Condensations Reactions

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

Acid-catalyzed isomerization reactions of alkanes as well as alkylation and condensation reactions are initiated by protolytic ionization. Available evidence indicates nonlinear but not necessarily triangular... 

The usual base or acid catalyzed aldol addition or ester condensation reactions can only be applied as a useful synthetic reaction, if both carbonyl components are identical. Otherwise complicated mixtures of products are formed. If two different aldehydes or esters are to be combined, it is essential that one of the components is transformed quantitatively into an enol whereas the other component remains as a carbonyl compound in the reaction mixture. 

Intramolecular condensation reactions to generate six-membered carbocycles are mentioned in section 1.12, the polyene cyclization in section 1.15. 

Functional group selectivity is often easy to achieve in reduction and condensation reactions since several highly selective reagents for reduction and for protection offunctional groups are available. 

Most syntheses of nitrogen heterocycles involve substitution and/or condensation reactions of nitrogen nucleophiles with difunctional halides or carbonyl compounds. Common nitrogen reagents are ... 

Syntheses by Functional Group Interconversions (Condensation Reactions)... 

Synthesis of large heterocycles usually involves condensation reactions of two difunctional molecules. Such molecules tend to polymerize. So far two special techniques have been described above to avoid this important side-reaaion , namely high dilution and use of templates. The general procedure to avoid polymerizations in reactions between difunctional molecules is, of course, the application of protecting groups as described in sections 4.1.2 and 2.6. 

In the synthesis of commercial sulfur-heterocycles two interesting reactions are used (i) diphenylamines may be connected by a sulfur bridge in the orfho-positions (ii) the amino grouping of sulfonamides undergoes condensation reactions with neighboring imino- and amide groups. 

Scheme 55) (235. 236). -The product obtained (77) is probably formed via the protonated form of the thiazole, whose reactivity is treated in Section IV, 1. The light-yellow leucobase (77) is reported to be oxidized by PbOj to the red-black carbinol (78) (236). This condensation reaction is also successful when benzaidehyde is replaced by formaldehyde, bis(2-amino-4-phenylthiazolyl-5)methane (79 i beine obtained (Scheme 56) (237). 

The guanidine (272) is very reactive in condensation reactions (Scheme 166) (487). 

The same 3-amino substituent is reactive in condensation reactions (Scheme 235) (701, 726-729). 2-Imino-3-amino-4-thia2oiine reacts, however, in the nicotinoylation reaction through its imino nitrogen (727). 

Thiazolium derivatives unsubstituted at the 2-position (35) are potentially interesting precursors of A-4-thiazoline-2-thiones and A-4-thiazoline-2-ones. Compound 35 in basic medium undergoes proton abstraction leading to the very active nucleophilic species 36a and 36b (Scheme 16) (43-46). Special interest has been focused upon the reactivity of 36a and 36b because they are considered as the reactive species of the thiamine action in some biochemical reaction, and as catalysts for several condensation reactions (47-50). 

Compound 36 when treated by sulfur under nitrogen, leads to the thiazoline-2-thione (37) (Scheme 16) (43). Oxidation by O- or air of 36 (43) or 38 (45, 46) leads to the corresponding thiazoiine (39 or 40). Consequently, condensation reactions using catalysts like 36 must be run in strictly oxygen-free atmosphere (47-50). The isolation of traces of 3-benzyl-4-methyl-A-4-thiazoline-2-one (42) as a product of the oxidation of... 

For 2-amino-4- m-nitrophenyl) seienazole, the yield is particularly high. This has been explained by the oxidizing effect of the nitro group, which liberates iodine from the hydrogen iodide eliminated in the condensation reaction. 

N-substituted selenoureas give the same condensation reaction (Scheme 9. Table X-3). 

The leader of DuPont s effort was Wallace H Carothers who reasoned that he could reproduce the properties of silk by constructing a polymer chain held together as is silk by amide bonds The neces sary amide bonds were formed by heating a dicar boxylic acid with a diamine Hexanedioic acid adipic acid) and 1 6 hexanediamme hexamethylenedi-amine) react to give a salt that when heated gives a polyamide called nylon 66 The amide bonds form by a condensation reaction and nylon 66 is an example of a condensation polymer... 

Condensation reaction (Section 15 7) Reaction m which two molecules combine to give a product accompanied by the expulsion of some small stable molecule (such as water) An example is acid catalyzed ether formation... 

Epoxy resin is prepared by the following condensation reaction ... 

Poly(butylene Terephthalate). Poly(butylene terephthalate) is prepared in a condensation reaction between dimethyl terephthalate and 1,4-butanediol and its repeating unit has the general structure... 

One type of polymerization reaction is the addition reaction in which successive repeat units add on to the chain. No other product molecules are formed, so the weight of the monomer and that of the repeat unit are identical in this case. A second category of polymerization reaction is the condensation reaction, in which one or two small molecules like water or HCl are eliminated for each chain linkage formed. In this case the molecular weight of the monomer and the... 

The polymer repeat unit arises from reacting together two different functional groups which usually originate on different monomers. In this case the repeat unit is different from either of the monomers. In addition, small molecules are often eliminated during the condensation reaction. Note the words usual and often in the previous statements exceptions to both statements are easily found. 

The product molecules have the functional groups formed by the condensation reactions interspersed regularly along the backbone of the polymer molecule ... 

Table 1.2 lists several examples of condensation reactions and products. Since the reacting monomers can contain different numbers of carbon atoms between functional groups, there are quite a lot of variations possible among these basic reaction types. 

Discussion of ladder polymers also enables us to introduce a step-growth polymerization that deviates from the simple condensation reactions which we have described almost exclusively in this chapter. The Diels-Alder reaction is widely used in the synthesis of both ladder and semiladder polymers. In general, the Diels-Alder reaction occurs between a diene [XVI] and a dienophile [XVll] and yields an adduct with a ring structure [XVlll] ... 

Haward et al.t have reported some research in which a copolymer of styrene and hydroxyethylmethacrylate was cross-linked by hexamethylene diisocyanate. Draw the structural formula for a portion of this cross-linked polymer and indicate what part of the molecule is the result of a condensation reaction and what part results from addition polymerization. These authors indicate that the crosslinking reaction is carried out in sufficiently dilute solutions of copolymer that the crosslinking is primarily intramolecular rather than intermolecular. Explain the distinction between these two terms and why concentration affects the relative amounts of each. 

This situation seems highly probable for step-growth polymerization because of the high activation energy of many condensation reactions. The constants for the diffusion-dependent steps, which might be functions of molecular size or the extent of the reaction, cancel out. 

Rhoda.mines, Rhodamines are commercially the most important arninoxanthenes. If phthalic anhydride is used in place of formaldehyde in the above condensation reaction with y -dialkylarninophenol, a triphenyknethane analogue, 9-phenylxanthene, is produced. Historically, these have been called rhodamines. Rhodamine B (Basic Violet 10, Cl45170) (17) is usually manufactured by the condensation of two moles of y -diethylaminophenol with phthahc anhydride (24). An alternative route is the reaction of diethylamine with fluorescein dichloride [630-88-6] (3,6-dichlorofluoran) (18) under pressure. 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

If, iastead of an acetoacetate equivalent, a malonyl derivative such as (39) were iavolved (37), appropriate condensation reactions would lead to the tropane [280-05-7] skeleton (40),... 

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