Hydrogen mechanism for

Over Cu/MgO the transfer from 2-propanol is much faster than molecular H2 addition (entry 1, Table 2), thus excluding the dehydrogenation-hydrogenation mechanism. For the other donors, the reaction needs to carried out at 140°C to reach reasonable rates. 

Scheme 14.15 Proposed hydrogenation mechanism for the 1,4-hydrogenation of dienes by [Cr(CO)3 (arene)] (45).

Scheme 14.18 Proposed 1,4-hydrogenation mechanism for the hydrogenation of dienes by cationic rhodium complexes [P2Rh(diene)]+A (54).

Scheme 17.8 Concerted ionic hydrogenation mechanisms for the hydrogenation of (a) ketones and (b) C02. The acidic ligand is shown as an alcohol in Scheme 17.7 b, but could equally well be water, a secondary amine, or a carboxylic acid.

SCHEME 13.24 Proposed arene hydrogenation mechanism for heterogeneous catalysts. 

Scheme 16.2. The hot hydrogen mechanism for ECD of peptides. Electron capture at a charge center (solvated to carbonyl groups) is followed by hydrogen atom transfer to a backbone carbonyl, resulting in cleavage of N-C bonds. (Reproduced from Ref. 180 with permission from Elsevier Science.)...

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

Mixtures of chlorine and hydrogen reaa only slowly in the dark but the reaction proceeds with explosive violence in light. A suggested mechanism for the photochemical chain reaction is ... 

In our hydrogen molecule calculation in Section 2.4.1 the molecular orbitals were provided as input, but in most electronic structure calculations we are usually trying to calculate the molecular orbitals. How do we go about this We must remember that for many-body problems there is no correct solution we therefore require some means to decide whether one proposed wavefunction is better than another. Fortunately, the variation theorem provides us with a mechanism for answering this question. The theorem states that the... 

Butanol and 2 butanol are converted to their corresponding bromides on being heated with hydrogen bromide Write a suitable mechanism for each reaction and assign each the appropriate symbol (SnI or Sn2)... 

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m the reaction of alcohols with hydrogen halides Figure 5 6 portrays a three step mechanism for the acid catalyzed dehydration of tert butyl alcohol Steps 1 and 2 describe the generation of tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

FIGURE 6 1 A mechanism for heterogeneous catalysis in the hydrogenation of alkenes... 

In the mechanism for alkene hydrogenation shown m Figure 6 1 hydrogen atoms are transferred from the catalyst s surface to the alkene Although the two hydrogens are not transferred simultaneously they both add to the same face of the double bond... 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

According to the proposed mechanism for biological 0x1 dation of ethanol the hydrogen that is transferred to the coenzyme comes from C 1 of ethanol Therefore the dihydropyridme ring will bear no deuterium atoms when CD3CH2OH IS oxidized because all the deuterium atoms of the alcohol are attached to C 2... 

A mechanism for the cleavage of diethyl ether by hydrogen bromide is outlined m Figure 16 4 The key step is an 8 2 like attack on a dialkyloxonmm ion by bromide (step 2)... 

FIGURE 16 4 The mechanism for the cleavage of ethers by hydrogen halides using the reac tion of diethyl ether with hydrogen bromide as an example... 

The chiral recognition mechanism for these types of phases was attributed primarily to hydrogen bonding and dipole—dipole interactions between the analyte and the chiral selector in the stationary phase. It was postulated that chiral recognition involved the formation of transient five- and seven-membered association complexes between the analyte and the chiral selector (117). 

Dual Function Catalytic Processes. Dual-function catalytic processes use an acidic oxide support, such as alumina, loaded with a metal such as Pt to isomerize the xylenes as weH as convert EB to xylenes. These catalysts promote carbonium ion-type reactions as weH as hydrogenation—dehydrogenation. In the mechanism for the conversion of EB to xylenes shown, EB is converted to xylenes... 

---