Summary Carbocation Reactions

Rearrange to a more stable carbocation, then react further. 

React with its own leaving group to return to the reactant R + X -- 

React with a nucleophile to form a substitution product (S l) R + Nuc  

Lose a proton to form an elimination product (an alkene) (El)  

Give the substitution and elimination products you would expect from the following reactions. 

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First-Order Elimination The E1 Reaction 258 Key Mechanism 6-8 The E1 Reaction 258 Mechanism 6-9 Rearrangement in an E1 Reaction 261 Summary Carbocation Reactions 262 6-18 Positional Orientation of Elimination Zaitsev s Rule 263 6-19 Second-Order Elimination The E2 Reaction 265 Key Mechanism 6-10 The E2 Reaction 266 6-20 Stereochemistry of the E2 Reaction 267... 

Carbocation rearrangements, 94 Carbocation reactions, 45, 114 Carbonic acid derivatives, 358 Carbonyl group, reduction of, 261 resonance structures, 333 Carboxylic acids, summary of chemistry, 356 Catechol, 432 Center of symmetry, 69 Chair and boat forms, 168 Chemical shift, 2ilff Chiral center, 71... 

In summary, the reaction mechanism consists of two steps (i) the protonation of the double bond resulting in the formation of stable carbocation and (ii) the attack of the nucleophile (Br") on the positive carbon of the same carbocation. 

In Summary Carbocations formed in solvolysis reactions are not only trapped by nucleophiles to give SnI products but also deprotonated in an elimination (El) reaction. In this process, the nucleophile (usually the solvent) acts as a base. 

In summary, there now exists a body of data for the reactions of carbocations where the values of kjkp span a range of > 106-fold (Table 1). This requires that variations in the substituents at a cationic center result in a >8 kcal mol-1 differential stabilization of the transition states for nucleophile addition and proton transfer which have not yet been fully rationalized. We discuss in this review the explanations for the large changes in the rate constant ratio for partitioning of carbocations between reaction with Bronsted and Lewis bases that sometimes result from apparently small changes in carbocation structure. 

In summary, transifion-metal-catalyzed alkene-polymerization reactions highlight the metal-induced electrophilic activation of C—C n bonds to form carbo-cation-like alkene complexes. Considerations involving substituent pi-donor or pi-acceptor strength (i.e., tendency toward carbocation formation) will be useful in similarly rationalizing polymerization reactions (4.105) for more general alkenes. 

In summary, a good initiator will yield a stable, singly charged anion and a cation with a high reactivity (large kf) towards the alkenes concerned, via simple, well defined reactions. The remainder of this paper is concerned with finding, by appropriate calculations, some carbocation salts which best meet the specified requirements. 

In summary, the Danheiser reactions are a family of [3 + 2] annulations that take place between allenylsilanes and diverse electrophiles. The products can be carbo-cycles or heterocycles. In all cases, the annulations proceed most efficiently when the allenylsilane has a non-hydrogen substituent on the carbon atom bearing the silicon. This is a consequence of the common mechanistic pathway that proceeds through a vinyl carbocation intermediate. 

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. 

Summary Bis-l,2-[(bromodiphenyl)methyl]-l,l,2,2-tetramethyldisilane (3) was prepared by bromination of the corresponding (diphenylmethyl)disilane with NBS in CCU A solution of 3 in CHCb loses HBr and forms a silacycle 3,3,5,5-Tetraphenyl-l-oxa-2,4-disila-cyclopentane (10) was found as the reaction product of 3 with H2O. The formation of the silacycles can be explained by an intramolecular rearrangement of the disilane 3 with a carbocation intermediate. Single crystal X-ray diffraction analysis has been performed on two silacycles. 

In summary, it appears that bromination usually involves a complex that collapses to an ion pair intermediate. The ionization generates charge separation and is assisted by solvent, acids, or a second molecule of bromine. The cation can be a P-carbocation, as in the case of styrenes, or a bromonium ion. Reactions that proceed through bromonium ions are stereospecific anti additions. Reactions that proceed through open carbocations can be syn selective or nonstereospecific. 

In summary, the results discussed above reveal that an ether oxygen in a medium or a bicyclic ring system may offer significant anchimeric assistance in reactions in solvolytic displacement reactions. Because of inductive effects, steric effects, field effects, and internal return, anchimeric assistance, when present, is often difficult to demonstrate kinetically. Products formed from bicyclic and tricyclic oxonium ions probably more often result from participation after carbocation formation. Participation by oxygen apparently occurs to the exclusion of transannular hydride shifts in medium ring systems, but other types of participation such as homoallylic or formation of cyclopropyl carbinyl cations will most likely occur to the exclusion of ether oxygen participation. 

In summary, EL-177 is a new and effective herbicide with a wide spectrum of activity on many broadleaf and grass weeds. EL-177 is being developed as a preemergent herbicide on corn and as a postemergent herbicide on cereals. A number of synthetic routes for the preparation of EL-177 have been developed. The application of a regioselective t-butylation reaction not only provided a more economic synthesis of EL-177, but through the use of tertiary carbocation chemistry an expansion of the 5-cyano-1-alky 1-lli-pyrazole-4-carboxamide series was accomplished. 

In Summary Treatment of alcohols with strong acid leads to protonalion to give alkyl-oxonium ions, which, when primary, undergo 8 2 reactions in the presence of good nucleophiles. AUcyloxonium ions from secondary or tertiary alcohols convert into carbocations, which furnish products of substitution and elimination (dehydration). 

In Summary Alkenes can be made by dehydration of alcohols. Secondary and tertiary systans proceed through carbocation intermediates, whereas primary alcohols can undergo E2 reactions from the intermediate alkyloxonium ions. All systems are subject to rearrangement and thus frequently give mixtures. 

The summary given at the end of Section 27-3 highlighted the fact that S l and El reactions always occur together. We also know that once the carbocation in these reactions is formed, it is susceptible to attack from a variety of species. Consequently, the S l and El reactions of haloalkanes are not often used for synthesizing other organic compounds. 

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