Alkyl substituents charge stabilization

This reaction can be understood by assuming that in it the carbocation intermediate is formed which leads to product II, which is more stable than the carbocation intermediate leading to product I (Following fig.). It is possible to predict the more stable carbocation by counting the number of alkyl groups attached to the positive centre. The more stable carbocation on the right has three alkyl substituents attached to the positively charged carbon whereas the less stable carbocation on the left only has one such alkyl substituent. The stability of carbocation is 3 >2 > 1. 

These substituent effects are presumably due to stabilization of the cation that is generated by protonation at the center carbon. Even if the allylic conjugation is not effective in the transition state, the aryl and alkyl substituents can stabilize the charge that develops. 

Carbocations are stabilized by alkyl substituents attached directly to the positively charged carbon Alkyl groups are electron releasing sub stituents Stability increases in the order... 

Draw the structures of the two carbocation intermediates that might form during the reaction of 2-methylpropene with HBr (Problem 5.40). We ll see in the next chapter that the stability of carbocations depends on the number of alkyl substituents attached to the positively charged carbon—the more alkyl substituents there are, the more stable the cation. Which of the two carbocation intermediates you drew is more stable ... 

In the solvolysis of secondary alkyl sulfonates, competition between nucleophilic solvation and electron donation by the substituents results in a significantly solvent-dependent p, which varies from — 9 to — 1 on going from the non-nucleophilic hexafluoro-2-propanol to 80% aqueous ethanol (Bentley et al, 1981). In contrast, the p -invariance for alkene bromination in H20, M70, MeOH and AcOH [equations (22)-(25)] seems to imply a perfect balance between the two types of charge stabilization. However, this conclusion is probably risky since the nucleophilicities of the solvents implied in (22)-(25) do not vary markedly. Data in non-nucleophilic fluorinated solvents would therefore help to fill the gap in our knowledge. 

The alkoxy substituent allows a delocalization of the positive charge. If the substituent were not present (e.g., in ethylene), the positive charge would be localized on the single a-carbon atom. The presence of the alkoxy group leads to stabilization of the carbocation by delocalization of the positive charge over two atoms—the carbon and the oxygen. Similar delocalization effects occur with phenyl, vinyl, and alkyl substituents, for example, for styrene polymerization ... 

Whereas substitution of hydrogen directly attached to the cationic center by alkyl or aryl has a large stabilizing effect, substituents at more remote positions are more subtle in their influence. We might expect, on the basis of the ability of alkyl groups to stabilize charge, that cation 6 should be more stable than cation 5. This prediction is indeed correct for the isolated ions in the gas phase. In... 

The first carbene compound to be well characterized was prepared in 1966 and was one of many Fischer-Type Carbene Complexes to be reported (see equation 7). Fischer carbenes are characterized by heteroatom substituents at the carbene carbon, stabilization by a low-valent metal center, and a partial positive charge at the carbene carbon. In contrast, Schrock-Type Carbene Complexes, or alkylidenes," that have alkyl substituents, are found on metal centers in higher oxidation states, and are nucleophihc at carbon. Many Fischer carbenes are known for chromium, whereas chromium alkylidenes are much less common. Monohalocarbenes of chromium, for example, (OC)5Cr=C(F)NEt2, have also been extensively investigated." Two carbene reactions of note for their application to organic synthesis are the cycloaddition of alkenes with carbene complexes and the reaction of aromatic carbenes with aUcynes to yield complexed naphthols (the Dotz reaction ). ... 

Reactions in acid often involve the formation of carbocations—trivalent, positively charged carbon atoms—as intermediates. The order of stability of carbocations containing only alkyl substituents is 3° > 2° > 1° > CHj. Cation stability is influenced by several factors ... 

The consistancy of the observed /4-factors, the magnitude of the activation entropies, the a correlations in the substituent effects at the a- and /8-carbon positions, the collective influence of all three substitutable centers on the reaction rates, the importance of charge stabilization in the transition state, and the primary deuterium isotope effects on the alkyl acetic acid ester decomposition, all favor the concerted polar 6-center transition state shown below (IV). However, an alternative possibility involving intimate ion-pair formation has been proposed by Scheer et al.. ... 

If one compares the 1,2,3-triphenylcyclopropyl anion as well as the cyclopropyl anion 320" with the many more cyclopropyl anions not showing the ring-opening reaction such as all cyclopropyl anions with H or alkyl groups at C(l) like the parent 323 or Walborsky s 324" it is immediately clear that substituents which stabilize a negative charge at both carbon atoms which become terminal centers of the allyl anion facilitate the ring scission. If the cyclopropyl anion is prepared by deprotonation of a cyclopropane it is... 

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