Radicals group increments

Group increments for silicon compounds based on the data reviewed here, and derived in accordance with the group additivity scheme29-31, are shown in Table 27. Radical and atomic heats of formation for non-silicon-containing species used in this chapter to derive bond dissociation energies are shown in Table 28. Miscellaneous inorganic silane heats of formation are included in Table 29. 

The major value of free radical group increments is in the prediction of stabilities of proposed radical and biradical intermediates in various thermal and photochemical reactions. For example, we might want to determine whether an observed thermal rearrangement occurs homolytically, via biradical intermediates, or by a concerted, pericyclic process. One valuable piece of information is AH° of the proposed biradical intermediate. If it is too high for the biradical to lie on the reaction path, then the biradical route can be rejected. We will see examples of this type of analysis in Chapter 15. 

Group Increment Values for Free Radicals (kcal/mol) ... 

A tertiary radical such as (CHiliC is more stable than an analogous primary radical suchasCH3CH2CH2CH2. Yet, in Table 2.7, the [ —(C)(H)2] group increment is more stabilizing (actually, less destabilizing) than the ( C—(0)3] group increment. Explain this. 

One of the most useful free radical rearrangements is the ring closure of the 5-hexenyl radical to form cyclopentylcarbinyl rather than cyclohexyl radical. Use group increments to estimate the relative energies of the 5-hexenyl, cyclopentylcarbinyl, and cyclohexyl radicals. 

Hydrolyzed Polyacrylamide. HPAM (6) can be prepared by a free-radical process ia which acrylamide is copolymerized with incremental amounts of acryUc acid or through homopolymerization of acrylamide followed by hydrolysis of some of the amide groups to carboxylate units. 

The moiety denoted as I is the initiator group. It can be as simple as a free radical or as complicated as a transition metal atom bonded to organic ligands and located on a catalytic support. The next step in the polymerization is propagation, i.e., the repeated insertion of monomer units into the chain to create an incrementally longer chain... 

An additional polar interaction called multidipole interaction is observed in reactions of peroxyl radicals with polyatomic alcohols [55], A few polar O—H groups interact with the polar reaction center C H O in such systems. A few examples of such interaction are given here [17]. Multidipole interaction sufficiently changes the thermoneutral activation energy of the reaction HOO + alcohol [54] and can be characterized by increment A A E. ... 

Another factor that influences the reactivity of two polar reactants, acylperoxyl radical with aldehyde, is the polar interaction of carbonyl group with reaction center in the transition state. Aldehydes are polar compounds, their dipole moments are higher than 2.5 Debye (see Section 8.1.1). The dipole moment of the acylperoxyl radical is about 4 Debye (/jl = 3.87 Debye for PhC(0)00 according to the quantum-chemical calculation [54]). Due to this, one can expect a strong polar effect in the reaction of peroxyl radicals with aldehydes. The IPM helps to evaluate the increment Ain the activation energy Ee of the chosen reaction using experimental data [1], The results of Acalculation are presented in Table 8.10. 

Volk and Schnitzer (1973) concluded that variations in the functional group components and spectral properties of humic acids from a group of Florida mucks indicated that higher rates of humification were related to (1) greater amounts of carboxyl, phenolic hydroxyl, quinone, and ketonic carbonyl groups (2) fewer alcoholic hydroxyl groups and aliphatic structures, as per IR evidence and (3) increments in EJEf, ratios and free-radical contents as revealed by ESR spectroscopy (Table 12). 

Following Bozzelli and Ritter (Lay et al., 1995 Ritter and Bozzelli, 1991), the thermochemistry of free radicals is estimated by adding Hydrogen Bond Increments (HBI) to the energy of the corresponding stable molecule where an H has capped the radical site. The HBI groups are also stored in a functional group tree. 

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