Aliphatic ethers, protonation

Aliphatic ethers are protonated in strong acids, at low temperatures the exchange rates of the acidic proton are slow enough to permit direct observation by NMR spectroscopy.133 Temperature-dependent NMR spectral studies allow us again to follow the kinetics of ether cleavage to form alkyl cations [Eq. (3.18)]. 

Similar to alcohols, aliphatic ethers [54], thiols [55], and sulfides are also protonated on oxygen or sulfur, respectively, at -60 °C in magic acid carbocations are subsequently formed upon raising the temperature. Promoted sulfides, excluding tertiary alkyl, are resistant to cleavage up to -i-70 °C [56]. 

No pA aH value is available for O-protonation of furan, but it is probably much less basic at oxygen than an aliphatic ether. Acid-catalysed deuteration occurs at an a-positionf 3/4-deuterio-furans are not obtained because, although P-protonation probably occurs, the cation produced is more susceptible to water, leading to hydrolytic ring opening. An estimate of p/faH -10.0 was made for the 2-protonation of 2,5-di-t-butylfuran, which implies a value of about -13 for furan itself. An a-protonated cation, stable in solution, is produced on treatment of 2,5-di-t-butylfuran with concentrated sulfuric acid. ... 

Similar to alcohols, aliphatic ethers, thiols, and sulfides are also protonated on oxygen or sulfnr, respectively, at — 60 °C in... 

Some typical results are shown in Table 1. Polymers are soluble in organic solvents such as benzene, toluene, THF and chlorinated hydrocarbons. The IR spectrum of these polymers displays characteristic absorption bands at 1640 cm (C=C stretching), 1600 and 1500 cm"l (phenyl ring vibrations), 1235 cm" (phenyl ether stretching) and 1015 cm (aliphatic ether stretching). The NMR spectrum of these polyethers recorded in CDCl at room temperature exhibits multiplets between 7.20 and 6.67 ppm (aromatic protons) and peaks at 6.02 ppm (-CH=CH-), at 4.50 ppm (-CH -0) and 1.60 ppm (-CH ). A small peak is observed at 4.05 ppm which can be attributed to the protons of the chloromethyl end groups. This peak is absent in the spectrum of sample 6 (Table 1). 

The acidity function will be on its best behavior in aqueous mineral acid solutions where ionization is essentially complete, although even here similar aniline-type indicators do not show exactly parallel changes of indicator ratio with acidity (336). Several other groups of neutral bases give plots of the logarithm of the indicator ratio (henceforth alluded to as log Q) vs. Ho with slopes very far from unity. In the cases of the aryl olefins (85) some azulenes (231) 1, 3, 5-tri-methoxybenzene (213) and some indole bases (185-187,357) protonation produces a carbonium ion which exhibits quite different activity coefficient behavior than that of bases such as anilines, ketones, or aromatic ethers. Other compounds for which deviations from unit slope have been reported are the amides (98) (a very serious case), some aliphatic ethers (12), and phenols (11). 

The qualitative fact that ethers behave as bases to Lewis acids is nearly as old as organic chemistry itself (277). Ether oxonium salts have been isolated (252) and careful studies of their physical properties make it plain that a ptoton is coordinated to the ether oxygen. Several cryoscopic studies (134,196) indicate that aliphatic ethers, with the exception of several negatively substituted ones, are completely protonated in concentrated sulfuric acid. Aromatic ethers are apt to be too insoluble for good cryoscopic work and the problem of rapid decomposition makes exact measurements of f-factors in sulfuric acid difficult for ethers in general. 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

The product has the following spectral properties infrared (KBr) cm.-1 3103 and 3006 (aromatic C—H), 2955, 2925, and 2830 (aliphatic C—H stretching), 1257 and 1032 (aromatic methyl ether), 841 and 812 (C—H out-of-plane bending of isoxazole C4—H and 4-substituted phenyl) proton magnetic resonance (trifluoroaeetic acid) 5, multiplicity, number of protons, assignment 3.98 (singlet,... 

The analysis of 1H NMR spectra of aliphatic and aromatic polyanhydrides has been reported by Ron et al. (1991), and McCann et al. (1999) and Shen et al. (2002), and 13C NMR has been reported by Heatley et al. (1998). In 1H NMR, the aliphatic protons have chemical shifts between 1 and 2 ppm, unless they are adjacent to electron withdrawing groups. Aliphatic protons appear at about 2.45 ppm when a to an anhydride bond and can be shifted even further when adjacent to ether oxygens. Aromatic protons typically appear with chemical shifts between 6.5 and 8.5 ppm and are also shifted up by association with anhydride bonds. The sequence distribution of copolymers can be assessed, for example in P(CPH-SA), by discerning the difference between protons adjacent to CPH-CPH bonds, CPH SA bonds, and SA-SA bonds (Shen et al., 2002). FTIR and 111 NMR spectra for many of the polymers mentioned in Section II can be found in their respective references. 

The 1,4-photoaddition of aliphatic amines with benzene via photoinduced electron transfer was first reported by Bryce-Smith more than 30 years ago [375-378], In the photoreaction of triethylamine with benzene, the proton transfer from the radical cation of triethylamine to the radical anion of benzene is proposed as a probable pathway (Scheme 113). In the case of tertiary amines, the photoaddition is accelerated by the addition of methanol or acetic acid as a proton source. Similar photoaddition of diethyl ether to benzene takes place assisted by trifluoroacetic acid, where methanol is not affective [379], In these photoreactions, a-hydrogen next to the heteroatom moves to the radical anion of benzene as a proton, followed by radical ccoupling to give 1,4-addition products. Similar photoaddition of amines to the benzene ring has been reported by Ohashi et al. [380,381],... 

The experimental KIEs were determined for the aliphatic Claisen rearrangement in p-cymene at 120°C and for the aromatic Claisen rearrangement either neat at 170°C or in diphenyl ether at 220°C. Changes in 2H, 13C or 170 composition were determined for unreacted substrates. For carbon analysis of allyl vinyl ether the C5 carbon was used as an internal standard. The C4 atom and rneta aryl protons were used as references in analysis of allyl phenyl ether. The 170 analysis was based on a new methodology. The results are summarized in Table 1, along with predicted isotope effects calculated for experimental temperatures by means of different computational methods. The absolute values of predicted isotope effects for C4 and C5 atoms varied with theoretical level and all isotope effects were rescaled to get reference effects equal to 1.000. 

The reactivity of an organic compound toward eaq depends on its functional groups because the main hydrocarbon chain is non-reactive. Aliphatic alcohols, ethers, and amines are also nonreactive (k 106 M 1 s-1), although alkylammonium ions show a slight reactivity and can transfer a proton to the hydrated electron. Isolated double bonds are practically nonreactive, for ethylene k <2-5 X 106 M -1 s-1, but conjugated systems or double bonds with an electron withdrawing group attached to them are very reactive. For example, butadiene and acrylic acid react with practically diffusion controlled rates ( 10 0 M -1 s-1). 

For PEMFCs, the solid electrolytes are polymer membranes polymers modified to include ions, usually sulfonic groups. One of the most widely used membranes today is the polymer Nafion , created by the DuPont company. These membranes have aliphatic perfluorinated backbones with ether-linked side chains ending in sulfonate cation exchange groups [6, 7], Nafion is a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether [8] and has a semi-crystalline structure [9], This structure (which resembles Teflon ) gives Nafion long-term stability in oxidative or reductive conditions. The sulfonic groups of the polymers facilitate the transport of protons. The polymers consist of hydrophilic and hydrophobic domains that allow the transport of protons from the anode to the cathode [10, 11],... 

Korosi94 proposed that the initial amidino acid is formed as shown in Scheme 14. According to this scheme the reaction is initiated by proton transfer from the amino acid to the lactim ether, being easier with aromatic and heterocyclic than with aliphatic amino acids. The... 

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