Carbocations, stability rearrangement

This IS a frequently used proce dure for the preparation of alkenes The order of alcohol reactivity paral lels the order of carbocation stability R3C > R2CH > RCH2 Benzylic al cohols react readily Rearrangements are sometimes observed... 

Each of the following carbocations can rearrange to a cation with special stabilization. Indicate likely routes for the rearrangement to a more stable species for each ion. 

How Does Carbocation Stability Control the Beckmann Rearrangement Reaction ... 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

In thermal free radical cracking, fi agmentation to small molecules occurs readily. In catalytic cracking carbocations rearrange more because the differences in carbocation stabilities are greater than the differences in free radical stabilities. 

Ammal SC, Yamataka H. How does carbocation stability control the beckmann rearrangement reaction In KK Laali, editor. Recent developments in carbocation and onium ion chemistry, ACS Symposium series, Vol. 965. New York American Chemical Society 2007, Chapter 17. 

SNl Favored for sterically hindered R carbocation is stabilized by polar solvent (30>2°>1°), therefore carbocation may rearrange racemic mixture first-order kinetics (formation of R+ determines the reaction rate). 

Scheme4.59. Ring opening of epoxides by nucleophilic attack or acid or base-mediated rearrangement. R = carbocation-stabilizing, sterically demanding group.

H OH —c—c— 1 1 h2so4 or h3po4 A V II n / E1 conditions dehydration Zaitsev s rule rate depends on carbocation stability watch for rearrangement... 

In general, we should expect rearrangements in reactions involving carbocations whenever a hydride shift or an alkyl shift can form a more stable carbocation. Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations. 

Like other El reactions, alcohol dehydration follows an order of reactivity that reflects carbocation stability 3° alcohols react faster than 2° alcohols, and 1° alcohols are the least reactive. Rearrangements of the carbocation intermediates are common in alcohol dehydrations. In most cases, Zaitsev s rule applies The major product is usually the one with the most substituted double bond. 

As already pointed out in systems with increased carbocation stabilization capabilities, mixed-mode rearrangement processes with varying participation of the ion-pair extreme come into play (Scheme 13). The rearrangement of the czs-configured sulfinate 50 (racemic 1 1 mixture of S epimers) delivers 70% of the expected czs-sulfone rac-51 and 30% of rac-52 obviously via ion-pair recombination [39]. As expected, increasing the leaving group ability of... 

Hague, A., Ghatak, A., Ghosh, S., Ghoshal, N. A Facile Access to Densely Functionalized Substituted Cyclopentanes and Spiro Cyclopentanes. Carbocation Stabilization Directed Bond Migration in Rearrangement of Cyclobutanes. J. Org. Chem. 1997, 62, 5211-5214. 

El must pass three tests a carbocation stability usually better than secondary, a polar solvent, and a leaving group with a p aHL of zero or less. Any route that goes via a carbocation like El may have rearrangement as a complication (Section 4.7). 

Strikingly, solvolysis of the parent (tosyloxymethyl)cyclopropane under identical conditions led predominantly to unrearranged cyclopropylmethanol. No rearrangement to a cyclobutane was observed either with carbocation stabilizing groups on the methyl carbon. ... 

The construction of a quantitative scale of carbocation stabilities based on hydride or halide exchanges would require the systematic overlapping equilibration" " of a large number of species (as in reactions (11) and (12)). This was the time-honored procedure for the construction of the gas-phase proton basicity scale." In the present case, this is difficult to achieve because of the widely different stabilities involved and because of the possibility of rearrangement of the ions. 

When benzene reacts with l-chloro-2,2-dimethylpropane, a primary carbocation rearranges to a tertiary carbocation. Thus, there is a greater increase in carbocation stability and, therefore, a greater amount of rearranged product—100% of the product (under all reaction conditions) has the rearranged alkyl substituent. 

---