Group rate constant

Table 16.3 Group Rate Constants (A prim, k, ktert, kOH) and Group Substituent Factors, F(X), at 298 K for H Abstraction at C-H or O-H bonds a...

Each of these four second-order rate constants can be estimated from the structure of the compound of interest using group rate constants and substituent factors that have been derived from a large set of experimental data. This method has proven to be quite successful for prediction of k n0. for compounds that are well represented in this database (i.e., predictions within a factor of 2 to 3). Larger deviations are found,... 

H-Atom Abstraction from C-H and O-H Bonds. The estimation of kH0. (H-abstr.) is based on group rate constants (see Table 16.3) for -CH3 ( prim), -CH2- (ksec), >CH-(kteTt), and -OH ( 0H) that reflect the presence of the standard substituent -CH3 [i.e., kltn is the second-order rate constant for the abstraction of the tertiary C-H in (CH3)3C-H]. These standard values are then modified by multiplication with factors F(X) for substituents connected to the group considered ... 

Table 16.4 Group Rate Constants for HO Addition to Double and Triple Bonds at 298 K a...

HO Interaction with N-, P-, and S-Containing Groups. A rather limited data set is available for quantification of the interaction of HO with nitrogen-, phosphorus-and sulfur-containing functional groups. The group rate constants for some func-... 

Table 16.7 Group Rate Constants for HO" Interaction with N-, P-, and S- ...

In summary, when applied to the type of compounds from which the group rate constants and substituent factors given in Tables 16.3 to 16.7 have been derived, the estimation of k H0. from structure yields quite satisfactory results. As is demonstrated by the examples given in Illustrative Example 16.3, in many cases only one or two structural moieties dominate the overall k HQ. value. Therefore, calculation of k HQ. can often be simplified by taking into account only a few terms of the overall estimation equation (Eq. 16-25). 

Estimate the ho. and /I/2jHo- (for [HO ]ss = 106 molecule - cm 3) values at 25°C of the following compounds using the group rate constants and substituent factors and constants given in Section 16.3 and in Tables 16.3 to 16.6. Compare the estimated values of knQ. with the experimental values given in parentheses. Try to explain any larger discrepancies (i.e., > factor 3). 

The calculation of H-atom abstraction rate constants from C-H and O-H bonds is based on the estimation of group rate constants for H-atom abstraction from -CH3, -CH2-, >CH-and -OH groups (Atkinson, 1986). The rate constants for H-atom abstraction from -CH3, -CH2- and >CH- groups depends on the identity of the substituents attached to these groups, with... 

Note that these group rate constants and substituent group factors should be used only for homologs of the compounds from which these factors were derived (Kwok and Atkinson, 1995 Koch et al., 1996). For example, the group rate constants k, N, k>NH and k N,l2 and the substituent factors F(-NH2), F(>NH) and F(>N-) are appropriate only for alkyl-substituted amines and not for amines containing halogen or other hetero-atoms (in particular, see Koch et al., 1996). For the phosphorothioamidates (with ->P(=S)N< structures) and... 

Group Rate Constants at 298 K for the Reactions of OH Radicals with N-, S- and P-Containing Organic Compounds. 

OH radical reaction with chloropyrofos is anticipated to proceed via OH radical addition to the pyridine ring, OH radical "interaction" with the P=S group [with a group rate constant k >P=S (Table 14.4)], and H-atom abstraction from the two -OCH2CH3 groups bonded to the P atom. The total OH radical reaction rate constant is given by ... 

K Group Rate Constants for N03 Radical and 03 Addition to >C=C< Structural Units3... 

For example, the method Atkinson (1986) proposed requires a reliable rate constant database for the various classes of organic compounds in order to derive the various group rate constants and substituent factors. As may be expected, this estimation technique is reasonably reliable when used within its derivation database (Kwok and Atkinson, 1995). Thus, Kwok and Atkinson (1995) observed that the 298 K rate constants were predicted to within a factor of 2 of the experimental values for 90% of the 485 organic compounds that were considered and for which experimental data were available [and rate constants for alkyl nitrates are still predicted to within a factor of 2 after re-evaluation to take into account the recent kinetic and mechanistic data of Talukdar et al. (1997)]. 

Hydrolysis no hydrolyzable groups, rate constant at pH 7 is zero (Kollig et al. 1987). 

At present it is difficult to see just what factors govern the ease of electron exchange among hydrated metal ions. A typically perplexing series is the M(II)-- -M(III) electron exchange reactions of metal ions of the first transition group. Rate constants (25° C) have been found to lie in the following order ... 

Unidentate Leaving Groups.—Rate constants and activation parameters... 

Substrate-Aryl Groups Rate Constants References... 

The Z group both modifies the reactivity of the macromonomer transfer agent (1) towards radical addition and conveys stability to the intermediate radical (2). Z is most often an ester or an aryl group. Rate constants for radical addition to the macromonomer transfer agents are generally similar to that of the common monomers they resemble 10 -10 M s ) and transfer... 

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