Reagents and Mechanisms

Light alkanes do not undergo exothermic proton transfer reactions with H3O. However, as the molecular mass increases, the proton affinity of the alkane increases and therefore for larger alkanes the proton transfer process eventually becomes exothermic. Arnold and coworkers have estimated that the endothermic/exothermic crossover point occurs at hexane, such that all heavier alkanes should have clear exothermic proton transfer reactions with H3O+ [10]. For heptane and higher alkanes, fast reaction with H3O+ is seen but the rate coefficient falls short of that predicted from the collision-limiting models presented in Section 2.23.2. Furthermore, reaction is dominated not by proton transfer but instead by association, that is 

In contrast to non-cyclic alkanes, reaction of H3O+ with cyclic alkanes is exothermic, even for cyclopropane (proton affinity = 750 kJ moP ). Reaction is by simple non-dissociative proton transfer in the case of cyclopropane, but larger cycUc alkanes can undergo dissociative channels. For example methylcyclohexane reaction is dominated by H2 ejection (90% of the products are produced in this reaction channel) [18]. 

Reactions between short chain alkenes and H3O+ are more favourable thermodynamically but the reaction with ethene is still (marginally) endothermic and relatively slow [32]. However, propene and higher alkenes undergo proton transfer at the collisional rate and the small alkenes show 100% production of the protonated molecule. However, dissociative channels are important for 1-heptene and higher alkenes [33]. 

The proton transfer reaction between H3O+ and acetylene (C2H2) is substantially endothermic and therefore cannot occur [32,33]. However, propyne (C3H4), which has a proton affinity greater than H2O, undergoes fast non-dissociative proton reaction with H3O+. 

In aromatic hydrocarbons with saturated sidechains the protonation takes place at the aromatic ring. This leads to very simple behaviour in their reaction with H3O +, showing fast reaction (collision limited) with 100% production of the protonated parent species [33]. 

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Up-to-date information about reactions, reagents, and mechanisms has been added throughout. 

In Chapter 12, we discuss the oxidation and reduction of alkenes and alkynes, as well as compounds with polar C—X o bonds— alcohols, alkyl halides, and epoxides. Although there will be many different reagents and mechanisms, discussing these reactions as a group allows us to more easily compare and contrast them. 

Every new reaction in Chapter 21 involves nucleophilic addition, so the challenge lies in learning the specific reagents and mechanisms that characterize each reaction. 

The reaction of diazomethane and its derivatives with ketones has been a rich source of chemistry. The major part of the developmental work with regard to reagents and mechanisms emerged from extensive studies in this area. Relative to aldehydes, aliphatic ketones are much less reactive to diazomethane and... 

The large number of reagents that are available for animation necessitated a deviation from the standard Organic Reactions format. The section on Reagents and Mechanisms includes discussion and exemplification of each reagent or reagent class as well as comments on mechanism, particularly in context of reagent-substrate combinations that can lead to more than one product. Stereochemistry is discussed in the relevant sections of Scope and Limitations. 

Procedures are listed by type of reagent in the same order as in the section on Reagents and Mechanisms. 

This is connnonly known as the transition state theory approximation to the rate constant. Note that all one needs to do to evaluate (A3.11.187) is to detennine the partition function of the reagents and transition state, which is a problem in statistical mechanics rather than dynamics. This makes transition state theory a very usefiil approach for many applications. However, what is left out are two potentially important effects, tiiimelling and barrier recrossing, bodi of which lead to CRTs that differ from the sum of step frmctions assumed in (A3.11.1831. 

The rate of addition depends on the concentration of both the butylene and the reagent HZ. The addition requires an acidic reagent and the orientation of the addition is regioselective (Markovnikov). The relative reactivities of the isomers are related to the relative stabiUty of the intermediate carbocation and are isobutylene 1 — butene > 2 — butenes. Addition to the 1-butene is less hindered than to the 2-butenes. For hydrogen bromide addition, the preferred orientation of the addition can be altered from Markovnikov to anti-Markovnikov by the presence of peroxides involving a free-radical mechanism. 

Carbanions are very useful intermediates in the formation of carbon-carbon bonds. This is true both for unstabilized structures found in organometallic reagents and stabilized structures such as enolates. Carbanions can participate as nucleophiles both in addition and in substitution reactions. At this point, we will discuss aspects of the reactions of carbanions as nucleophiles in reactions that proceed by the 8 2 mechanism. Other synthetic aj lications of carbanions will be discussed more completely in Part B. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

The reduction of iminium salts can be achieved by a variety of methods. Some of the methods have been studied primarily on quaternary salts of aromatic bases, but the results can be extrapolated to simple iminium salts in most cases. The reagents available for reduction of iminium salts are sodium amalgam (52), sodium hydrosulfite (5i), potassium borohydride (54,55), sodium borohydride (56,57), lithium aluminum hydride (5 ), formic acid (59-63), H, and platinum oxide (47). The scope and mechanism of reduction of nitrogen heterocycles with complex metal hydrides has been recently reviewed (5,64), and will be presented here only briefly. 

Thus one of the transferred hydrogens conies from the aluminum reagent, and the other one from the solvent. In addition to the mechanism via a six-membered cyclic transition state, a radical mechanism is discussed for certain substrates. ... 

Coniine, molecular model of. 28 structure of, 294 Conjugate acid, 49 Conjugate base, 49 Conjugate carbonyl addition reaction, 725-729 amines and, 727 enamines and, 897-898 Gilman reagents and, 728-729 mechanism of, 725-726 Michael reactions and, 894-895 water and. 727 Conjugated diene, 482... 

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