Lone pairs carbocations

Resonance stabilization can also make n -electron donation much more effective by avoiding the formation of a sextet carbocation. Lone-pair donation from tire oxygen of enol derivatives is very important to the good donor ability of these compounds. The resulting oxonium ion has all valence octets (although positively charged) and is thus stabilized over sextet canonical forms. 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

The lone pair on oxygen cannot be directly involved m carbocation stabilization when attack is meta to the substituent... 

Adjacent atoms with one or more lone pairs of electrons strongly stabilize a carbocation. Table 1.13 (p. 30) indicates the stabilization of flie methyl cation by such... 

Figure 7.6 The structure of dichlorocarbene. Electrostatic potential maps show how the positive region (blue) coincides with the empty p orbital in both dichlorocarbene and a carbocation (CH3+). The negative region (red) in the dichlorocarbene map coincides with the lone-pair electrons.

Figure 16.15 Carbocation intermediates in the nitration of chlorobenzene. The ortho and para intermediates are more stable than the meta intermediate because of electron donation of the halogen lone-pair electrons.

Carbocations may also be generated by protonation of lone pair electrons, if the protonated atom is converted into a better leaving group thereby and ionisation thus promoted ... 

The vinyl halide product is then able to react with a further mole of HX, and the halide atom already present influences the orientation of addition in this step. The second halide adds to the carbon that already carries a halide. In the case of the second addition of HX to RC CH, we can see that we are now considering the relative stabilities of tertiary and primary carbocations. The halide s inductive effect actually destabilizes the tertiary carbocation. Nevertheless, this is outweighed by a favourable stabilization from the halide by overlap of lone pair electrons, helping to disperse the positive charge. 

Aliphatic ketones are oxidised in both acetonitrile [1,2] and trifluoracetic acid [3] at potentials less positive than required for the analogous hydrocarbons. The oxidation process is irreversible in both solvents and cyclic voltammetry peak potentials are around 2.7 V V5. see. Loss of an electron from the carbonyl oxygen lone pair is considered to be the first stage in the reaction. In acetonitrile, two competing processes then ensue. Short chain, a-branched ketones cleave the carbon-carbonyl bond to give the more stable carbocation, which is then quenched by reaction with... 

Step 3 Forming a carbocation is difficult however, tertiary Ccirbocations, such as this one, can form as intermediates, or species that exist for a short time during the reaction. (See Figure 2-12.) The positive chcirge on the carbon makes this a strong electrophile that seeks a lone pair. In the final step of this mechanism, the carbocation accepts a lone pair from the chloride ion generated in the first step. The transfer is lone pair to bond. 

In the preceding mechanism, the carbocation was an intermediate (a species that exists for a short time during the reaction). The form of the intermediate is often essential to understanding the mechanism. The curved arrows help you in drawing the intermediate. Because you can use curved arrows in only three ways (bond to lone pair, bond to bond, and lone pair to bond), you have limited options for drawing intermediates. 

In Mechanisms 1 and 2 the reactions are considered to proceed via an equilibrium concentration of the free base. Mechanism 3, Scheme (4), which was first proposed by Wenkert and Liu [10], starts with protonation at Nb, the most basic site and so the most likely to be protonated first. After protonation, the C-3 - Nb bond is cleaved due to participation of the indole nitrogen lone pair, giving a carbocation intermediate. Ring reclosure is assisted by the Nb lone pair to effect the inversion at C-3. 

The capture of carbocations by alcohols involves a similar donation of a lone pair of electrons on oxygen to the vacant 2p atomic orbital of the sp2-hybridized, sextet carbocation. Note that charge must be conserved so the first formed product is a positively charged oxonium ion. 

Carbocations with a-heteroatom substituents such as trimethylsilyl and nitro groups that lack a stabilizing lone pair of electrons have also been prepared and studied.375,510... 

The carbocation is stabilised by delocalisation of the lone-pair electrons on the adjacent heteroatom. 

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