Electrostatic potential maps allyl cation

Electrostatic potential map for delocalized allyl cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

Draw Lewis structures for allyl cation. Where is the positive charge Examine atomic charges as well as the electrostatic potential map for localized and delocalized forms of allyl cation. Which carbon (s) carries the charge in each ... 

FIGURE 10.1 An electrostatic potential map for allyl cation. The middle carbon (red region) has the least positive charge of the three carbons the end carbons (blue regions) have the most positive charge. 

Figure 10.1 gives an orbital overlap view of bonding in allyl cation. The planar structure of H2C=CHCH2" (Figure 10.1a) provides a framework that allows for continuous overlap of the 2p orbitals of the three adjacent 5/7 -hybridized carbons (Figure lO.lfo and c). Until now, we have only seen rr orbitals involving two adjacent carbons. Conjugated systems are characterized by extended rr orbitals that encompass three or more atoms. The electrostatic potential map (Figure 10.Id) shows the equal sharing of positive charge between the first and third carbons of 2C...

Bonding in allyl cation, (a) All of the atoms of H2C=CHCH2 lie in the same plane. Each carbon is sp -hybridized. (b) The alignment of the tt component of the double bond and the vacant p orbital permits overlap between them, (c) A tt orbital encompasses all three carbons of H2C=CHCH2. The two electrons in this orbital are delocalized over three carbons, id) An electrostatic potential map shows the positive charge to be shared equally by the two end carbons. 

The electrostatic potential map (bottom) shows the unequal charge distribution in a methyl-substituted allyl cation. 

The carbocation intermediate in these reactions is a single species, a resonance hybrid. This type of carbocation, with a carbon-carbon double bond adjacent to the positive carbon, is called an allylic cation. The parent allyl cation, shown below as a resonance hybrid, is a primary carbocation, but it is more stable than simple primary ions (such as propyl) because its positive charge is delocalized over the two end carbon atoms as shown in the electrostatic potential map accompanying eq. 3.34. 

Part Three. The benzyl (and allyl) halides are a special case they have resonance. To see how the charge is delocalized in the benzyl carbocation, request two plots the electrostatic potential mapped onto a density surface and the LUMO mapped onto a density surface. Submit these for calculation at the AMI semiempirical level. On a piece of paper, draw the resonance-contributing structures for the benzyl cation. Do the computational results agree with the conclusions you draw from your resonance hybrid ... 

Allylic carbocations, like allylic radicals (Section 8.6), have a double bond next to the electron-deficient carbon. The allyl cation is the simplest allylic carbocation. Because the allyl cation has only one substituent on the carbon bearing the positive charge, it is a primary allylic carbocation. Allylic carbocations are considerably more stable than comparably substituted alkyl carbocations because delocalization is associated with the resonance interaction between the positively charged carbon and the adjacent tt bond. The allyl cation, for example, can be represented as a hybrid of two equivalent contributing structures. The result is that the positive charge appears only on carbons 1 and 3, as shown in the accompanying electrostatic potential map. 

Electrostatic potential map for the allyl cation. The positive charge (blue) is on carbons 1 and 3. 

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