Carbocation intermediate forming

Write a structural formula for the carbocation intermediate formed in the dehydration of each of the alcohols in Problem 5 14 (Section 5 10) Using curved arrows show how each carbocation is deprotonated by water to give a mixture of alkenes... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

The first step in alkylation is the generation of a carbocation (carbo-nium ion). When an olefin is the alkylating agent, a carbocation intermediate forms. 

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... 

Aromatic compounds such as benzene react with alkyl chlorides in Ihe presence of AlCl i catalyst to yield alkylbenzenes. The reaction occurs through a carbocation intermediate, formed by reaction of the alkyl chloride with AICI3 (R—Cl + A1CI 1 - U+ + AICl4 ). How can you explain the observaiion that reaction of benzene with 1-chloropropane yields isopropylbenzene as the major product ... 

The following model is that of an allylic carbocation intermediate formed by protonation of a conjugated diene with HBr. Show the structure of the diene and the structures of the final reaction products. 

The carbocation intermediate formed in the first step of the addition is trigonal plannar and is achiral. 

FIGURE 3. The C-metal a-n conjugation in the carbocation intermediate formed in the hydride transfer... 

The nature of the substituents on the allene can have an impact on the outcome of a [2 + 2] cycloaddition reaction, as was illustrated by the Lewis acid catalyzed cycloadditions of l-thioaryl-3,3-dimethylallene (24) and 1 -methyl- 1-trimethylsilylallene to various 2-alkoxy-p-benzoquinones 25 (e.g. equation 8)17. The reactions were considered to proceed via carbocation intermediates formed by nucleophilic attack of the thioallene on the Lewis acid activated quinone. At lower temperatures, these carbocations closed to cyclobutanes 26, whereas at higher temperatures, the thermodynamically more stable benzofurans 27 were formed. 

The selectivity of ionic hydrogenation strongly depends on the stability of the carbocation intermediate formed. It is possible to carry out selective monodeutera-tion by deuterated silanes of trialkylsusbtituted alkenes at the tertiary carbon atom.219 The transformation of the optically active (—)-30 (63% optical purity) yielded the cyclized saturated product with the same optical purity as the starting diene 220... 

The acid-catalyzed reactions proceed through a carbocation intermediate formed by protonation of the double bond, followed by nucleophilic attack of HN3 and subsequent loss of a proton (equation 181). 

Anodic oxidation of carboxylic acids bearing a trimethylsilyl group on the -position gives exclusively terminal olefins in rather good yields. The reaction seems to proceed via a carbocation intermediate formed by the oxidative elimination of CO2 (equation 42)46. 

The carbocation intermediate formed in the addition of hydrogen iodide to the alkene is one which can rearrange by a methyl group migration. 

These addition reactions are similar to the addition reactions of alkenes. However, the reaction is much slower for an alkyne, because alkynes are less reactive. Alkynes can be expected to be more nucleophilic, as they are more electron rich in the vicinity of the multiple bond, that is, six electrons in a triple bond as compared to four in a double bond. However, electrophilic addition to an alkyne involves the formation of a vinylic carbocation (Following fig.). This carbocation is much less stable than the carbocation intermediate formed during electrophilic addition to an alkene. 

Inhibitors of squalene synthase have recently received considerable attention in the search for new antihypercholesterolemic agents. BP-based inhibitors were designed as mime tics of the putative carbocation intermediate formed from FPP in the initial step of the squalene synthase reaction. (See also mechanism of action of nitrogen-containing BPs and Fig. 15.5.) The BP function served as a stable surrogate for the pyrophosphate moiety [27]. 

The following structure represents the carbocation intermediate formed in the addition reaction of HCl to an alkene. Draw the structure of the alkene (gray = C, ivory = H). 

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