Of elimination reactions

Reaction (11) will increase the apparent fraction of tin nonspecific route for calomel formation and consequently decrease the enrichment. The increase in enrichment and the decrease in [Pg.233]

The reaction generator will produce so many possible reactions that the program has to filter out those which are obviously unimportant. The two basic techniques are the elimination of very endothermic reactions, and the elimination of reactions which are too slow. 

We presented here a conceptual framework and algorithms for the synthesis of pathways or mechanisms given a set of steps. The algorithms have been applied to catalytic reaction systems and to biochemical pathways. The basic approach is based on successive processing and elimination of reaction intermediates that should not appear in the net stoichiometry of the overall reactions accomplished. 

A simplified method to find the conversions in the two reactions is available as will be shown below, but a general method which can solve any ehemieal equilibrium problem is preferred. For this purpose two methods may be used. The first is minimisation of the Gibbs free energy [316], whereas the other one is the solution for conversions [468]. The first one may be attractive from a theoretical point of view and it is readily combined with phase equilibrimn, but the last one is preferred in catalysis, since no combination of reactions may proceed in all cases. The set of equations in Table 1.2 may be solved using the Newton-Raphson method with the conversions as independent variables. Some of the components (higher hydrocabons or oxygen) may almost disappear in the final mixture so it is necessary to handle elimination of reactions with almost complete conversion. 

Other publications have mentioned an air-breathing fuel ceU in their titles (e.g., Jaouen et al., 2005), but it must be pointed out that on the whole, the important problem of fuel cell operation with a passive air snpply from the ambient atmosphere and a passive elimination of reaction prodncts and heat has not been investigated sufficiently. 

Claisen reaction Condensation of an aldehyde with another aldehyde or a ketone in the presence of sodium hydroxide with the elimination of water. Thus benzaldehyde and methanal give cinnamic aldehyde, PhCH CH-CHO. 

Condensation polymerization differs from addition polymerization in that the polymer is formed by reaction of monomers, each step in the process resulting in the elimination of some easily removed molecule (often water). E.g. the polyester polyethylene terephthalate (Terylene) is formed by the condensation polymerization (polycondensation) of ethylene glycol with terephthalic acid ... 

The next simplest loop would contain at least one reaction in which three electron pairs are re-paired. Inspection of the possible combinations of two four-electron reactions and one six-electron reaction starting with CHDN reveals that they all lead to phase preseiwing i p loops that do not contain a conical intersection. It is therefore necessary to examine loops in which one leg results in a two electron-pair exchange, and the other two legs involve three elechon-pair exchanges fip loops). As will be discussed in Section VI, all reported products (except the helicopter-type elimination of H2) can be understood on the basis of four-electron loops. We therefore proceed to discuss the unique helicopter... 

Figure 3-3. Representative, simple examples of a substitution, an addition, and an elimination reaction showing the number, n, of reaction partners, and the change in n, An, during the reaction.

HORACE used alternating phases of classification (which topological or physicochemical features are required for a reaction type) and generalization (which features are allowed and can be eliminated) to produce a hierarchical classification of a set of reaction instances. 

The stereochemistry of reactions can also be treated by permutation group theory for reactions that involve the transformation of an sp carbon atom center into an sp carbon atom center, as in additions to C=C bonds, in elimination reactions, or in eIcctrocycHc reactions such as the one shown in Figure 3-21. Details have been published 3l]. 

Elimination of halogen by sodium (Wurtz s reaction) gives a higher hydrocarbon. 

This elimination of the diazonium group is therefore a very valuable reaction, as it affords almost the only method by which nitro and primary amino groups directly attached to the benzene ring can be eliminated. 

This Reaction should be carefully distinguished from the Claisen Conden-tation, which is the condensation of an ester, under the influence of sodium ethoxide, with (i) another ester, (ii) a ketone, or (iii) a nitrile, with the elimination of alcohol. For details of this condensation, see Ethyl Acetoacetate, p. 264. 

The presence of the base brings about the irreversible elimination of hydrogen chloride between the acid chloride and the acid the resulting p3rridine hydrochloride precipitates out as the reaction progresses. 

An important general method of preparing indoles, known as the Fischer Indole synthesis, consists in heating the phenylhydrazone of an aldehyde, ketone or keto-acld in the presence of a catalyst such as zinc chloride, hydrochloric acid or glacial acetic acid. Thus acrtophenone phenylhydrazone (I) gives 2-phenyllndole (I V). The synthesis involves an intramolecular condensation with the elimination of ammonia. The following is a plausible mechanism of the reaction ... 

It has been tentatively suggested that one mechanism underlies the Willgerodt reaction and the Kindler modification of it. A labile intermediate is first formed which has a carbon—carbon bond in the side chain. The scheme is indicated below it postulates a series of steps involving the addition of ammonia or amine (R = H or alkyl), elimination of water, re addition and eUmination of ammonia or amine until the unsaturation appears at the end of the chain then an irreversible oxidation between sulphur and the nitrogen compound may occur to produce a thioamide. 

Polymers can be classified as addition polymers and condensation polymers. Addition polymers are formed by iiitermolecular reactions of the monomeric units without the elimination of atoms or groups. An example is vinyl chloride, which can be made to combine with itself to yield polyvinyl chloride ... 

Note that for 4.42, in which no intramolecular base catalysis is possible, the elimination side reaction is not observed. This result supports the mechanism suggested in Scheme 4.13. Moreover, at pH 2, where both amine groups of 4.44 are protonated, UV-vis measurements indicate that the elimination reaction is significantly retarded as compared to neutral conditions, where protonation is less extensive. Interestingy, addition of copper(II)nitrate also suppresses the elimination reaction to a significant extent. Unfortunately, elimination is still faster than the Diels-Alder reaction on the internal double bond of 4.44. 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Another reaction in the last step is the syn elimination ofhydrogen with Pd as H—Pd—X, which takes place with alkyl Pd complexes, and the Pd hydride and an alkene are formed. The insertion of an alkene into Pd hydride and the elimination of, (3-hydrogen are reversible steps. The elimination of, 3-hydrogen generates the alkene, and both the hydrogen and the alkene coordinate to Pd, increasing the coordination number of Pd by one. Therefore, the / -elimination requires coordinative unsaturation on Pd complexes. The, 3-hydrogen eliminated should be syn to Pd. 

The elimination of 3-hydrogen of Pd alkoxide (17) to afford a carbonyl compound is a similar reaction. 

---