Proton-acceptor Site Selectivity

C-H bonds in n-alkanes, an observation that is supported by thermochemical data. The weaker protonation at the different irmer (non-penultimate) C-H bonds in n-alkanes apparently occurs to about the same extent at each bond. 

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Evidence for Proton-donor and Proton-acceptor Site Selectivity in the Symmetric Proton Transferfrom Alkane Radical Cations to Alkane Molecules... 

All chemical reactions in a biological cell take place with the direct participation of enzymes as catalysts. Enzymes are proteins, which are macromolecules composed of a combination of the 20 amino acids. Enzymes, as catalysts, are highly efficient and selective in binding small molecular species called ligands. A ligand that is acted upon by an enzyme to cause a chemical reaction is called a substrate. Only a certain, small portion of the amino acids that comprise an enzyme is involved in the catalytic reaction. This region is called the active site, and is directly involved in the formation of product(s). For example, the amino acid residues of proteins are greatly influenced by their local pH values, and the activity of proton acceptors and donors occurs in the active site. 

N, /[fi.W, Ni2 Afio ] ways. In a second step we have to select the A n acceptors 1 out of the acceptor population. This can be done in Af, /[AfnKA i - A n) ] ways. In a third step we have to select the B acceptors 2 out of the wA i acceptor population. However, once we have selected the B proton donors that participate in intramolecular bonds, we have also selected the molecules with the acceptor 2 sites that participate in the B indamolecular bonds. We will assume for simplicity that all w acceptor sites are equivalent for the intramolecular bonds. In each of these B molecules we must now select the acceptor 2 site for the intramolecular bond out of the w acceptor 2 population. For each molecule this can be done in h /[1 (w - 1) ] ways. Thus, for the B molecules it can be done in w /[l (w - 1) ] = w ways. Having selected the B acceptor 2 sites we must now select, out of the remaining (wNi - B) acceptor 2 population, the N12 that will participate in the intermo-lecular bonds. This can be done in (wN -B)U[(wNi - B- A/12)WiiH ways. The andN12 bonds can be done in ways while the B bonds in only one way after we have selected both the... 

Evidence for the occurrence of asymmetric proton transfer in mixed n-alkane crystals has been obtained in close cormection with the demonstration of site selectivity in the donor and acceptor processes. Such evidence can only be gathered properly from species that are related to the solute (higher alkane) molecule, because any site selectivity from matrix spedes is completely wiped out by an overwhelming amount of nonselective processes (see above). With respect to the site of proton donation, proton transfer from solute radical cations to matrix molecules was therefore studied, whereas, with respect to the site of proton acceptance, proton transfer from matrix radical cations to solute molecules was investigated. 

From Fig. 5.10 it is evident that penultimate C-H bonds in octane, which for reasons of energetics have the greatest propensity to act as proton acceptor, are also structurally favored over the interior C-H bonds with respect to proton acceptance from planar chain-end C-H bonds in the heptane radical cations. Both structural and thermodynamic factors thus favor the penultimate position and this translates into a very high selectivity with respect to this site (C2/C3 5.3, C2/C4 13.4, at 3 mol%). As a matter of fact, the minor formation of 3- and 4-chlorooctane can (largely) be attributed to random processes related to reaction (5.17). 

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