Carbocations alkenyl cations

Acid sites catalyse reactions by forming carbocations. Alkenyl cations have been seen by NMR and allylic ones by IR. Simple secondary or tertiary carbenium ions have yet to be observed. There are many questions here. For example, why is isobutane formed aiid why is there an induction period in n-hexane cracking ... 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

Evidence from a variety of sources, however, indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations, and their involvement in these additions has been questioned. For example, although electrophilic addition of hydrogen halides to alkynes occurs more slowly than the corresponding additions to alkenes, the difference is not nearly as great as the difference in carbocation stabilities would suggest. 

The SN1 pathway requires formation of a carbocation. We saw earlier (Section 13-9) that alkenyl halides are unreactive toward displacement reactions. The Ss-2 pathway is poor for the same reason we stated above for benzene compounds. In addition, alkenyl cations are high-energy species because the placement of a positive charge on an -hybridized carbon is very unfavorable. The same issue arises in the case of benzene The phenyl cation is a high-energy species and forms only with difficulty because it contains an s/r-hybridized. positively charged carbon atom. 

The interaction of an acid with an alkenyl monomer can generate ionic chain carriers, but also covalent products with varying degrees of polarity. It has been shown that in certain systems these ester molecules can propagate the growth of a polymer chain, while in others they are inactive. Another source of covalent species in cationic polymerisation is the collapse (recombination) of the ionic pair or the X displacement from the anion to the carbocation discussed in the previous section. 

The stabilities of most other stable carbocations can also be attributed to resonance. Among these are the tropylium, cyclopropenium, " and other aromatic cations discussed in Chapter 2. Where resonance stability is completely lacking, as in the phenyl (CeH ) or vinyl cations, the ion, if formed at all, is usually very short lived. Neither vinyl nor phenyl cation has as yet been prepared as a stable species in solution. However, stable alkenyl carbocations have been generated on Zeolite Y. ... 

Alkenyl (vinyl), aryl, and alkynyl carbocations are particularly unstable with respect to alkyl carbocations. Let s compare the isopropyl cation with the iso-propenyl cation. In the latter, the central C has two cr bonds, one it bond, and one empty orbital, so it is sp-hybridized (linear). Both ions are stabilized by the C(sp3)-H cr bonds of the CH3 group on the right. In the isopropyl cation there is an additional interaction with C(sp3)-H cr bonds on the left, whereas in the isopropenyl cation there is an additional interaction with C(sp2)-H cr bonds on the left. Because C(sp2) orbitals are lower in energy than C(sp3) orbitals, the... 

C(sp2)-H cationic center. In fact, 2° alkenyl carbocations (R.2C=CR) are only about as stable as 1° alkyl carbocations, and 1° alkenyl carbocations (R2C=CH) are only about as stable as the CH3+ carbocation. 

Common error alert If your mechanism has an alkenyl, alkynyl, or aryl carbocation as an intermediate, it is almost certainly incorrect. (One major exception aryldiazonium ions ArNz lose N2 to give aryl cations Ar. )... 

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