Indiscriminate attacks

We have shown by our ISO brightness and Ak measurements that the lignin was attacked by the reactive species created by the y irradiation. The results indicate that the reactive species created attack indiscriminately the phenolic hydroxyl or coniferalde-hyde groups and the quinone groups. The decrease in physical properties is associated with die attack on the micro fibrils cellulose. However, at 3000 krad/h, the increase in the ISO brightness is smaller than the one observed around 300 krad/h for the TMP, while the physical properties of the pulp are affected more profoundly at 3000 krad/h than at 300 krad/h. The physical properties of the P-TMP behave similarly, while the increase in brightness is negligible at 3000 krad/h for P-TMP. 

Selectivity is an especially tough challenge in the case of autoxidation of small olefins in fluid phase because of diversion of the radical chain reaction by termination steps that result in the formation of oxy radicals. These are very reactive species that attack indiscriminately the starting hydrocarbon and primary products. The photochemical reaction in zeolites opens up much better controlled oxidation of low olefins, which are illustrated here briefly for the commercially important case of propylene oxidation. 

Ozone attacks the rings of PAHs rather indiscriminately with fission of the rings to prodnce aldehyde gronps. There has been concern, however, since the products may be more harmful than their precursors. In the studies that are used as illustration, in vitro gap junctional intracellular communication (GJIC) was used to assess adverse alteration on the expression of genes at the transcription, translational, or posttranslational level ... 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

As shown principally by the propane and butane experiments, CH2 from CH2CO is generally less indiscriminate than CH2 from CH2N2. In this respect CH2 from diazirine is similar to CH2 from CH2CO. Only in the liquid phase experiments, with CH2N2 as the methylene precursor, is random attack on C—H bonds found. 

The eourBe of addition of phenols to epicMorohydrin has been a subject of some controversy, since one might suppose the Btrongly nucleophilic character of phenoxide ion to cause indiscriminate attack on the chlorine-substituted carbon atom and tbe epoxide ring... 

Because hydroxyl radicals have indiscriminate reactivity, they can react with almost all types of organic and inorganic compounds. Most aromatic compounds undergo radical attack on the aromatic ring in a manner similar to that of benzene systems. The products and the rate constants for hydroxyl radical attack on aromatic compounds are listed in Table 5.11. The data were obtained from the pulse radiolysis studies (Buxton et al., 1988). 

Ketenes are highly reactive electrophiles but not nearly so indiscriminate as carbenes. When the properties of diazoacetyl photoaffinity reagents were evaluated the Wolff rearrangement was found to be a major problem accounting for 30 to 60% of the products arising from O-esters and 100% of the products fromS-esters. For instance, diazoacetyl-chymotrypsin gave rise to O-carboxymethyl serine formed by the attack of water on the ketene (Shafer et al., 1966) (Fig. 3.10). 

Moreover, the slight differences in reactivity of C—H bonds in alkanes to free radicals lead to indiscriminate attack of the hydrocarbon chains. Improvements in the efficiency and selectivity in the conversion of saturated hydrocarbons under relatively mild conditions is a desirable goal. This objective may be achieved by the selective activation of C—H bonds in alkanes by the use of suitable metal catalysts. The latter condition obtains in the selective microbiological hydroxylation of saturated hydrocarbons (see Section V.A), in which the enzyme probably interacts with the hydrocarbon via metal-catalyzed redox reactions. 

Chromic acid in aqueous sulfuric acid attacks the benzene ring in a large number of strychnine and brucine derivatives, leading to the isolation of products in which the aromatic ring has been oxidized away. Doubtless, as the low yields indicate, attack of the other, nonaromatic, portion of the molecules also occurs, but this is presumably so indiscriminate that the isolation of homogeneous products has so far failed. The following is a selection of the more important and interesting chromic acid oxidations. 

An insertion-type mechanism for the S( Z)) atom reaction is further supported by the indiscriminative nature of the attack on the various paraffinic C—H bonds. For the propane and isobutane molecules it was found that the two possible isomeric mercaptans were formed in a statistical ratio, i.e., n-PrSH/iso-PrSH 5 3.0, and iso-BuSH/f-BuSH iii 9.0. From methylene chemistry it is well known that singlet CH2 in its insertion reactions may be completely indiscriminative, but the abstractive attack of free radicals upon C—H bonds is known to be very sensitive to bond order and strongly increases in the sequence 0° < 1° < 2° < 3°. Insertive interactions with C—H bonds have also been postulated for singlet and triplet carbon atoms, NH, C2O radicals, and recently for the reactions of 0( Z)) atoms with alkanes. ... 

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