Carbocation rearrangements mechanism

Since the problem of the carbocation rearrangement mechanism has been much attention to in the literature including the reviews, this section will briefly deal only with some aspects of the matter. 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

Evidence for the Mechanism of Electrophilic Additions Carbocation Rearrangements... 

Isoborneol (Problem 27.37) is converted into camphene on treatment with dilute sulfuric acid. Propose a mechanism for the reaction, which involves a carbocation rearrangement. 

Propose a mechanism for the biosynthesis of the sesquiterpene trichodiene from famesyl diphosphate. The process involves cyclization to give an intermediate secondary carbocation, followed by several carbocation rearrangements. 

The most common examples of rearrangements involve an electron-deficient atom, and pre-eminent amongst these are carbocations. Since carbocations are a feature of the SnI and El mechanisms, it follows that rearrangements can be side-reactions of these types of transformation. The driving force in carbocation rearrangements is to form a more stable carbocation. 

A-9. Solvolysis of the compound shown occurs with carbocation rearrangement and yields an alcohol as the major product. Write the structure of this product, and give a mechanism to explain its formation. 

The dehydration reaction has some limitations. Because the mechanism is El and involves a carbocation, rearrangements are possible. Figure 10.7 shows an example of a dehydration involving a carbocation rearrangement. In addition, the reaction is not... 

Follow the steps listed in the preceding Visual Summary of Key Reactions section. Identify the leaving group, the electrophilic carbon, and the nucleophile (or base). Then determine which mechanism is favored (see Section 9.7). Watch out for stereochemistry where important, regiochemistry in elimination reactions, and carbocation rearrangements when the mechanism is SN1 or El. 

This chapter presented many reactions. Although most of them follow the same general mechanism in which an electrophile adds to the carbon-carbon double bond, there are several variations on this mechanism. In addition, the regiochemistry and/or the stereochemistry of the reaction may be important. Complications, such as carbocation rearrangements, may occur. You will have an easier time remembering all these details if you organize the reactions according to the three variations of the mechanism that they follow. 

The mechanism for this reaction, shown in Figure 22.5, involves a carbocation rearrangement that occurs by an allowed [1,2] sigmatropic shift. The product of this rearrangement, a protonated ketone, is considerably more stable than the initial carbocation, so the migration is quite favorable. Another example of the pinacol rearrangement is provided in the following equation ... 

Primary cations can never be observed by NMR—they are too unstable. But secondary cations can, provided the temperature is kept low enough, sec-Butyl chloride in SO2CIF at -78 °C gives a stable, observable cation. But, as the cation is warmed up, it rearranges to the f-butyl cation. Now this rearrangement truly is a carbocation rearrangement the starting material is an observable car-bocation, and so is the product, and we should just look at the mechanism in a little more detail. 

The mechanism follows the usual path cyclization of linalyl diphosphate, followed by attack of the n electrons of the second double bond, produce an intermediate carbocation. A carbocation rearrangement occurs, and the resulting carbocation reacts with water to form an alcohol that is oxidized to give a-fenchone. 

Aristolochene, a hydrocarbon found in both pepper and tobacco, is bio-s> nthesized by the following pathway. Add curved arrows to show the mechanism of each step. Which steps involve alkene electrophilic addition(s), and which involve carbocation rearrangement(s) (The abbreviation H—A stands for an unspecified acid, and "Base" is an unspecified base in the enzyme.)... 

The dehydration of 3,3-dimethyl-2-butanol illustrates the rearrangement of a 2° to a 3° carbocation by a 1,2-methyl shift, as shown in Mechanism 9.3. The carbocation rearrangement occurs in Step [3] of the four-step mechanism. 

A 2° alcohol reacts with HBr by an SnI mechanism. Because substitution converts a 2° alcohol to a 3° alkyl halide in this example, a carbocation rearrangement must occur. 

Sometimes carbocation rearrangements can change the size of a ring. Draw a stepwise, detailed mechanism for the following reaction. 

Because syn addition to the double bond occurs and no carbocation rearrangements are observed, carbocations are not formed during hydroboration, as shown in Mechanism 10.5. The proposed mechanism involves a concerted addition of H and BH2 from the same side of the planar double bond the it bond and H-BH2 bond are broken as two new o bonds are formed. Because four atoms are involved, the transition state is said to be four-centered. 

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