Group Size Effects

The influence of the group size on the rate of generation of alkyl radicals has been investigated for the same reactions as mentioned in Table 112a 27. Most information is available on the thermolysis of t-azoalkanes 20 (R1 -R3 = alkyl)28.  

Qualitatively the saijie reactivity pattern was observed for the decomposition of sym. azonitriles 20 (R1 = CN, R2, R3 = alkyl)29 and several symmetrically and un-symmetrically substituted azo compounds30. A selection of these results is found in Table 2. It is apparent from these data that the thermal stability of 20 decreases as the size of the groups R1—R3 increases. Riichardt et al. have observed that a linear relationship exists between the thermolysis rates of Table 2 and the Sn 1-solvolysis rates of corresponding f-alkyl-p-nitrobenzoates 27 in 80% acetone-water28 d. The 

In comparison with the decomposition of frans-azoalkanes 20 a much larger group size effect has been found for the thermolysis rates of a few cis-azoalkanes 24. Due to the repulsion of the free electron pairs on the two nitrogen atoms and due to steric interaction between the cis oriented alkyl groups cis azoalkanes 24 decom- 

The rates of homolytic fragmentation of peroxyesters 25 are also enhanced when the size of the side chains R1 —R3 = alkyl is increased. This is shown for several examples in Table 4. The rate enhancing effect is smaller than for the azoalkane thermolyses 

It is somewhat contradictory and not yet fully understood why the back strain effect on the rate of perester decompositions is so large. We had reasoned before from the discussion of conformational effects that the Ca-CO-bond of 25 is only stretched to a small extent at transition state. From an analysis of bond energies5,18) it becomes questionable if the homolysis of C-N-bonds (as in 20 ) and C-C-bonds (as in 25) is likely to be directly comparable5 12a 18). In addition the extent of Ca-CO-cleavage at the transition state of fragmentation of 25 may well be itself dependent on the 

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Cold Habitats. Because of considerations of surface area relative to body mass, animals that live in cold habitats tend to have larger body sizes and smaller extremities (especially ears and legs) compared to their counterparts in warmer habitats. Animals that live in cold habitats also have a greater amount of insulation, such as fat, fur, or feathers. Behavioral adaptations include gathering in groups, which effectively decreases the exposed surface area of each individual. 

A detailed study on the size effect of gold has been carried out by the group of Tsukuda [102]. Figure 33... 

The results of the kinetic study and model discrimination show that insertion of SM is rate-controlling. Two reasons may explain why this step is ratecontrolling. First, the protection group in om SM is very bulky, making the reaction slow, which is consistent with literature data (8) showing the size effect on reactivity. Second, a free aniline group in the SM could bond with Rh and reduce the catalyst reactivity. 

Recently, Porter et al. (1986b, 1988) have reported the synthesis of both meso- and ( )-forms of a series of two-chain carbonyl diacids made by joining two pentadecanoic acid units by a carbonyl group at the 3,3, 6,6, 9,9 and 12,12 positions, 3,5-didodecyl-4-oxoheptanedioic acid (C-15 3,3 ), 6,8-dinonyl-7-oxotridecanedioic acid (C-15 6,6 ), 9,11-dihexyl-10-oxononadecanedioic acid (C-15 9,9 ) and 12,14-dipropyl-13-oxopentacosanedioic acid (C-15 12,12 ), respectively. The diacids were used to probe further the question of stereochemical preference in two-chain amphiphiles. The method used for examining the diastereomeric preference was equilibration by base-catalyzed epimerization in homogeneous, bilayer and micellar media. This method allows for stereoselection based on hydrophobic/hydrophilic considerations rather than classic steric size effects. 

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