Amines bond energies

Neglecting the possibility that the bond energy in primary amines be slightly less than in secondary and tertiary amines, we take the average value 2.88 v. e. for C N. 

Huisgen has stated that the driving force behind the 1,3-dipolar addition is stronger the more the loss of T-bond energy in the reactants is overcompensated by the energy of the two new bond energy is O-N < N-N < C-N, azides do not add at all to aldehydes and ketones and add with more difficulty to nitriles than to olefins. Phenyl azide, for instance, adds preferentially to the C-C double bond of acrylonitrile.194 103 This is also the reason why the condensation products of aldehydes and primary amines, which essentially exist in the Schiff-base structure 46a, react in the tautomeric enamine form 46b.2 ... 

The ability of various molecules to act as H atom donors in photoreduction varies greatly with the Z-H bond energy and with the electron density on the abstractable H atom. Molecules with electron donor groups (amino, hydroxy, etc.) near the H-donor GH group are very efficient. These include some amines, amides, alcohols and ethers hydrocarbons are substantially poorer H-donors and pure aromatics like benzene show very little reactivity. Acetonitrile H3C-CN is a very poor H-atom donor because its C-H bonds are made dipolar by the action of the strong electron acceptor CN so that the electron density on H is low. Water itself is an extremely bad H-atom donor in view of both the high OH bond energy and the acidic character of these bonds. 

Stabilization of the exciplex by the estimated bond energy of the amine dimer cation radical, 0.7 eV (129), would render intersystem crossing to form t endothermic by 0.2 eV as shown in Fig. 9 and enhance the rate of nonradiative decay. 

Although hydronium ion (H30+) (Chapter 8) and dioxygen (02) (Chapter 9) are the most studied of the molecules and ions without metal atoms, several of the molecules that contain sulfur, nitrogen, or carbon also are electroactive. The results for representative examples are presented to illustrate the utility of electrochemical measurements for die evaluation of the redox thermodynamics and bond energies for non-metal-containing molecules. In particular, die electrochemistry for several sulfur compounds [S8, S02, HS(CH2)3SH], nitrogen compounds [-NO, HON=0, N20, H2NOH, hydrazines (/ NHNH/ ), amines, phenazine], and carbon compounds (C02, CO, NCT) is summarized and interpreted. 

Hydrazines and Amines. These substrates are directly oxidized in a base-free matrix (Me2SO or MeCN) at platinum or glassy-carbon electrodes with the potential determined primarily by the / N—H bond energy, and secondarily by die basicity of the substrate (Figures 11.9 and 11.10) 15,16... 

In contrast, when hydroxide ion HO- is present, it is more easily oxidized than the amine substrates. In MeCN, in the absence of substrate, HO- is oxidized at +0.7-0.9 V versus SCE. However, with hydrazines and amines present, as in the case of H2NOH [Eq. (11.25)], the N—H bonds are homo-lytically cleaved by the HO product of HO- oxidation. The latter s oxidation potential is shifted by the difference in the HO—H and RN—H bond energies (—AGbf). Thus, the oxidation of PhNHNHPh is shifted by —1.7V when HO-becomes the electron-transfer mediator with PhNH2 the shift is —1.1 V ... 

TABLE 11.1 Oxidation Potentials for 1-5 mM Hydrazines and Amines in MeCN [0.1 M EtfNiClO ] and in the Presence of an Equivalent of HO- Evaluation of UN—H Bond Energies (—AGBF)... 

This class of compounds is kinetieally stabilized by bulky tm-butyl groups (3-6) or amine groups (7,8). In 3, 7, and 8, HF elimination should be possible, but it is hindered by the very strong SiF bond energy. These mono(silyl)hydrazines are very stable molecules and show no tendency to undergo condensation at room temperature. 

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