. 

The greatly diminished stabilization of the silylenium compared to the carbenium structure by it substituents is connected with the known low effectiveness of 2p-3p (it) conjugation. Calculations were made on the =Si+ cation substituted with a phosphorus group with the hope of showing a superior 3p-3p (tt) conjugation of P—Si+ moiety (26,28). A smaller inductive destabilization by phosphorus than by electronegative substituents of the second period (N, O) was also expected. However, all... 

As seen in Table 14 gas phase protonation of cyclopropylamine is 0.9 kcal mol" more favorable than for methylamine but 4.4 kcal mol" less favorable than for isopropylamine. A possible explanation of the latter large difference is stabilization of cyclopropylamine by the conjugative interaction shown in Figure 3, Section III.D, and inductive destabilization of the cation by cyclopropyl. 

Bulky groups such as methoxymethyl (which is also inductively destabilizing to a cation) resulted in mixtures of the two regioisomeric products from cyclization at the substituted and unsubstituted position. When the a substituent R is an aryl group such as p-tolyl, despite its cation-stabilizing properties, the regio- and stereocontrol of the reaction is lost due to its bulk thus three isomers 67,68, and 69, of the tricyclic product were isolated. While the first step of the cyclization process, the H abstraction, has been... 

Common error alert Lone-pair-bearing heteroatoms are usually electronegative, too, but when there is a competition between an inductive destabilizing effect and a resonance stabilization effect, resonance always wins ... 

The stability of fluorinated carbocations [1] is determined by a delicate equilibrium between inductive destabilization and mesomeric stabilization of the positive charge. a-Fluoro substituents stabilize the positively charged carbon by TT-donation... 

Because of their strong inductive destabilization, few salts of a- or /i-fluorocarbe-nium ions have been isolated and characterized. Despite persistent attempts to isolate salts of the CF3+ cation, this most simple a-fluorocarbenium ion has so far only been subject of numerous theoretical studies [3]. Recently, the more stable dimethylfluorocarbenium ion, in the form of its hexafluoroarsenate salt (MeyCf Asf, ), has been characterized by X-ray crystallography [4]. 

Trifluoromcthyl hypofluorite exhibits a similar stereochemical behaviour to acetyl hypofluorite in its reactions with stilbene123, cytosine, and uracil124. The tendency towards ath addition has been attributed to the inductive destabilization of a vicinal carbocation which is not mitigated by cyclic fluoronium ion formation. This short-lived carbocation induces ion pairing and neighboring group participation123. 

The fact that vanadyl tetraphenylporphine was less reactive is not inconsistent with this rationalization although in this instance all four methine carbons have phenyl substituents. It is likely that the formation of the mono-substituted carbonium ion would be inhibited because of steric interactions at the methine position involved. However, a more important consideration is that the benzene ring attached to the methine carbon is not coplanar with the macrocyclic porphyrin ring. The extent of deviation from planarity is enough that a positive charge generated on the methine carbon would receive no resonance stabilization but considerable inductive destabilization as a consequence of the unsaturated substituent. 

---