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Carbocations high-energy

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... [Pg.276]

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. [Pg.381]

In the El mechanism, the leaving group has completely ionized before C—H bond breaking occurs. The direction of the elimination therefore depends on the structure of the carbocation and the identity of the base involved in the proton transfer that follows C—X heterolysis. Because of the relatively high energy of the carbocation intermediate, quite weak bases can effect proton removal. The solvent m often serve this function. The counterion formed in the ionization step may also act as the proton acceptor ... [Pg.383]

According to the Hammond postulate (Section 6.10), any factor that stabilizes a high-energy intermediate also stabilizes the transition state leading to that inlermediate. Since the rate-limiting step in an S l reaction is the spontaneous, unimolecLilar dissociation of the substrate to yield a carbocation, the reaction is favored whenever a stabilized carbocation intermediate is formed. The more stable the carbocation intermediate, the faster the S l reaction. [Pg.376]

A common feature of these intermediates is that they are of high energy, compared to structures with completely filled valence shells. Their lifetimes are usually very short. Bond formation involving carbocations, carbenes, and radicals often occurs with low activation energies. This is particularly true for addition reactions with alkenes and other systems having it bonds. These reactions replace a tt bond with a ct bond and are usually exothermic. [Pg.861]

We consider the relatively high pKA values of 6-8 to be typical value for a cation-quinone methide equilibrium. The formation of a resonance-stabilized aromatic carbocation is one reason for these high pKA values. Another reason is the high energy of the quinone methide. The thermodynamic cycle shown in... [Pg.257]

Saturated aliphatic compounds represent the other extreme. Since they do not have n electrons ionisation, the molecular ion requires removal of electrons from o orbitals and so high energies. Therefore the formation of molecular ion is difficult. Further, since, the formation of molecular ion is difficult and since this is quite unstable, it cleaves easily into more stable carbocations. [Pg.269]

Recentiy published crystal structures of antibody 4C6, an antibody that catalyzes another cationic cyclization reaction (Figure 6), revealed that this antibody has exquisite shape complementarity to its eliciting hapten 5. The active site contains multiple aromatic residues which shield the high-energy intermediate from solvent and stabilize the carbocation intermediates through cation-7r interactions. [Pg.327]

Since the reaction rate is determined by how well the transition state of the rate determining step stabilised. In a situation in which a high energy intermediate is formed (i.e. the carbocation), the transition state leading to it will be closer in character to the intermediate than the starting material. Therefore, any factor that stabilises the intermediate carbocation also stabilises the transition state and consequently increases the reaction rate. [Pg.203]

Reactive intermediate (Section 8.6) A high-energy, reactive species, such as a carbocation, that is formed along a reaction pathway. Under most conditions it has a very short lifetime. Reduction reaction (Section 10.14) A reaction that results in a decrease in oxygen content of the compound and/or an increase in hydrogen content... [Pg.1276]

Effect of the Substrate The structure of the substrate (the alkyl halide) is an important factor in determining which of these substitution mechanisms might operate. Most methyl halides and primary halides are poor substrates for SnI substitutions because they cannot easily ionize to high-energy methyl and primary carbocations. They are relatively unhindered, however, so they make good Sn2 substrates. [Pg.256]

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. [Pg.1297]


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See also in sourсe #XX -- [ Pg.539 ]




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