Carbocations p orbitals

In the conformation shown, only the methyl-group C—H that is parallel to the carbocation p orbital can show hyperconjugation. 

Hyperconjugation has a profound effect on structure and stability of cyclohexyl cations. An elegant study combined theoretical results with experimental data to confirm that different hyperconjugative stabilization patterns lead to the formation of two equilibrating conformers of the 1-methyl-1-cyclohexyl cation where the carbocation p-orbital is oriented either pseudoaxiaUy or pseudoequatoriaUy. 

In the case of a carbocation, the unfilled orbital is the vacant p orbital of the carbocation, and the filled orbitals are C — H or C — C sigma bonds at the carbons adjacent to the p orbital of the carbocation. Sharing of electron density from adjacent C—H or C — C sigma bonds with the carbocation p orbital delocalizes the positive charge. 

Tertiary carbocations are stabilized because sigma bonds at three adjacent carbons contribute electron density to the carbocation p orbital by hyperconjugation (Section 6.1 IB). Secondary and primary carbocations have less stabilization by hyperconjugation. A methyl carbocation has no stabilization. Formation of a relatively stable carbocation is important in an S l reaction because it means that the free energy of activation for the... 

Figure 6.7 shows a stylized representation of hyperconjugation between a sigma bonding orbital and an adjacent carbocation p orbital. 

SnI displacements normally give mixtures of stereoisomeric products. The high-energy carbocation intermediate reacts with the first nucleophilic species it encounters, regardless of which lobe of the carbocation p orbital the nucleophile approaches. The example shown below is a very unusual... 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

The preferred alignment of orbitals for a 1,2-hydride or 1,2-alkyl shift involves coplanarity of the p orbital at the carbocation ion center and the a orbital of the migrating group. 

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... 

Hyperconjugation, as it i termed, implies that the electron pair associated with out-of-plane CH bond is donated into the empty p Orbital at the carbocation center. 

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.

A great deal of evidence has shown that carbocations are planar. The divalent carbon is 5p2-hybridized, and the three substituents are oriented to the corners of an equilateral triangle, as indicated in Figure 6.9. Because there are only six valence electrons on carbon and all six are used in the three a bonds, the p orbital extending above and below the plane is unoccupied. 

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 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 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.

Before seeing how electrophilic aromatic substitutions occur, let s briefly recall what we said in Chapler 6 about electrophilic alkene additions. When a reagent such as HCl adds to an alkene, the electrophilic hydrogen approaches the p orbitals of the double bond and forms a bond to one carbon, leaving a positive charge at the other carbon. This carbocation intermediate then reacts with the nucleophilic Cl- ion to yield the addition product. 

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. 

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... 

However, in this structure, boron does not have an octet. It has an empty p orbital, (very similar to a carbocation, except there is no positive charge here). Therefore, borane is very reactive. In fact, it reacts with itself to give dimeric structures, called diborane ... 

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