And disproportionation

Chromates(V), (CrO ) ". Dark green species which hydrolyse and disproportionate to Cr III) and Cr(VI) (K2Cr04 plus KOH in melt). 

Kaptein R and Oosterhoff J L 1969 Chemically Induced dynamic nuclear polarization III (anomalous multiplets of radical coupling and disproportionation products) Chem. Phys. Lett. 4 214-16... 

Zhen-Ming Hu and NakatsujI H 1999 Adsorption and disproportionation reaction of OH on Ag surfaces dipped adcluster model study Surf. Sci. 425 296-312... 

The aluminium monochloride vapour is unstable when cooled and disproportionates (p.77) below 1100 K thus ... 

Germanium forms divalent compounds with all the halogens. Germaniunil 1) chloride can be prepared by passing the vapour of germanium(IV) chloride (see below) over heated germanium. The reaction is reversible and disproportionation of germanium(II) chloride is complete at about 720 K at atmospheric pressure ... 

When titanium dissolves in dilute hydrochloric acid, a violet solution containing titanium(III) ions is formed. This solution rapidly decolorises acidified aqueous potassium permanganate at room temperature. Titanium(IV) chloride is a colourless covalent liquid completely hydrolysed by water. Titanium(III) chloride forms hydrated titanium(III) ions in water and disproportionates when heated in a vacuum. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Tlie formation of initiator radicals is not the only process that determines the concentration of free radicals in a polymerization system. Polymer propagation itself does not change the radical concentration it merely changes one radical to another. Termination steps also occur, however, and these remove radicals from the system. We shall discuss combination and disproportionation reactions as modes of termination. 

Combination and disproportionation are competitive processes and do not occur to the same extent for all polymers. For example, at 60°C termination is virtually 100% by combination for polyacrylonitrile and 100% by disproportionation for poly (vinyl acetate). For polystyrene and poly (methyl methacrylate), both reactions contribute to termination, although each in different proportions. Each of the rate constants for termination individually follows the Arrhenius equation, so the relative amounts of termination by the two modes is given by... 

Xylenes Produetion Via Toluene Transalkylation and Disproportionation. The toluene that is produced from processes such as catalytic reforming can be converted into xylenes via transalkylation and disproportionation. Toluene disproportionation is defined as the reaction of 2 mol of toluene to produce 1 mol of xylene and 1 mol of benzene. Toluene transalkylation is defined as the reaction of toluene with or higher aromatics to produce xylenes ... 

Deall lation, Transall lation, and Disproportionation. The action of aluminum chloride also removes alkyl groups from alkylbenzenes (dealkylation, disproportionation) (12). Alkylbenzenes, when heated with AlCl, form mixtures of benzene and polyalkylated benzenes ... 

Iron hahdes react with haHde salts to afford anionic haHde complexes. Because kon(III) is a hard acid, the complexes that it forms are most stable with F and decrease ki both coordination number and stabiHty with heavier haHdes. No stable F complexes are known. [FeF (H20)] is the predominant kon fluoride species ki aqueous solution. The [FeF ] ion can be prepared ki fused salts. Whereas six-coordinate [FeCy is known, four-coordinate complexes are favored for chloride. Salts of tetrahedral [FeCfy] can be isolated if large cations such as tetraphenfyarsonium or tetra alkylammonium are used. [FeBrJ is known but is thermally unstable and disproportionates to kon(II) and bromine. Complex anions of kon(II) hahdes are less common. [FeCfy] has been obtained from FeCfy by reaction with alkaH metal chlorides ki the melt or with tetraethyl ammonium chloride ki deoxygenated ethanol. 

Fig. 3. Resin acids in rosin sizes, R = CH(CH2)2- The rosin acids are represented here as abietic acid [514-10-3] (1) and levopimaric acid [79-54-9] (2). In rosin there are other isomers and disproportionation products. The product of reaction with fumaric acid (3) is levopimaric acid— fumaric acid adduct...

Koch Chemical Company is the only U.S. suppHer of all PMBs (except hexamethylbenzene). Its process has the flexibility of producing isodurene, prehnitene, and pentamethylbenzene, should a market develop. Koch s primary process (20) is based on isomerization, alkylation, and disproportionation conducted in the presence of a Friedel-Crafts catalyst. For the synthesis of mesitylene and hemimellitene, pseudocumene is isomerized. If durene, isodurene, or prehnitene and pentamethylbenzene are desired, pseudocumene is alkylated with methyl chloride (see Alkylation Friedel-CRAFTSreactions). 

Early studies of the condensation reaction on the monomer level did not give the full picture of this process and only in the 1980s was polycondensation of siloxanols studied by using oligomeric model compounds (76,77). These studies revealed that in the presence of strong protic acids three processes must be considered linear condensation (eq. 14), cyclization (eq. 15), and disproportionation (eq. 16). 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

Table 9 is a summary of world toluene supply and demand for 1996. North America, Asia, and Western Europe dominated the world s toluene business ia 1996. The three regions together accounted for over 85% of world production, imports, exports, and actual consumption, respectively. North America led ia production and consumption, while Asia led ia imports and exports. Table 10 presents the world toluene supply and demand. The worldwide demand for toluene increased by 7% from 1993 to 1994 and from 1994 to 1995, consecutively, because of higher hydrodealkylation (HDA) and disproportionation (TDP) operations, plus strong demand for all other derivatives. Over 70% of toluene is derived from a single source, catalytic reformate. 

Petroleum-derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed ia the steam cracking of olefins (35). 

The principal chemical uses of BTX are illustrated in Figure 1 and Hsted in Table 1 (2). A very wide range of consumer products from solvents to fibers, films, and plastics are based on BTX. The consumption of BTX is approximately in the proportions of 67 5 28, respectively. However, no BTX process gives BTX in these proportions. The economic value of benzene and xylenes (especially -xylene) is normally higher than that of toluene. Because of this, processes that convert toluene to benzene by hydrodealkylation (3) and disproportionate toluene to benzene and xylenes (4) have been commercialized. In addition, reforming processes that emphasize production of either benzene or -xylene [106 2-3] have been described (5). Since these are not classified as BTX processes they are not discussed in detail here. 

The net result of photosynthesis is reduction of carbon dioxide to form carbohydrates. A key intermediate is phosphoglyceric acid, from which various simple sugars are produced and disproportionated to form other carbohydrates. 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

On the other hand, not only initial colour but colour change (discolouration) of the resin under UV light and heat is important. Colour retention of a resin is related to the chemical stability and increases as the degree of non-aromatic conjugated unsaturation of the resin molecule decreases. Thus, for rosins a high level of abietic-type resin acids lead to relatively unstable resins. Hydrogenation and disproportionation as well as esterification provide improved stability and colour retention to rosins. 

Free hypophosphorous acid is obtained by acidifying aqueous solutions of hypophosphites but the pure acid cannot be isolated simply by evaporating such solutions because of its ready oxidation to phosphorous and phosphoric acids and disproportionation to phosphine and phosphorous acid (Fig. 12.16). Pure H3PO2 is obtained by continuous cxiraction from aqueous solutions into Ft 0 it forms white crystals mp... 

The relative importance of combination and disproportionation may be predicted using the following guidelines 145... 

Watanabe et al,25-5 52s applied AMS dimer (116) as a radical trap to examine the reactions of oxygen-centered radicals (e.g. r-butoxy, cumyloxy, benzoyloxy). AMS dimer (116) is an addition fragmentation chain transfer agent (see 6.2.3.4) and reacts as shown in Scheme 3,96. The reaction products are macromonomers and may potentially react further. The reactivity of oxygen centered radicals towards 116 appears to be similar to that of S.2 1 Cumyl radicals are formed as a byproduct of trapping and are said to decay mainly by combination and disproportionation. 

Even though the absolute rate constant for reactions between propagating species may be determined largely by diffusion, this does not mean that there is no specificity in the termination process or that the activation energies for combination and disproportionation are zero or the same. It simply means that this chemistry is not involved in the rate-determining step of the termination process. 

The relative importance of combination and disproportionation in relevant model systems and in polymerizations of some common monomers is considered in Sections 5,2.2.1 and 5.2.2.2 respectively. The significance of the termination mechanism on the course of polymerization and on the properties of polymers is discussed briefly in Section 5.2.2 and is further discussed in Section 8.2. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Knowledge of kui/kii is also important in designing polymer syntheses. For example, in the preparation of block copolymers using polymeric or multifunctional initiators (Section 7.6.1), ABA or AB blocks may be formed depending on whether termination involves combination or disproportionation respectively. The relative importance of combination and disproportionation is also important in the analysts of polymerization kinetics and, in particular, in the derivation of rate parameters. 

Hensley ei al.,n reported the only direct experimental observation of head-to-head linkages in PS by 2D INADEQUATE NMR on, 3C-enriched PS. The method did not enable these groups to be quantified with sufficient precision for evaluation of W tc- Zammit el a/.130 studied chain distribution of low molecular weight PS prepared with AIBN initiator by MALDI-TOF. Separate distributions of chains formed by combination and disproportionation were observed. They estimated kJkK. at 90 °C to be 0.057. 

---