Benzylic cations stability

Effects of oxygen substitutents in an aromatic ring upon an exocyclic rather than endocyclic carbocation charge center have also been measured. The possibility of comparing HO, MeO, and O substituent effects for the benzylic cations is provided by recent studies of quinone methides, including the unsubstituted / -quinone methide 23, which may be considered as a resonance-stabilized benzylic cation with a /xoxyanion substituent. 

Reactions at Saturated Carbons trans ting junction as the new formed bond would be pseudoequatorial. The five-membered ring D is produced because the resonance-stabilized benzyl cation 240 is formed in preference to the six-membered ring homobenzylic cation 241. 

In Section 3.5.1, it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated alkenes react with Br2 hut not... 

In Section 3.5.1 it was mentioned that Br2 and Cl2 form resonance-stabilized benzyl cation intermediates with styrene derivatives and that gem-dialkylated olefins react with Br2 but not with Cl2 via halonium ions. Because C—Cl bonds are shorter than C—Br bonds, chloronium ions presumably have a higher ring strain than bromonium ions. Accordingly, a /3-chlorinated tertiary carbenium ion is more stable than the isomeric chloronium ion, but a /3-brominated tertiary carbenium ion is less stable than the isomeric bromonium ion. 

Compounds containing aromatic rings tend to fragment at the carbon (called a benzylic carbon) next to the aromatic ring. Such a cleavage forms a resonance-stabilized benzylic cation. 

In each case, the bromine atom ends up on the more substituted carbon, and the mechanism explains why. There are the two possible outcomes for protonation of styrene by HBr, but you should immediately be able to spot which is preferred, even if you don t know the outcome of the reaction. Protonation at one end gives a stabilized, benzylic cation, while protonation at the other would give a highly unstable primary cation, and therefore does not take place. The benzylic cation gives the benzylic alkyl bromide. 

Similarly, several nitriles (such as CICH2CN and o-MeCsHtCN) which tend to react poorly in the conventional reaction now reacted most efficiency (98 and 83%, respectively) with the stabilized benzyl cation. Tertiary carbenium ions were so stabilized that no Ritter reaction took place and the behavior of secondary ions depended on the particular case (Scheme S3). Complexed propargyl alcohol (111) was converted into the complexed acetamide (112) without encountering the customary acid-catalyzed rearrangement problems (equation 48). 

In this case the intermediate formed by cleavage of the cyclopropane ring is a stabilized benzyl cation which does not rearrange prior to cyclization. It was found that alkoxy substituents in the aromatic rings were essential for formation of tetralones in useful yields. ... 

Mass Spectrometry The most common mass spectral fragmentation of alkylben-zene derivatives is the cleavage of a benzylic bond to give a resonance-stabilized benzylic cation. For example, in the mass spectrum of -butylbenzene (Figure 16-18),... 

Mercuric salt cleavage of o- and p-nitrophenylcyclopropanes takes the expected course (Scheme 35) and bromination in alcohol similarly follows a course which may be rationalized by cleavage of the substituted cyclopropane bond to the stabilized benzylic cation which is quenched with solvent. ... 

When the carbenium ion can be stabilized in a manner other than with a bridging bromine, the preference for anti addition diminishes. For example, cis- and frans-l-phenyl-propene give only 73% and 83% products from anti addition, respectively, while the remaining products are from syn addition (Eqs. 10.17 and 10.18). Here, there is an equilibrium between two forms of the carbenium ion a cyclic bromonium ion and an open, resonance stabilized benzyl cation. Syn addition and anti addition products can arise from the open form due to bond rotation, but only anti addition products can arise from the cyclic form. 

---