Acidic protons, exchange

Various types of fuel cells have been developed to generate power according to the applications and load requirements (Chaurasia, 2000). There are several types of electrolyte, which plays a key role in the different types of fuel cells. It must permit only the appropriate ions to pass between the anode and cathode. The main electrolyte types are alkali, molten carbonate, phosphoric acid, proton exchange membrane (PEM), and solid oxide. The first three are liquid electrolytes, the last two are solids. 

Polymerization of THF initiated with fluorosulfonic acid (FSO3H) was studied by H-NMR in A singlet due to the acidic proton, exchangeable during poly-... 

B. Gupta, M. Staub, G.G. Scherer, and D. Grman. Surface nonhomogeneity in radiation grafted FEP-g-polystyrenesulfonic acid proton-exchange membranes. Journal of Polymer Science Part A Polymer Chemistry 33, 1545-1549 1995. 

From the second half of twentieth century, acidic (proton exchange) Polymer Electrolyte Membrane Fuel Cells (PEMFC) have attracted much attention due to their potential as a clean power source for portable applications (alcohol feed). 

Wu, X., He, G., Gu, S., Hu, Z., Yao, P. Novel interpenetrating polymer network sulfonated poly(phthalazinone ether sulfone ketone)/polyaciylic acid proton exchange membranes for fuel cell. J. Membr. Scl. 295, 80-87 (2007)... 

Rate constants were measured at different temperatures and from the Eyring plots activation energies of proton exchanges were obtained as follows 7.9 kcal/mole for A-BU, 12.7 for A-U, 13.6 for A-TH and 15.3 for A-DH. The order is parallel to that of pk valves of the imino protons of uracil derivatives. The more acidic protons exchange more easily. 

Si K., Wycisk R., Dong D., Cooper K., Rodgers M., Brooker R, Slattery D., Litt M., Rigid-rod poly(phenylenesulfonic acid) proton exchange membranes with cross-linkable biphenyl groups for fuel cell applications. Macromolecules, 2013, 46(2), 422-M33. 

Studies of electrical interactions in proteins, polypeptides, and amino acids started over 60 years ago [1]. To a large extent, electrostatic properties of proteins are determined by the ability of certain amino acids to exchange protons with their environment and the dependence of these processes on pH. The proton occupies a special position as a promoter and iiuxliator in... 

Acidic Cation-Exchange Resins. Brmnsted acid catalytic activity is responsible for the successful use of acidic cation-exchange resins, which are also soHd acids. Cation-exchange catalysts are used in esterification, acetal synthesis, ester alcoholysis, acetal alcoholysis, alcohol dehydration, ester hydrolysis, and sucrose inversion. The soHd acid type permits simplified procedures when high boiling and viscous compounds are involved because the catalyst can be separated from the products by simple filtration. Unsaturated acids and alcohols that can polymerise in the presence of proton acids can thus be esterified directiy and without polymerisation. 

Cyanuric acid is a titrable weak acid (pffai — 6.88, pifa2 — H-40, pffas — 13.5) (10). The pH of a saturated aqueous solution of pure CA at room temperature is - 4.8. Thermodynamic properties of CA are given ia Table 1. Spectroscopic data are available (1 3). Proton nmr is of limited usefulness because of proton exchange and CA s symmetry and low solubiUty. Nuclear quadmpole resonance measurements ( " N) have been reported (12). 

The two-proton exchange in pairs of OH O fragments of various carbonic acid dimers... 

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

Much information on proton transfers has been obtained by NMR chemical exchange studies. An example is the proton exchange between neopentyl alcohol and acetic acid in acetic acid as the solvent. The reaction is... 

Methylcyclohexanone, pK 20, is typical of a weak acid that undergo H/D exchange. Identify the acidic protons of 2-methylcyclohexanone, i.e., those most susceptible to attack by base, as positions for which the value of the lowest-unoccupied molecular orbital (LUMO) is large. Use a LUMO map (the value of the LUMO mapped onto the electron density surface). Does this analysis correctly anticipate which of the anions obtained by deprotonation of 2-methylcyclohexanone is actually most stable Are any of the other ions of comparable stability, or are they aU much less stable ... 

LUMO map for 2-methylcyclohexanone reveals (in blue) acidic protons, susceptible to H/D exchange. 

As a catalyst sulfuric acid is most often used phosphoric acid, boron trifluoride or an acidic ion exchange resin have also found application. 1,1-disubstituted alkenes are especially suitable substrates, since these are converted to relatively stable tertiary carbenium ion species upon protonation. o ,/3-unsaturated carbonyl compounds do not react as olefinic component. 

Propylene glycol, glycolysis of polyurethanes with, 572 Propylene oxide (PO), glycolysis of polyurethanes with, 572-573 Propylene oxide (PO) polyols, 211, 223 Proton exchange membrane fuel cells (PEMFCs), 272-273 Proton NMR integrations, 386. See also H NMR spectroscopy Protonic acids, reactions catalyzed by, 67-68... 

Of course, such exchange is most successful for relatively acidic protons, such as those a to a carbonyl group, but even weakly acidic protons can exchange with bases if the bases are strong enough (see p. 228). 

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