Klein and co-workers have documented the remarkable lubricating attributes of polymer brushes tethered to surfaces by one end only [56], Studying zwitterionic polystyrene-X attached to mica by the zwitterion end group in a surface forces apparatus, they found /i < 0.001 for loads of 100 and speeds of 15-450 nm/sec. They attributed the low friction to strong repulsions existing between such polymer layers. At higher compression, stick-slip motion was observed. In a related study, they compared the friction between polymer brushes in toluene (ji < 0.005) to that of mica in pure toluene /t = 0.7 [57].  [c.447]

Glycine is present in aqueous solution as the "zwitterion NH CH,COO which is incapable of reacting with formaldehyde. When, however, sodium  [c.463]

Some ammo acids have side chains that bear acidic or basic groups As Table 27 3 indicates these ammo acids are characterized by three values The third pK reflects the nature of the side chain Acidic ammo acids (aspartic and glutamic acid) have acidic side chains basic ammo acids (lysine arginine and histidine) have basic side chains The isoelectric points of the ammo acids m Table 27 3 are midway between the pK values of the zwitterion and its conjugate acid Take two examples aspartic acid and lysine Aspartic acid has an acidic side chain and a pi of 2 77 Lysine has a basic side chain and a pi of 9 74  [c.1118]

The most stable structure of a neutral ammo acid is a zwitterion The pH of an aqueous solution at which the concentration of the zwitterion IS a maximum is called the isoelectric point (pi)  [c.1150]

Zaitsevs rule (Section 5 10) When two or more alkenes are capable of being formed by an elimination reaction the one with the more highly substituted double bond (the more sta ble alkene) is the major product Zwitterion (Section 27 3) The form in which neutral amino acids actually exist The ammo group is in its protonated form and the carboxyl group is present as a carboxylate  [c.1297]

In acyl halides R—CO—X Shorter bond in carboxylic acids and esters In zwitterion forms In 0=C= In isocyanates RN=C=0 In conjugated systems, as in partial triple bond 0=C—C=C In 1,4-quinones In metal acetylacetonates In calcite CaCOg 117.1(4) 123.3(5) 126(1) 116.0(1) 117(1) 121.5(5) 115(2) 128(2) 129(1)  [c.313]

Because of the zwitterion formation, mutual buffering action, and the presence of strongly acid components, soybean phosphoHpids have an overall pH of about 6.6 and react as slightly acidic in dispersions-in-water or in solutions-in-solvents. Further acidification brings soybean phosphoHpids to an overall isoelectric point of about pH 3.5. The alcohol-soluble fraction tends to favor oil-in-water emulsions and the alcohol-insoluble phosphoHpids tend to promote water-in-oil emulsions.  [c.99]

The 1,3-zwitterion appears to have some diradical as weU as 1,2-zwitterionic (carbonyl oxide) character  [c.117]

Hydrogen atoms in azolium ions can be removed easily as protons (e.g. 230—>232) exchange with deuterium occurs in heavy water. The intermediate zwitterion (e.g. 231) can also be written as a carbene, and in some cases this carbenoid form can be trapped or isolated as a dimer.  [c.70]

With 3- and 4-substituted isoxazoles the tautomeric form normally present is the XH tautomer, (13 X = O) and (14 X = O, N) respectively. However, other influences need to be considered as in cycloserine (IS), which exists as a zwitterion, as does 5-amino-3-hydroxy-isoxazole (16).  [c.11]

Ethylene. A possible loop for ethylene photolysis was presented in Figure 14. Experimentally, irradiation into the first absorption band [populating the B(1 B ) state] leads to cis-trans isomerization as well as to a H atom shift. The covalent A state lies at a very high energy in the planar form [79,80], but is the lowest excited singlet in the perpendicular one. This system was studied as early as 1985 by Ohmine [6], who reported a conical intersection that involves pyramidalization, and may lead to hydrogen-atom transfer. Fle calculated the conical intersection to be found along two coordinates—a phase inverting one (rotation around the double bond) and a phase preserving one, fomiing a methyl carbene (hydrogen-atom transfer) or a charge separated intermediate. Similar results were obtained more recently by the extensive studies of Ben-Nun and Martinez ([10] and references cited therein). Figure 33 shows a phase-inverting loop for the cis-trans isomerization, that leads also to a hydrogen-atom shift. The third anchor in this loop may appear as a carbene (CH3CH ) or as a zwitterion (CHjCHj), see Figure 1(4). The conical intersection encircled by the loop is termed CIh, for hydrogen-atom transfer.  [c.366]

The glycine so formed has to be separated from the ammonium chloride, and the salt-like nature of the zwitterion precludes "NHt CHtCOO the use of most organic solvents for this purpose.  [c.130]

Glycine is present in aqueous solution as the "zwitterion NHjCHjCOO which is incapable of reacting with formaldehyde. When, however, sodium  [c.463]

A very convenient method for activation and Sn2 substitution of alcoholic hydroxy groups is the Mitsunobu reaction. In this reaction a zwitterion resulting from addition of a phosphine or a phosphite ester to diethyl diazenedicarboxylate converts the hydroxy groups into good leaving groups. In the presence of zinc halides or of imides efficient conversions with nearly complete inversion of configuration to alkyl halides (P.-T. Ho, 1984) or to N-alkylated imides (O. Mitsunobu, 1972) are observed. With carboxylic acids an inversion-esterification of the alcohol is achieved (see p. 286). Phosphoric acid derivatives and car-  [c.160]

When the nitrogen atom is substituted by a nitrophenacyl group, OH attack gives the betainic zwitterion (Scheme 13). which is soluble in organic solvents (32). The stability of the C-betainic or ylid structure has been explained as an effect of resonance of the negative charge in the molecule (33, 34).  [c.33]

The isomerization of alkyiisothiazoles has been studied and leads to alkylthiazoles. The isomerization seems to occur by a zwitterion mechanism (Scheme 1).  [c.374]

The general mechanism of the rearrangement of aryl and diaryl-thiazoles seems to exclude the zwitterion route. Instead it takes place through bending of thiazoles bonds (98.213). Moreover, tricyclic sul-fonium cation intermediates, after irradiation of deuterated phenyl-thiazoles, have been suggested by several workers (98).  [c.378]

Table 27 2 includes a column labeled pi which is the isoelectric point of the ammo acid The isoelectric point, also called the isoionic point, is the pH at which the ammo acid has no net charge It is the pH at which the concentration of the zwitterion is a maximum At a pH lower than pi the ammo acid is positively charged at a pH higher than pi the ammo acid is negatively charged For the ammo acids m Table 27 2 pi is the average of pA i and pK 2 and lies slightly to the acid side of neutrality  [c.1118]

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6).  [c.493]

The zwitterion (6) can react with protic solvents to produce a variety of products. Reaction with water yields a transient hydroperoxy alcohol (10) that can dehydrate to a carboxyUc acid or spHt out H2O2 to form a carbonyl compound (aldehyde or ketone, R2CO). In alcohoHc media, the product is an isolable hydroperoxy ether (11) that can be hydrolyzed or reduced (with (CH O) or (CH2)2S) to a carbonyl compound. Reductive amination of (11) over Raney nickel produces amides and amines (64). Reaction of the zwitterion with a carboxyUc acid to form a hydroperoxy ester (12) is commercially important because it can be oxidized to other acids, RCOOH and R COOH. Reaction of zwitterion with HCN produces a-hydroxy nitriles that can be hydrolyzed to a-hydroxy carboxyUc acids. Carboxylates are obtained with H2O2/OH (65). The zwitterion can be reduced during the course of the reaction by tetracyanoethylene to produce its epoxide (66).  [c.494]

Unsaturated compounds undergo ozonization to initially produce highly unstable primary ozonides (15), ie, 1,2,3-trioxolanes, also known as molozonides, which rapidly spHt into carbonyl compounds (aldehydes and ketones) and 1,3-zwitterion (16) intermediates. The carbonyl compound-zwitterion pair then recombines to produce a thermally stable secondary ozonide (17), also known as a 1,2,4-trioxolane (44,64,125,161,162).  [c.117]

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extmded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (17) as shown with itself (dimerization) to form cycHc diperoxides (4) or with protic solvents, such as alcohols, carboxyUc acids, etc, to form a-substituted alkyl hydroperoxides. The latter can form other peroxidic products, depending on reactants, reaction conditions, and solvent.  [c.117]

There is evidence that dioxirane is an intermediate product in the low temperature ozonization of ethylene and is probably formed from the diradical resonance isomer of the 1,3-zwitterion (164).  [c.118]

Ozone cracking is a physico-chemical phenomenon, and many factors are involved in explaining the effect of ozone attack on elastomers (2—7). From a chemical point of view, ozone attack on olefinic double bonds causes chain scission and the formation of the decomposition products shown in Figure 1 (8,9). These chemical reactions are beheved to be similar for both small olefins and unsaturated mbbers. The first step is the formation of a relatively unstable primary ozonide (or molozonide) (1), which cleaves to an aldehyde or ketone (2) and a carbonyl oxide (or zwitterion) (3). Subsequent recombination of (2) and (3) produces a secondary ozonide (or just ozonide) (4). In small olefins, ozonide formation is generally a facile process. In stretched mbber, however, ozonide formation is more difficult, since the cleaved intermediates (2) and (3) may be forcefiiUy separated to reheve the stress. Ozonides are reasonably stable in neutral environments, but they decompose readily under the influence of heat or various reducing agents to yield such chain scission products as aldehydes, ketones, acids, and alcohols. Polymeric peroxides (5) may be formed initially from the carbonyl oxide, but these are unstable and eventually decompose to yield chain scission products. The rate of chain scission is increased in the presence of active hydrogen (eg, water), probably because of the reaction with carbonyl oxides to form reactive hydroperoxides (6).  [c.236]

Oxiranones (a-lactones) 81JA686, 80AG(E)276), e.g. (6), are highly reactive, readily polymerizing (Scheme 18), possibly via a zwitterion (18). Such a species (19) would also account for the rearranged products (20) and (21) from (22 Scheme 19).  [c.103]

Methylideneoxiranes (allene oxides Section react with nucleophiles as if ring opening occurs to give a zwitterion (e.g. 51 or 52), which may be captured by the nucleophile before (Scheme 42) or after (Scheme 43) isomerization to a cyclopropanone.  [c.109]

See pages that mention the term Zwitterion : [c.434]    [c.130]    [c.435]    [c.620]    [c.244]    [c.39]    [c.84]    [c.97]    [c.1118]    [c.1118]    [c.1275]    [c.117]    [c.470]    [c.297]    [c.4]    [c.136]    [c.100]    [c.103]    [c.168]    [c.177]    [c.281]   
Practical organic chemistry (1960) -- [ c.130 ]

Practical organic chemistry (1978) -- [ c.130 ]