Epoxidation states

Leaves were dark-adapted therefore, there is no detectable level of zeaxanthin. Concentrations are nmol pigment (mol chi a — b), the P value from one factor ANOVA is displayed below each column. V-A-Z = xanthophyll pool (violaxanthin, antheraxanthin, zeaxanthin) EPS = epoxidation state. Reprinted with permission from P. J. Ralph et al. [76]. 

F/g. 14. Characterization of (left panel) the relative levels of the D1 and LHCIl proteins during the transition from summer to winter (1993 to 1994), and (right panel) the relative level of the D1 protein, the epoxidation state, EPS, of the xanthophyll cycle (0.5A+V)/(V+A+Z), and the efficiency of open PS II units (F /F determined after a 30 min period of dark adaptation) during the transition from winter to summer (1994) from needles of Scots pine growing in Sweden. Data redrawn from Ottander et al. (1995). 

Fig. 2. Simple two state allosteric model for qE in which the switch between quenched and unquenched conformation is driven by the effect of protonation and de-epoxidation. In this model LHCll can exist in two states, an unquenched conformation and a quenched conformation. Because the affinity of proton and zeaxanthin binding is greater in the quenched state, the ApH and de-epoxidation state will determine the equilibrium between these states. The existence of co-operativity indicates that these two states consist of a group of LHCll subunits which interact and change conformation in concert. The changed conformation, and intersubunit interaction, may give rise to quenching process itself In this model one or moreofthe different LHCll components may be involved.

This general formulation makes various predictions about qE, which have been confirmed by experimental observation. Synergism between DEPS and ApH has been found in the de-epoxidized state, the ApH requirement for qE is reduced compared to the epoxidized state at high ApH maximum quenching can be observed even at very low DEPS and at low ApH the stimulation of qE by DEPS is greatest (Rees et al., 1989 Noctor et al., 1991). Violaxanthin de-... 

Fig. 5. Determination of the relative efficiency of binding of violaxanthin to different LriCII complexes (A) and the de-epoxidation states of CP29 compared to thylakoids, and different preparations of LHCIIb (B). A-band refers to an oligomeric LHCII preparation prepared by gentle detergent solubilization of thylakoids, and A-band trimer, the LHCIIb trimer prepared from it (Ruban et al., 1999).

The synthon will be, in a lower valeney state - if X is C then it will be a carbene or the synthetic equivalent of a carbene. Let s see how this disconnection works out for epoxides. Taking X = O first we have... 

Peroxy acid and alkene Transition state for oxygen transfer from the OH group of the peroxy acid to the alkene Acetic acid and epoxide ... 

Dieldrin [60-57-1] or l,2,3,4,10,10-hexachloro-l,4,4t ,5,8,8t -hexahydro-6,7-epoxy-l,4- <7o, Aro-5,8-dimethanonaphthalene (34) (mp 176°C, vp 0.4 mPa at 20°C) is formed from aldrin by epoxidation with peracetic or perben2oic acids. It is soluble in water to 27 / g/L. Aldrin and dieldrin have had extensive use as soil insecticides and for seed treatments. Dieldrin, which is very persistent, has had wide use to control migratory locusts, as a residual spray to control the Anopheles vectors of malaria, and to control tsetse flies. Because of environmental persistence and propensity for bio accumulation, registrations in the United States were canceled in 1974. 

The trans isomer is more reactive than the cis isomer ia 1,2-addition reactions (5). The cis and trans isomers also undergo ben2yne, C H, cycloaddition (6). The isomers dimerize to tetrachlorobutene ia the presence of organic peroxides. Photolysis of each isomer produces a different excited state (7,8). Oxidation of 1,2-dichloroethylene ia the presence of a free-radical iaitiator or concentrated sulfuric acid produces the corresponding epoxide [60336-63-2] which then rearranges to form chloroacetyl chloride [79-04-9] (9). 

The most important chemical reaction of chi orohydrin s is dehydrochloriaation to produce epoxides. In the case of propylene oxide. The Dow Chemical Company is the only manufacturer ia the United States that still uses the chlorohydrin technology. In 1990 the U.S. propylene oxide production capacity was hsted as 1.43 x 10 t/yr, shared almost equally by Dow and Arco Chemical Co., which uses a process based on hydroperoxide iatermediates (69,70). More recentiy, Dow Europe SA, aimounced a decision to expand its propylene oxide capacity by 160,000 metric tons per year at the Stade, Germany site. This represents about a 40% iacrease over the current capacity (71). 

Crystallinity is low the pendent allyl group contributes to the amorphous state of these polymers. Propylene oxide homopolymer itself has not been developed commercially because it cannot be cross-baked by current methods (18). The copolymerization of PO with unsaturated epoxide monomers gives vulcanizable products (19,20). In ECH—PO—AGE, poly(ptopylene oxide- o-epichlorohydrin- o-abyl glycidyl ether) [25213-15-4] (5), and PO—AGE, poly(propylene oxide-i o-abyl glycidyl ether) [25104-27-2] (6), the molar composition of PO ranges from approximately 65 to 90%. 

More definitive evidence for the formation of an oxirene intermediate or transition state was presented recently by Cormier 80TL2021), in an extension of his earlier work on diazo ketones 77TL2231). This approach was based on the realization that, in principle, the oxirene (87) could be generated from the diazo ketones (88) or (89) via the oxocarbenes 90 or 91) or from the alkyne (92 Scheme 91). If the carbenes (90) (from 88) and (91) (from 89) equilibrate through the oxirene (87), and if (87) is also the initial product of epoxidation of (92), then essentially the same mixture of products (hexenones and ketene-derived products) should be formed on decomposition of the diazo ketones and on oxidation of the alkyne this was the case. 

Interesting developments were also taking place in the field of thermosetting resins. The melamine-formaldehyde materials appeared commercially in 1940 whilst soon afterwards in the United States the first contact resins were used. With these materials, the forerunners of today s polyester laminating resins, it was found possible to produce laminates without the need for application of external pressure. The first experiments in epoxide resins were also taking place during this period. 

Cyclic aliphatic epoxide resins" were first introduced in the United States. Some typical examples of commercial materials are shown in Table 26.6. 

Epoxide resin laminates are of particular importance in the aircraft industry. It has been stated that the Boeing 757 and 767 aircraft use 1800 kg of carbon fibre/ epoxide resin composites for structural purposes per aeroplane. The resin has also been used with Aramid fibres for filament-wound rocket motors and pressure vessels. The AV-18 fighter aircraft is also said to be 18% epoxide resin/cc bon fibre composite. The resins are also widely used both with fibres and with honeycomb structures for such parts as helicopter blades. 

Epoxide resins reinforced with carbon and Aramid fibres have been used in small boats, where it is claimed that products of equal stiffness and more useable space may be produced with a 40% saving in weight over traditional polyester/ glass fibre composites. Aramid fibre-reinforced epoxide resins have been developed in the United States to replace steel helmets for military purposes. Printed circuit board bases also provide a substantial outlet for epoxide resins. One recent survey indicates that over one-quarter of epoxide resin production in Western Europe is used for this application. The laminates also find some use in chermical engineering plant and in tooling. 

Chemical Designations - Synonyms Epoxidized tall oil, octyl ester Chemical Formula Mixture. Observable Characteristics - Physical State (as shipped) Liquid Color. Pale yellow Odor Mild. Physical and Chemical Properties - Physical State at IS X and 1 atm. Liquid Molecular Weight 420 (approx.) Boiling Point at 1 atm. Not pertinent Freezing Point Not pertinent Critical Temperature Not pertinent Critical Pressure Not pertinent Specific Gravity (est.) 1.002 at 20 °C (liquid) Vcpor (Gas) Specific Gravity Not pertinent Ratio of Specific Heats of Vcpor (Gas) Not pertinent Latent Heat of Vaporization Not pertinent Heat of Combustion Data not available Heat of Decomposition Not pertinent. 

Regioselectivity in opening of a-functionalized epoxides by treatment with trimethylamine dihydrofluoride has been observed. Thus, cw-isophorol epoxide gives exclusively 3-fluoro-l,2-diol, whereas from the trans isomer, 2-fluoro-l,3-diol is obtained as the main product together with 3-methylenecyclohexane-l,2-diol. This behavior has been discussed m terms of the influence of a-substituents on the transition state conformations [7 J] (equations 11 and 12). 

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