Resonance carbocation stabilization

Another structural feature that increases carbocation stability is the presence, adjacent to the cationic center, of a heteroatom bearing an unshared pair," for example, oxygen," nitrogen," or halogen. Such ions are stabilized by resonance ... 

Therefore in general the carbocation stability is increased due to resonance. 

When allenes are attacked by electrophilic reagents,95 Markovnikov s rule would predict that the attack should be at the end of the system, since there are no hydrogens in the middle. Attack at the center gives a carbocation stabilized by resonance, but not immediately. 

Since fluorine is the most electronegative element, it should inductively destabilize carbocations. The stability of fluoromethyl cations in the gas phase decreases in the order CFH2+ > CF2H+ > CF3+ > CH3+. The trend in solution, however, could be different, due to solvent effects, ion pairing, and so on. Indeed, fluorine has been shown to provide stabilization for carbocations. The existence of CH3CF2+, in contrast to the elusive ethyl cation CH3CH2+, is a clear evidence that replacement of H atoms by F atoms provides stabilization for carbocations.524 Furthermore, it was found that in perfluorobenzyl cation C6F5CF2+ fluorine atoms in resonance positions (ortho and para) are more deshielded than those in meta positions.536 This indicates carbocation stabilization by back-donation. 

Hehre et 3 . who have used ion-cyclotron resonance techniques to obtain criteria of aliphatic and aromatic carbocation stabilities. 

The principles of inductive effects and resonance effects, first introduced in Section 18.6, can now be used to predict carbocation stability. 

Carbocations stabilized by resonance with a lone pair are more stable than those stabilized only by resonance with a carbon-carbon double bond, which in turn are more stable than those stabilized only by alkyl group substitution. The effects are additive three of a lesser type of stabilization are at least as good as one of the better type. 

Carbocations can also be stabilized through resonance by neighboring lone pairs or pi-electrons. In general, this stabilization is greater than one degree of substitution, so a secondary carbocation stabilized by resonance will be more stable than a tertiary carbocation with no resonance stabilization, and a primary carbocation stabilized by resonance will be more stable than a secondary carbocation with no resonance stabilization. 

Resonance forms are classical structures used to describe a more complex system they do not actually exist. The species is more accurately described by a resonance hybrid which can be imagined as an average of the resonance forms. Resonance always stabilizes a system. Each atom in a resonance stabilized system has a p-orbital. Allylic carbocations are stabilized by delocalization of the positive charge. 

Figure 2.7 Effect on carbocation stability of resonance stabilization by conjugation with pi bonds.

To involve allylic resonance in stabilizing the carbocation intermediate formed when the electrophile bonds to C-2, the benzene-like character of the other ring is sacrificed. 

Carbocations stabilized by resonance are usually prominent in mass spectra. Several ways that resonance-stabilized carbocations can be produced are outlined in the following list. These examples begin by illustrating the likely sites for initial ionization (ir and nonbonding electrons), as well. 

Draw all of the contributing resonance structures and the resonance hybrid for the carbocation that would result from ionization of bromine from 5-bromo-1,3-pentadiene. Open the computer molecular model at the book s website depicting a map of electrostatic potential for the pentadienyl carbocation. Based on the model, which is the most important contributing resonance structure for this cation Is this consistent with what you would have predicted based on your knowledge of relative carbocation stabilities ... 

Radical stability parallels carbocation stability Benzylic and allylic are more stable than tertiary, tertiary more stable than secondary, secondary more stable than primary, and primary more stable than methyl. The determining factors are resonance and hyperconjugative stabilization. 

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