Bridgehead free radical stability. The decomposition of t-butylperoxyesters of the 1-adamantyl, 1-bicyclo[2•2•2]octyl, and 1-norbornyl systems

1968 ◽  
Vol 46 (19) ◽  
pp. 3099-3103 ◽  
Author(s):  
Leonard B. Humphrey ◽  
Bruce Hodgson ◽  
Richard E. Pincock

The t-butylperoxyesters of 1-adamantyl, 1-bicyclo[2•2•2]octyl, and 1-norbornyl carboxylic acids thermally undergo first order, free radical decomposition in cumene with relative rate constants at 80° of 1.0, 0.07, and 0.001 respectively. As measures of the stability of the derived bridgehead free radicals, the rates span a range much narrower than that for carbonium ion formation at these strained positions. The results are consistent with suggestions that free radicals have smaller force constants for out-of-plane bending than do the corresponding carbonium ions.

1998 ◽  
Vol 76 (7) ◽  
pp. 1064-1069 ◽  
Author(s):  
X -Z Zhang ◽  
R C Francis ◽  
D B Dutton ◽  
R T Hill

Peracetic acid (Pa) is now being used for totally chlorine-free delignification and bleaching of wood pulps. During the process, metals desorb from the pulp fibers into the aqueous solution. Of the relevant metals in bleaching systems, cobalt(II) and vanadium(V) are the most potent in wastefully decomposing Pa to O2. In the present study, radical scavenging by N,N'-(5-nitro-1,3-phenylene)-bisglutaramide (GAMID) indicates that free-radical mechanisms are operative. Kinetic data support a free-radical decomposition mechanism previously described for Co(II). A similar type of mechanism, involving VO(OH)2(aq) and VO(OH)3(aq), is postulated to partially explain catalysis by vanadium.Key words: peracetic acid, peroxymonosulfuric acid, transition metals, free radicals, acetoxy radical, peracetoxy radical.


2004 ◽  
Vol 467-470 ◽  
pp. 1111-1116 ◽  
Author(s):  
Lasar S. Shvindlerman ◽  
Günter Gottstein ◽  
Anthony D. Rollett

We present a new analysis of the relative rate of growth or shrinkage of grains in a two-dimensional network, based on the classical Von Neumann-Mullins (VN-M) analysis. We find that an analysis of the stability of the grain shape during shrinkage or growth shows that any change in the regular 2D grain leads to changes in the shape. We also re-examine a recent analysis that claims to have invalidated the VN-M relationship, but find that it is still valid, and that the cited analysis, in fact, confused a second order correction with a first order problem, partly because their derivation was in error. The erroneous magnitude of the discrepancy led them to use unphysical issues to explain the discrepancy. The way in which the curvature is distributed along the perimeter of a grain only gives rise only to second order corrections to the rate of change of area as a function of grain topology (number of sides).


1954 ◽  
Vol 32 (2) ◽  
pp. 63-70 ◽  
Author(s):  
R J Cvetanović ◽  
E. Whittle

The effect of fluctuations in free radical concentrations on the values of relative reaction rate constants calculated from steady state expressions has been evaluated for a number of idealized model cases, representative of various conditions usually encountered in experiments.


2019 ◽  
Vol 15 ◽  
pp. 746-751
Author(s):  
Zehong Liang ◽  
Henna Koivikko ◽  
Mikko Oivanen ◽  
Petri Heinonen

Five different 2-alkoxypropan-2-yl groups are introduced as acid-labile protecting groups for the 5’- and 3’-hydroxy groups of a 2’-deoxynucleoside. All studied protecting groups were readily introduced with good to excellent yields using the appropriate enol ether as a reagent and 0.5 to 1 mol % p-toluenesulfonic acid as a catalyst. The protected compounds could be purified by silica gel column chromatography without degradation. The compatibility of these protecting groups in parallel use with benzoyl and silyl groups was verified. The stabilities of the different alkoxy acetal protecting groups were compared by following the kinetics of their hydrolysis at 25.0 °C in buffered solutions through an HPLC method. In the pH range 4.94 to 6.82 the hydrolysis reactions are of first order in the hydronium ion. The rate of hydrolysis correlates with the electron-donating or electron-withdrawing ability of the corresponding alkoxy group. The studied 2-alkoxypropan-2-yl groups and the relative rate constants for their cleavage from the 5’-hydroxy group of 2’-deoxythymidine were: cyclohexyloxy (k rel = 7.7), isopropoxy (7.4), methoxy (1), benzyloxy (0.6) and 2,2,2-trifluoroethyloxy (0.04). The attachment of the same groups to the 3’-hydroxy group are from 1.3 to 1.9-fold more stable. The most reactive of these acetone-based acetal groups are faster removed than a dimethoxytrityl group, and they are easier to cleave completely in solution. The structural variation allows steering of the stability and lipophilicity of the compounds in some range.


The reaction between oxygen atoms and cyanogen has been studied in a flow system at temperatures up to 400 °C in the pressure range 1.0 to 5.0 mmHg. This reaction is first order in both reactants, and has an activation energy of 11.0 ± 2.0 kcal/mole; this is associated with the initial attack of an oxygen atom on cyanogen for which the exponential factor is close to 4 x 10 13 cm 3 mole -1 s -1 . It has not been possible to choose between mechanisms in which the initial steps are O + C 2 N 2 = NCO + CN (6) and O + NCO = NO + CO, (7) or O + C 2 N 2 = NCN + CO (8) and O + NCN = NO + CN. (9) Subsequent reactions include CN + NO = CO + N 2 , (11) O + CN = N + CO. (12) In the presence of molecular oxygen reactions (10) and (7) become important CN + O 2 = NCO + O. (10) Strong CN emission accompanying the reaction is shown to be largely due to the reaction O+O+CN = O 2 + CN*. (20) Quenching of this emission by various additives is associated with their reaction with CN for which relative rate constants are deduced.


1952 ◽  
Vol 30 (4) ◽  
pp. 295-310 ◽  
Author(s):  
R. U. Lemieux ◽  
Carol Brice

The stannic chloride catalyzed anomerization of the pentaacetyl-D-gluco-pyranoses in chloroform solution is specific for the C1-acetoxy group. The reactions involve complete dissociation of the C1-carbon atom to acetoxy group bond with an intermediate formation of carbonium ions. The initial step of the beta to alpha rearrangement is a rapid dissociation, involving the participation of the C2-acetoxy group, to a resonance-stabilized carbonium ion with the lactol carbon atom occupied in the α-configuration. The rate-controlling step in the reaction appears to be the rearrangement of this ion to other ions which are capable of recombining with acetate ion to yield the α-acetate. The α-acetate is highly stable, as compared to the β-anomer, and the dissociation of the C1-carbon atom to acetoxy group bond is the rate-controlling step in its rearrangement. The stability of the α-acetate toward a variety of acidic reagents which readily dissociate the β-form is pointed out. For example, although the α-acetate is highly stable toward titanium tetrachloride, the reaction of the β-anomer with this reagent, to yield tetraacetyl-β-D-glucopyranosyl chloride, is extremely fast. This product is unstable under the reaction conditions used and rearranges to the α-form at a measurable rate. 1,2,3,4-Tetraacetyl-β-D-glucopyranose with stannic chloride in chloroform solution yielded triacetyl-D-glucosan <l, 5 > β <1, 6>.The alkaline hydrolysis of triacetyl-D-glucosan <1, 5> α <1, 2> yielded D-glucosan <1, 5> β <1, 6>. The mechanisms of these reactions are discussed.


2012 ◽  
Vol 204-208 ◽  
pp. 3061-3067 ◽  
Author(s):  
Hou Jun Kang ◽  
Yue Yu Zhao ◽  
Hai Ping Zhu

Cable-arch structure is a combined structure, which utilizes flexibility of cable and rigidity of arch. Cable-arch structure has been widely used in bridge engineering. In this work, we proposed a novel mechanical model of cable-arch structure. The out-of-plane buckling and in-plane buckling were studied using the energy method. The formula of critical loads of both the first order out-of-plane buckling and in-plane buckling were derived using Rayleigh-Ritz method. An example is ultimately investigated numerically. The results indicate that the cable can improve considerably the out-of-plane and in-plane stability of arch. Therefore, the research about the stability of cable-arch structure are both valuable not only in theoretical research but also in design of engineering structure.


2012 ◽  
Vol 1 (10) ◽  
pp. 79 ◽  
Author(s):  
G. Raja* ◽  
Ivvala Anand Shaker ◽  
Inampudi Sailaja ◽  
R. Swaminathan ◽  
S. Saleem Basha ◽  
...  

Natural antioxidants can protect the human body from free radicals and retard the progress of many chronic diseases as well as lipid oxidative rancidity in foods. The role of antioxidants has protected effect against free radical damage that may cause many diseases including cancer. Primary sources of naturally occurring antioxidants are known as whole grains, fruits, and vegetables. Several studies suggest that regular consumption of nuts, mostly walnuts, may have beneficial effects against oxidative stress mediated diseases such as cardiovascular disease and cancer. The role of antioxidants has attracted much interest with respect to their protective effect against free radical damage that may cause many diseases including cancer. Juglans regia L. (walnut) contains antioxidant compounds, which are thought to contribute to their biological properties. Polyphenols, flavonoids and flavonols concentrations and antioxidant activity of Leaves, Stems and Nuts extract of Juglans regia L. as evaluated using DPPH, ABTS, Nitric acid, hydroxyl and superoxide radical scavenging activity, lipid peroxidation and total oxidation activity were determined. The antioxidant activities of Leaves, Stems and Nuts extract of Juglans regia L. were concentration dependent in different experimental models and it was observed that free radicals were scavenged by the test compounds in all the models.


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