Vinylic carbocation, electronic structure

Vinylic carbocation, electronic structure of, 263 electrostatic potential map of. 

A mechanism for the formation of the minor product is presented below. The alkyne attacks ICl, resulting in the formation of a new C-I bond, and a vinyl carbocation (see below for a justification for formation of a vinyl carbocation in this case). This carbocation then serves as an electrophile in an electrophihc aromatic substitution. The n electrons from the methoxyarene attack this carbocation, resulting in the formation of a new C-C bond, and a resonance-stabilized sigma complex. This intermediate is highly conjugated and has at least seven reasonable resonance structures in addition to the four shown. Deprotonation of the sigma complex restores aromaticity, thus yielding the product. 

Despite the successful prediction of chemical shifts for a great structural variety of carbocations some difficulties have been encountered for vinyl cations.47 The effect of electron correlation, basis sets and geometry on calculated NMR spectra of vinyl cations has been studied in some detail also for the parent vinyl cation in its linear form.48 Comparative experimental and computational NMR studies, however, have... 

The cationic polymerization of vinyl isobutyl ether at —40°C produces stereoregular polymers (structure 5.21). The carbocations of vinyl alkyl ethers are stabilized by the delocalization of p valence electrons in the oxygen atom, and thus these monomers are readily polymerized by cationic initiators. Poly(vinyl isobutyl ether) has a low Tg because of the steric hindrance offered by the isobutyl group. It is used as an adhesive and an impregnating resin. 

Trivalent ( classical ) carbenium ions contain an, vp2-hybridized electron-deficient carbon center that tends to be planar in the absence of constraining skeletal rigidity or steric interference. (It should be noted that ip-hybridized, linear acyl cations and vinyl cations also show substantial electron deficiency on carbon.) The carbenium carbon contains six valence electrons, and thus it is highly electron-deficient. The stmcture of trivalent carbocations can always be adequately described by using two-electron, two-center bonds (Lewis valence bond structures). 

---