Sequence reaction

Once the electroactive species has been identified, a far more exacting task is to uncover the sequence of elementary steps leading to stable products. In discussing and exemplifying this problem, we shall deal only with direct processes since the indirect ones in principle have reasonably well understood counterparts in homogeneous systems. 

It is an important postulate of electrochemistry that electrons are always transferred one by one (Semenov, 1958), and hence the first discrete intermediate from a direct electron transfer (79) will be a radical cation or anion. 

It has already been pointed out (Section 1) that the chemistry of radical ions, having hardly penetrated to the textbook level in organic chemistry, is little known, and we shall therefore briefly summarize their reactions here (for more detailed treatments, see Bard et al., 1976 Dorfman, 1970 Szwarc, 1968, 1969). Shown below are the elementary acts (denoted C or E cf. p. 25) in which radical ions have been shown to participate  

It should be noted that radical cations have a dual reactivity toward a nucleophile, depending on the properties of the latter as a base. It is possible to distinguish between these alternatives by cyclic voltammetry in cases of relatively stable radical cations (Parker and Eberson, 1969a) and by their reactivity toward pyridine nucleophiles of varying steric demands. The principle is illustrated in Fig. 6, which 

Relative Reactivities0 of Radical Cations Toward Different Lutidines  

Since in general two isomers are obtained from 3(5)-substituted pyrazoles and from indazoles, some effort has been directed in trying to obtain regiospecific reactions for instance, Seela 91HCA315 has discussed the problem of the regioisomerism in the glycosylation of indazole and 

In azoles containing at least two annular nitrogen atoms, one of which is an NH group and the other a multiply bonded nitrogen atom, electrophilic attack occurs at the latter nitrogen. Such an attack is frequently followed by proton loss from the NH group, e.g. (75)— (76). If the electrophilic reagent is a proton, this reaction sequence simply means tautomer interconversion (see Section 2.4.5.1.1), but in other cases leads to the product. 

Since the electrophilic reagent attacks the multiply bonded nitrogen atom, as shown for (77) and (78), the orientation of the reaction product is related to the tautomeric structure of the starting material. However, any conclusion regarding tautomeric equilibria from chemical reactivity can be misleading since a minor component can react preferentially and then be continually replenished by isomerization of the major component. 

In addition to reaction sequences of type (75) — (76), electrophilic reagents can attack at either one of the ring nitrogen atoms in the mesomeric anions formed by proton loss (e.g. 79— 80 or 81 see Section 3.4.1.3.6). Here we have an ambident anion, and for unsymmetrical cases the composition of the reaction product (80) + (81) is dictated by steric and electronic factors. 

---

Example 2.5 tert-Butyl hydrogen sulfate is required as an intermediate in a reaction sequence. This can be produced by the reaction between isobutylene and moderately concentrated sulfuric acid ... 

Enzymes act by lowering the overall activation energy of a reaction sequence by involving a series of intermediates, or a mechanism, different from the spontaneous uncatalysed reaction. 

Derive the steady-state rate law corresponding to the reaction sequence of Eqs. XVIII-40-XVIII-44, that is, without making the assumption that any one step is much slower than the others. See Ref 234. 

Write a possible reaction sequence for the photochemical oxidation of aqueous CN ion on Ti02. 

Let us consider tire oxidation of Fe(s) to Fe (solvated), which can be described by tire following reaction sequence [36, 40] ... 

The reaction mechanisms of plasma polymerization processes are not understood in detail. Poll et al [34] (figure C2.13.6) proposed a possible generic reaction sequence. Plasma-initiated polymerization can lead to the polymerization of a suitable monomer directly at the surface. The reaction is probably triggered by collisions of energetic ions or electrons, energetic photons or interactions of metastables or free radicals produced in the plasma with the surface. Activation processes in the plasma and the film fonnation at the surface may also result in the fonnation of non-reactive products. 

Deposition of Si02 has been achieved by exposing the substrate to the binary reaction sequence SiCL-l-2H20 —> Si02+ 4HC1. This is divided into the following half-reactions in which species at the surface are inaicated by asterisks [95] ... 

George S M, Sneh O, Diiion A C, Wise M L, Ott A W, Okada L A and Way J D 1994 Atomic iayer controiied deposition of SiO, and Ai,0, using ABAB.. . binary reaction sequence chemistry Appl. Surf. Sc/. 82/83 460-7... 

Ott A W, McCariey K C, Kiaus J W, Way J D and George S M 1996 Atomic iayer controiied deposition of Ai203 fiims using binary reaction sequence chemistry App/. Surf. Sc/. 106 128-36... 

Ott A W, Kiaus J W, Johnson J M and George S M 1997 Ai203 thin fiim growth on Si(IOO) using binary reaction sequence chemistry Thin Solid Films 292 135-44... 

To become familiar with a knowledge-based reaction prediction system To appreciate the different levels in the evaluation of chemical reactions To know how reaction sequences are modeled To understand kinetic modeling of chemical reactions To become familiar with biochemical pathways... 

When reaction rate equations can be given for the individual steps of a reaction sequence, a detailed modeling of product development over time can be made ... 

We have already explored this route (frame 184) and found that the reaction sequence actually is ... 

Perhaps the most sensational synthesis of chiysanthemic add uses this strategy. You ma r remember that TM 31 is usually made from the adduct of acetylene and acetone. Draw out the stages of this reaction sequence. 

TrialkyIboranes (p. 9), which can be synthesized from olefins and diborane, undergo alkyl coupling on oxidation with alkaline silver nitrate via short-lived silver organyls. Two out of three alkyl substituents are coupled in this reaction. Terminal olefins may be coupled by this reaction sequence in 40 - 80% yield. With non-terminal olefins yields drop to 30 - 50% (H.C. Brown, 1972C, 1975). 

Cis-olefins or cis./rjns-dienes can be obtained from alkynes in similar reaction sequences. The alkyne is first hydroborated and then treated with alkaline iodine. If the other substituents on boron are alkyl groups, a cis-olefin is formed (G. Zweifel, 1967). If they are cir-alkenyls, a cis, trans-diene results. The reactions are thought to be iodine-assisted migrations of the cis-alkenyl group followed by (rans-deiodoboronation (G. Zweifel, 1968). Trans, trans-dienes are made from haloalkynes and alkynes. These compounds are added one after the other to thexylborane. The alkenyl(l-haloalkenyl)thexylboranes are converted with sodium methoxide into trans, trans-dienes (E. Negishi, 1973). The thexyl group does not migrate. 

N. Trachtenberg, 1969). products or even as the major 1958). The reaction sequence with SeOj, except that the C—< the dehydrogenation of C—C other reagents have also been quinones. 

The reaction sequence is successful because reverse, ring-contraction reactions are unlikely and because only the final product contains a secondary lactam group, which is depro-tonated under the reaction conditions. 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

With the catalysis of strong Lewis acids, such as tin(IV) chloride, dipyrromethenes may aiso be alkylated. A very successful porphyrin synthesis involves 5-bromo-S -bromomethyl and 5 -unsubstituted 5-methyl-dipyrromethenes. In the first alkylation step a tetrapyrrolic intermediate is formed which cyclizes to produce the porphyrin in DMSO in the presence of pyridine. This reaction sequence is useful for the synthesis of completely unsymmetrical porphyrins (K.M. Smith, 1975). 

Enantiomerically pure tetroses, pentoses, and hexoses have been synthesized by the following reaction sequence (A.W.M. Lee, 1982 S.Y. Ko, 1983), which is useful as a repetitive two-carbon hotnologi-.ation in total syntheses of higher monosaccharides and other polyhydroxy compounds (1) Wittig reaction of a protected hydroxy aldehyde with (triphenylphosphor-... 

The two stage reaction sequence is called ozonolysis and is represented by the gen eral equation... 

Suggest reaction sequences and reagents suitable for carrying out each of the following con versions Two synthetic operations are required in each case... 

Deduce the identity of the missing compounds in the following reaction sequences Show stereochemistry in parts (b) through (d)... 

This reaction sequence is called the acetoacetic ester synthesis It IS a standard... 

Recognize too that the reaction sequence is one that is characteristic of p keto esters in general and not limited to just ethyl acetoacetate and its derivatives Thus... 

Ethyl acetoacetate behaves similarly to diethyl malonate in its reactivity toward a p unsaturated carbonyl compounds Give the structure of the product of the following reaction sequence... 

The acetoacetyl group is then transformed to a butanoyl group by the reaction sequence illustrated m steps 2 to 4... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

The net effect of this sequence is the destmction of 2 molecules of as the one is lost in NO2 formation and the O of equation 26 would have combined with O2 to form the other. In addition, the NO acts as a catalyst. It is not consumed, and therefore can participate in the reaction sequence many times. 

Fig. 2. Overall schematic of solid fuel combustion (1). Reaction sequence is A, heating and drying B, solid particle pyrolysis C, oxidation and D, post-combustion. In the oxidation sequence, left and center comprise the gas-phase region, tight is the gas—solids region. Noncondensible volatiles include CO, CO2, CH4, NH, H2O condensible volatiles are C-6—C-20 compounds oxidation products are CO2, H2O, O2, N2, NO, gaseous organic compounds are CO, hydrocarbons, and polyaromatic hydrocarbons (PAHs) and particulates are inerts, condensation products, and solid carbon products.

Metha.nol-to-Ga.soline, The most significant development in synthetic fuels technology since the discovery of the Fischer-Tropsch process is the Mobil methanol-to-gasoline (MTG) process (47—49). Methanol is efftcientiy transformed into C2—C q hydrocarbons in a reaction catalyzed by the synthetic zeoHte ZSM-5 (50—52). The MTG reaction path is presented in Figure 5 (47). The reaction sequence can be summarized as... 

A related reaction sequence, which proceeds through a Curtius rearrangement, allows the transformation of a-cyano acids into hydantoins (66) ... 

AH components of the reaction mixture, whatever their source, are subject to the same kind of radical attacks as the starting substrate(s). Any free-radical oxidation is inevitably a cooxidation of substrate(s) and products. The yields of final products are deterrnined by two factors (/) how much is produced in the reaction sequence, and (2) how much product survives the reaction environment. By kinetic correlations and radiotracer techniques, it is... 

Aldehydes are important products at all pressures, but at low pressures, acids are not. Carbon monoxide is an important low pressure product and declines with increasing pressure as acids increase. This is evidence for competition between reaction sequence 18—20 and reaction 21. Increasing pressure favors retention of the parent carbon skeleton, in concordance with the reversibiUty of reaction 2. Propylene becomes an insignificant product as the pressure is increased and the temperature is lowered. Both acetone and isopropyl alcohol initially increase as pressure is raised, but acetone passes through a maximum. This increase in the alcohoLcarbonyl ratio is similar to the response of the methanoLformaldehyde ratio when pressure is increased in methane oxidation. 

Catechol Derivatives. An elegant synthesis of trimethoxybenzaldehyde [86-81-7] (4) starting from quaiacol [90-05-1] (5) and formaldehyde (75) has been developed. The reaction sequence is as follows ... 

---