Dimerization and condensation

Dimerization and condensation of pentane-2,4-dione by molybdenum(VI) oxide tetrachloride (MoOC14) affords l,3,5,7-tetramethyl-2,4,6,8-tetraoxa-adamantane, the structure of which is determined by elemental analysis, IR, mass, and PMR spectroscopy.188... 

Some of the chemical transformations which can be responsible for changes in the boiling point of the solvent are disproportionation, dimerization, and condensation. Isomerization, which has been studied in detail by others (4,5) has not been mentioned because although isomerization significantly affects the quality of the solvent, boiling point shifts are minimal. 

Dimerization and condensation of alkyl aromatics and phenols produce higher boiling polycyclic aromatic hydrocarbons which may not be isolable as recycle solvent. 

Potassium superoxide dissolved in benzene witli the aid of 18-crown-6 was shown to oxidize hydrazines, hydrazones and related compounds into a variety of products. The products resulted from oxidation, dimerization and condensation. A mechanism is proposed to account for the products. 

Di- and Triisobutylcncs. Diisobutylene [18923-87-0] and tnisobutylenes are prepared by heating the sulfuric acid extract of isobutylene from a separation process to about 90°C. A 90% yield containing 80% dimers and 20% trimers results. Use centers on the dimer, CgH, a mixture of 2,4,4-trimethylpentene-1 and -2. Most of the dimer-trimer mixture is added to the gasoline pool as an octane improver. The balance is used for alkylation of phenols to yield octylphenol, which in turn is ethoxylated or condensed with formaldehyde. The water-soluble ethoxylated phenols are used as surface-active agents in textiles, paints, caulks, and sealants (see Alkylphenols). 

The 2,6-DHMP condensation produced only one dimer and a significant amount of trimer as depicted in Scheme 8. The structure of the trimer was not reported. The reaction path is analogous to that of 2-HMP, but occurred at a faster rate. 2,6-DHMP was the only derivative to form a significant amount of trimer under the reaction conditions studied. This supports the idea that ortho-linked PF polymers should have a faster cure than others. It also points out the futility of attempting to manufacture an ortho-Ymkcd polymer under alkaline conditions. Extension of the polymerization process as depicted in Scheme 8 leads to a continual reduction in the amount of para functionality available for condensation as shown in Table 7. 

From a kinetic point of view, the initial reaction of condensation to form dimers is much faster than the subsequent condensation of these dimers and higher polymers. 

Allen et al mainly on the evaluation of data reported by previous workers, advanced structure (5) for 2-methylpyrrole dimer. These data were (a) the monobasic nature of the dimer and (b) the conversion by aqueous acid into an indole which they showed by elimination (i.e., by the synthesis of 2,5-, 2,6-, and 2,7-dimethylindoIes) to be 2,4-dimethyIindole (this compound has since been synthesized by Marion and Oldfield ). Structure (5) for the dimer was confirmed by Edwards and Smith by conversion, by way of the methiodide (6), into the pyrrolidinopyrrole (7), the structure of which was proved by synthesis l,5-dimethylpyrrolid-2-one was condensed with 2-methyI-pyrrole by means of POCI3 to give the cation (8), isomeric with the... 

Some types of reactions involving gases that have been studied in IFs are hydrogenations [16, 25-37 ], oxidations [38, 39], and hydroformylations [25, 40 5]. In addition, some dimerizations and allcylations may involve the dissolution of condensable gases (e.g., ethylene, propylene, isobutene) in the IF solvent [46-50]. 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

The Ag(lll)/NO system turns out to be rather complex for adsorption at 20K despite the fact that at 300K virtually no adsorption occurs, and one might therefore expect that at low temperature only physlsorptlon and condensation would occur. In fact, condensed NO exists as (NO)2 dimers (28), and a complex set of reactions leading to O, NO, N2O and NO2 species takes place when the temperature is raised as determined by combined XPS and TPD measurements (29). Following the SIMS cluster behavior during the reactions shows that several of the reaction species can be identified from the SIMS molecular clusters. 

The general mechanistic features of the aldol addition and condensation reactions of aldehydes and ketones were discussed in Section 7.7 of Part A, where these general mechanisms can be reviewed. That mechanistic discussion pertains to reactions occurring in hydroxylic solvents and under thermodynamic control. These conditions are useful for the preparation of aldehyde dimers (aldols) and certain a,(3-unsaturated aldehydes and ketones. For example, the mixed condensation of aromatic aldehydes with aliphatic aldehydes and ketones is often done under these conditions. The conjugation in the (3-aryl enones provides a driving force for the elimination step. 

To remove volatile impurities, the naterial was heated in vacuo at 135 °C. This treatment resulted in further condensation. Thus, the distillation residue was composed of a mixture of doubly-bridged dimers and tetramers, x = 1 and 2 as determined from the molecular weight, boron and nitrogen analyses and the volatile condensibles produced. 

Gas Recycle is a relatively simple operation. Rather than being circulated through a variety of pipes, pumps and columns, the catalyst remains in one place. A key control variable is maintaining a constant liquid level in the reactor. This is not as simple as it might first seem because in addition to butanal isomers forming, butanal condensation products including dimers and trimers also form to give what are collectively termed heavies . 

Fig. 5.1 A schematic projection of the 3n dimensional (per molecule) potential energy surface for intermolecular interaction. Lennard-Jones potential energy is plotted against molecule-molecule separation in one plane, the shifts in the position of the minimum and the curvature of an internal molecular vibration in the other. The heavy upper curve, a, represents the gas-gas pair interaction, the lower heavy curve, p, measures condensation. The lighter parabolic curves show the internal vibration in the dilute gas, the gas dimer, and the condensed phase. For the CH symmetric stretch of methane (3143.7 cm-1) at 300 K, RT corresponds to 8% of the oscillator zpe, and 210% of the LJ well depth for the gas-gas dimer (Van Hook, W. A., Rebelo, L. P. N. and Wolfsberg, M. /. Phys. Chem. A 105, 9284 (2001))...

The dimerization (Aldol condensation) takes place at temperatures of 175—250°F in the presence of a dilute solution of sodium hydroxide. After the reaction, the mixture is passed to a separator tank where the dimer is separated then sent to a reactor to be hydrogenated over a nickel catalyst. Reaction conditions have temperatures of 300°F and 2500 psi. Distillation of the effluent gives purified 2-EH in 95% yield. 

Ubiquitinated histones have been suggested to destabilize the interface between the H2A-H2B dimers and the H3-H4 tetramer [211], and to be depleted from highly condensed mitotic chromosomes and enriched in HI deficient chromatin [212]. In Drosophila, the inducible hsp70 and copia genes are ubiquitinated, which represents... 

While highly reactive formaldonitrone (3) was detected only recently, both the nitrosomethane (1) and the formaldehyde oxime (2) are also reactive compounds in the condensed phase 2 undergoes facile oligomerization , whereas 1 forms a dimer and isomerizes to 2 . Deprotonation of any of these isomers (1, 2, 3) would result in the corresponding simplest nitrosomethanide anion [H2C=N—O]. ... 

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