Hydroperoxide radicals, bond dissociation energy

Enthalpies of Formation AH (gas, 298 K) of Hydroperoxides and Peroxyl Radicals and C—02 Bond Dissociation Energies in Peroxyl Radicals... 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

Solar light may generate sufficient energy to polymeric materials in order to cleave RO-OH and R-OOH bonds in hydroperoxides. So the dissociation energy value of RO-OH bond in hydroperoxides is 176 kJ moF and that of R-OOH is 377 kJ moF [7]. That is why it is considered that during hydroperoxide irradiation the formation of RO- and HO- radicals is predominant. 

The electronic spectrum of the cyclohexylperoxyl radical has a maximum at A = 275 nm with molar absorption coefficient e = 2.0 x 103L mol-1 cm-1 [103]. The dissociation energy of the O—H bond in a hydroperoxide ROOH depends on the R structure [104-106] ... 

Chain propagation in an oxidized aldehyde is limited by the reaction of the acylperoxyl radical with the aldehyde. The dissociation energy of the O—H bond of the formed peracid is sufficiently higher than that of the alkyl hydroperoxide. For example, in hydroperoxide PhMeCHOOH, Z)0 H = 365.5 kJ mol-1 and in benzoic peracid... 

The first law of photochemistry [the Grotthus-Drapper principle (30)] states that for a photochemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry [the Stark-Einstein principle (3J)j states that a molecule can only absorb one quantum of radiation. The absorbed energy causes the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to formation of chromophoric groups such as carbonyls, carboxyls, qui-nones, peroxides, hydroperoxides, and conjugated double bonds. 

The calculated C=CO—OH bond energy is very weak, 7.1 kcal mol in C=CO—OH and 7.8 kcal mol in the methyl substituted CC=CO—OH the bond strengths are only on the order of a hydrogen bond. The ethynyl hydroperoxide moiety is highly unstable. This molecule will rapidly dissociate to form a ketenyl radical + OH ... 

The C=CO—OC and CC CO—OC bond energies in ethynyl peroxides are 3.27 and 3.72 kcal mof respectively but they are too small to exist physically at high temperature the ethynyl peroxide moiety is unstable. The HC COOCHs and CH3C=COOCH3 molecules will immediately dissociate to form a ketenyl radical (C H=C=0 or CC =C=0 and CH3O) with bond energies less than 4 kcal mof. This instability is similar in the ethynyl hydroperoxide molecules. 

The initiation step provides a radical source by thermal or photochemical dissociation of initiators, which then provides bromine radicals by reaction with Br2. Initiators are sometimes present in the alkene as allyl hydroperoxides which may be present due to inadvertent, prior autooxidation. Bromine or HBr may be present in trace amounts in NBS. Reaction of the bromine radical 20 with the substrate 1 proves selective for allylic or benzylic hydrogens due to the near thermoneutral nature of the reaction which breaks the C-H bond and forms the H-Br bond. Reaction of the formed carbon-centered radical 21 with Br2 provides the desired bromide 3 and Br 20. Hydrogen bromide 17 reacts with NBS to form succinimide 4 and resupplies the required low concentration of Br2. Alternatively, reaction of substrate radical 21 with NBS 2 provides product 3 and succinimidyl radical 22 (S ). Due to energy and kinetics considerations, abstraction of the allylic hydrogen by the S should be slower than abstraction of bromine from NBS by an allyl radical. In using solvents in which NBS, succinimide 4 or it s radical 22 are not very soluble, S is not the key chain-carrier. Byproducts and side-reactions can occur with S. ... 

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