Carbocations electronegative atom

Aldiough diese structures have a positive charge on a more electronegative atom, diey benefit from an additional bond which satisfies file octet requirement of the tricoordinate carbon. These carbocations are well represented by file doubly bonded resonance structures. One indication of file participation of adjacent oxygen substituents is file existence of a barrier to rotation about the C—O bonds in this type of carbocation. 

Different rate-determining steps are observed for the acid-catalyzed hydration of vinyl ethers (alkene protonation, ks kp) and hydration of enamines (addition of solvent to an iminium ion intermediate, ks increasing stabilization of a-CH substituted carbocations by 71-electron donation from an adjacent electronegative atom results in a larger decrease in ks for nucleophile addition of solvent than in kp for deprotonation of the carbocation by solvent. 

Both carbocations and carbanions are unstable reactive intermediates. A carbocation contains a carbon atom surrounded by only six electrons, and a carbanion has a negative charge on carbon, which is not a very electronegative atom. Carbocations (electrophiles) and carbanions (nucleophiles) can be intermediates in polar reactions— reactions in which a nucleophile reacts with an electrophile. 

Common error alert When a heteroatom stabilizes a carbocation by sharing its lone pair, it still has its octet and hence is neither electron-deficient nor electrophilic (Remember that when an electronegative atom has a formal positive charge and its full octet, the atoms adjacent to it are electrophilic.) Electronegative atoms like N, O, and F are capable of stabilizing carbocations by resonance exactly because they do not surrender their octet when they participate in resonance. 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

Significantly slower rates are found only for compounds that do not exhibit any aromatic ring or carbon-carbon double bond, and for aliphatic compounds with no easily abstractable H-atoms. Such H-atoms include those that are bound to carbon atoms carrying one or several electronegative heteroatoms or groups. (Note that the stabilization of a carbon radical (R ) is similar to that of a carbocation.) We will come back to such structure-reactivity considerations in Section 16.3, when discussing reaction of HO" with organic pollutants in the gas phase (i.e., in the atmosphere). 

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. 

Unlike a normal carbocation, all the atoms in a halonium ion have filled octets. The three-membered ring has considerable ring strain, however, which, combined with a positive charge on an electronegative halogen atom, makes the halonium ion strongly electrophilic. Attack by a nucleophile, such as a halide ion, opens the halonium ion to give a stable product. 

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