Vacant p-orbital

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

The positive charge on carbon and the vacant p orbital combine to make carbo cations strongly electrophilic ( electron loving or electron seeking ) Electrophiles are Lewis acids (Section 117) They are electron pair acceptors and react with Lewis bases (electron pair donors) Step 3 which follows and completes the mechanism is a Lewis... 

FIGURE 4 11 Combi nation of tert butyl cation and chloride anion to give tert butyl chloride In phase overlap between a vacant p orbital of (CHbIbC and a filled p orbital of Cr gives a C—Cl (T bond... 

An orbital overlap description of electron delocalization mil dimethylallyl cation H2C=CH—C(CH3)2 is given m Figure 10 2 Figure 10 2a shows the rr bond and the vacant p orbital as independent units Figure 10 2b shows how the units can overlap to give an extended rr orbital that encompasses all three carbons This permits the two rr electrons to be delocalized over three carbons and disperses the positive charge... 

This reanangement is shown in orbital terms in Figure 5.8. The relevant orbitals of the secondary car bocation are shown in structure (a), those of the transition state for reanangement in (b), and those of the tertiary carbocation in (c). Delocalization of the electrons of the C—CH3 a bond into the vacant p orbital of the positively charged car bon by hyperconjugation is present in both (a) and (c), requires no activation energy, and... 

O A hydrogen atom on the electrophile HBr is attacked by tt electrons from the nucleophilic double bond, forming a new C-H bond. This leaves the other carbon atom with a + charge and a vacant p orbital. Simultaneously, two electrons from the H-Br bond move onto bromine, giving bromide anion. 

Figure 6.9 The structure of a carbocation. The trivalent carbon is sp -hybridized and has a vacant p orbital perpendicular to the plane of the carbon and three attached groups.

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C-H bonds in the carbocation are aligned for hyperconjugation with the vacant p orbital on the positively charged carbon. 

The dichlorocarbene carbon atom is syj- -hybridized, with a vacant p orbital extending above and below the plane of the three atoms and with an unshared pair of elections occupying the third sp2 lobe. Note that this electronic description of dichlorocarbene is similar to that for a carbocation Section 6.9) with respect to both the sp2 hybridization of carbon and the vacant p orbital. Electrostatic potential maps further show this similarity (Figure 7.6). 

Figure 8.2 The structure of a secondary vinylic carbocation. The cationic carbon atom is sp-hybridized and has a vacant p orbital perpendicular to the plane of the tt bond orbitals. Only one R group is attached to the positively charged carbon rather than two, as in a secondary alkyl carbocation. The electrostatic potential map shows that the most positive (blue) regions coincide with lobes of the vacant p orbital and are perpendicular to the most negative (red) regions associated with the ir bond.

Hyperconjugation (Sections 6.6, 6.9) An interaction that results from overlap of a vacant p orbital on one atom with a neighboring C-H a bond. Hyperconjugation is important in stabilizing carbocations and in stabilizing substituted alkenes. 

A single-crystal. X-ray diffraction analysis of the structure has recently been performed that shows that the compound is, in fact, a tin-tin bonded dimer, having an Sn-Sn bond length of 276 pm, similar to that in hexaphenylditin this was interpreted in terms of overlap of a filled spaPy orbital with the vacant p orbitals on the other tin atom resulting in a "bent, weak, Sn-Sn double bond (332). 

In the interaction of a pair of atomic orbitals, two electrons form a bond and four electrons form no bond (Sect. 1.1). The snbstitnted carbocations are stabilized by the electron delocalization (hyperconjngation and resonance) through the interaction of the doubly occupied orbitals on the snbstitnents with the vacant p-orbital on the cation center. The exchange repulsion (Sect. 1.5) is cansed by four electrons. Now... 

Almost all known inorganic heterocychc molecules, where N, O and S atoms with lone pair orbitals are donors while B atoms with vacant p orbitals are acceptors, are classified into discontinuous conjugation. The donors and the acceptors are alternately disposed along the cyclic chain. The thermodynamic stabilities are controlled by the non-cycUc electron delocalization or by the number of neighboring donor-acceptor pairs, but not by the number of % electrons [83]. In fact, both 4n % and 4n + 2% electron heterocycles are similarly known [84,85] (Scheme 33), contradicting the Hueckel rule. 

Cyclic conjugation is continuous in l,2-dihydro-l,2-azaborine with one N-B bond (Scheme 34). The nitrogen atom with a lone pair is donor. The B atom with a vacant p orbital is acceptor. Whether the remaining C=C bonds are donors or accepters, the donors are disposed on one side of the cychc chain while the acceptors are on the other side. The orbital phase property or the number of electrons is important. The phase continuity or the six n electrons predicts that 1,2-dihydro-l-,2-azaborine could be aromatic. 

Two reasons may be offered for the enhanced /3-deuterium isotope effect in vinyl cations as compared with carbonium ions (193). As pointed out by Noyce and Schiavelli (21), in the transition state of a vinyl cation, the isotopically substituted C—H bond is ideally suited for overlap with the developing vacant p orbital, as the dihedral angle between the empty p orbital and C—H bonds is zero in the intermediate, as shown in structure 239. Shiner and co-workers (195)... 

Closely related to, but distinct from, the anionic boron and aluminum hydrides are the neutral boron (borane, BH3) and aluminum (alane, A1H3) hydrides. These molecules also contain hydrogen that can be transferred as hydride. Borane and alane differ from the anionic hydrides in being electrophilic species by virtue of the vacant p orbital and are Lewis acids. Reduction by these molecules occurs by an intramolecular hydride transfer in a Lewis acid-base complex of the reactant and reductant. 

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