Nucleophilic displacement reactions barriers

Polymers for these conductive systems may be synthesised by a variety of means including Ziegler-Natta polymerisation or nucleophilic displacement reactions. The molecules tend to be rigid because of the need for them to possess extended conjugation. This lack of free rotation about carbon-carbon bonds within the molecule imposes a high energy barrier to solvation, thus making these molecules difficult to dissolve. This lack of solubility in turn... 

Beam techniques may be used to probe energy barriers and chemistry driven by translational energy the flow reactor is used to study rates and mechanisms at thermal energies. Rates studied as a function of temperature reveal barriers to reaction. Rates studied as a function of solvation number reveal the kinetic role of solvate, in the absence of bulk solvent. We illustrate this behavior for the nucleophilic displacement reaction... 

The pentamer of tetrafluoroethene (66) (Scheme 29) is an unusual example of type (96) and reacts readily with nucleophiles [129] (Scheme 65). In contrast, (66) undergoes a remarkable reaction with aqueous triethylamine, producing the dihydrofuran derivative (101) and the process formally involves a direct intramolecular displacement of fluorine from a saturated site and a mechanism has been advanced (Scheme 66) which accounts for the product formed [126]. Understandably, this process is not easily accepted [3, 130] because it has essentially no precedent. Indeed, it is well established that nucleophilic displacement from saturated sites in fluorocarbons occurs only in exceptional circumstances. Consequently, other mechanisms have been advanced, which seem no more convincing [3, 130]. It should be remembered that the major point in favour of the step (100) to (101) (Scheme 66) is that the nucleophile is generated in close proximity to the reaction centre because of the special geometry of this situation. Consequently, much of the otherwise high energy/entropy barrier has already been overcome in this case. 

These results extend our knowledge of the competition between basicity and nucleophilicity. In solution, proton transfer preempts nucleophilic displacement wherever it is possible thermodynamically. This situation is the simple consequence of low barriers for the former and high barriers for the latter. In the gas phase, proton transfer also preempts nucleophilic displacement. Here, there are no barriers for proton transfer—reaction occurs on every collision—and low barriers for nucleophilic displacement. [The relationship between the barriers in the two phases has been explored in a series of important papers by Brauman and co-workers and reviewed (25, 26).] What has therefore been established in the gas phase is that proton transfer always wins when it is exothermic and that nucleophilic displacement is only efficient when proton transfer is endothermic (10). What is added here is that endothermic proton transfer can still win, when energy is available. If the explanation advanced is correct, this situation is only possible where the actual proton transfer remains exothermic. 

Why no reaction is found for the triply hydrated reactant is now apparent. If the product ion has to be Cl-, this channel is now endothermic by 6 kcal/mol. The more hydrated the reactant ion, the more endothermic will be the reaction channel leading to the Cl- product. Hydration quenches the reaction if the reactant has more than two waters of hydration. When the reactant has only one or two waters, Figure 7 suggests why the reactivity decreases as shown in Figure 6. Addition of the two waters successively decreases the exothermicity this result, in turn, raises the barrier for nucleophilic displacement—through application of Marcus theory (24, 25). 

As with Mb/Hb, metHr is the thermodynamically stable form in aerobic solutions, and a metHr reductase has been reported. The rate of autoxidation of oxyHr decreases by approximately tenfold with a one-unit decrease in pH in the acidic region, consistent with reaction (6a). Certain exogenous anions, particularly azide, catalyze autoxidation of oxyHr by nucleophilic displacement of the peroxo ligand and/or by delivery of a proton to the coordinated peroxo. The slow rates of reactions (6a) and (6b) in the absence of these anions, however, indicate a substantial kinetic barrier to autoxidation. Presumably, autoxidation requires entry of solvent into the 02-binding pocket of oxyHr, i.e., transient, simultaneous occupancy of the pocket by both bound O2 and solvent. Consistent with this mechanism, the L98 variants of Hr in which either smaller or more polar side chains were substituted all led to significant increases in autoxidation rates.Thus, the main function of L98 and the other hydrophobic residues shown in Figure 17 may be as hydrophobic and steric barriers to solvent-induced autoxidation. 

Nevertheless, the reaction conditions required by strategy A, as well as strategy B, for the nucleophilic displacement of the halide ions in fragment g are a barrier to increasing spectacularly the yield of the catenation reaction. Indeed, the stability of the precatenane species Cu(3.4) or Cu(3)2 in basic medium is rather limited, especially at high temperature. A new approach was developed later, which utilizes a... 

These results clearly show that the potential energy surface can contain a series of minima. The fact that selectivity in re-attack by the F ions can be observed indicates that the differences between the energy barriers for the secondary reactions control the distribution of the final products. The multistep character of these processes is further illustrated by the reactions observed when enolate anions are used as reactant ions. The ambident enolate anions may react with methyl pentafluorophenyl ether at the carbon or the oxygen site. If they react with the carbon site at the fluorine-bearing carbon atoms, then the molecule in the F ion/molecule complex formed contains relatively acidic hydrogen atoms so that proton transfer to the displaced F ion may occur. An example is given in (47) where the enolate anion, generated by HF loss, is not observed. An intramolecular nucleophilic aromatic substitution occurs instead and leads to a second F ion/ molecule complex. The F" ion in this complex then re-attacks the substituted benzofuran molecule formed, either by proton transfer or SN2 substitution. 

Sn2 reactions of allylic halides with good nucleophiles (Section 6-8) are faster than those of the corresponding saturated haloalkanes. Two factors contribute to this acceleration. One is that the allylic carbon is attached to a relatively electron-withdrawing sp hybridized carbon (as opposed to sp Section 13-2), making it more electrophilic. The second is that overlap between the double bond and the p orbital in the transition state of the Sn2 displacement (see Figure 6-4) is stabilizing, resulting in a relatively low activation barrier. 

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