Phenols, H-bonding

Returning now to matters that are more properly within the spectral scope, we refer to the studies on phenolic H-bonding of Tsubomura (1952), who reported that with phenol and a variety of H-bond acceptors (in dilute hydrocarbon solution), there was no direct relation between the wavelength... 

Chem. 28, 22-30 (1956). IR phenolic H bonds in coals, carbonaceous materials. 

H-Bond Acceptor-Donor (HBAD) Phenols Aromatic acids Aromatic amines Alpha H nitriles Iniines Monocarboj hc acids Other monoacids Peracids Alpha H nitros Azines Primary amines Secondary amines n-alcohols Other alcohols Ether alcohols... 

The decrease in rate was proportional to the concentration of dioxane in the reaction mixture. An equivalent concentration of p-xylene (whose dielectric constant is similar to that of dioxane) produced a smaller decrease, consistent with simple dilution of the reactants. It was, therefore, hypothesized that dioxane forms an H-bonded molecular complex with phenol, the complexed form of the phenol being unreactive. The data could be accounted for with a 2 1 stoichiometry (phe-nokdioxane). This argument was supported by experiments with tetrahydrofuran, which also decreased the rate, but which required a 1 1 stoichiometry to describe the rate data. 

The influence of an ort/io-imidazole substituent on the bond dissociation energy of the O—H bond in phenol was studied by DFT calculations [00IJQ714]. The imidazole ring is twisted with respect to the phenol ring by 59° and causes a decrease of the bond dissociation energy by about -1 kcal/mol with respect to the unsubstituted molecule only. 

Phenol, C6H5OH, is a stronger acid than methanol, CH3OH, even though both contain an O-H bond. Draw the structures of the anions resulting from loss of H+ from phenol and methanol, and use resonance structures to explain the difference in acidity. 

Alcohols and phenols have nearly the same geometry around the oxygen atom as water. The R-O—H bond angle has an approximately tetrahedral value (109° in methanol, for example), and the oxygen atom is sp3-hybridized. 

The reaction is generally believed to proceed via the formation of ionic acylam-monium intermediate compounds (Reaction 1, Scheme 2.27). The equilibrium constant of the acylammonium formation depends mostly on steric and resonance factors, while the basicity of the tertiary amine seems to play a secondary role.297 In die case of the less basic compounds, such as acidic phenols, and of strong tertiary amines, such as Uialkylamines, the reaction has been reported to proceed through a general base mechanism via the formation of hydroxy-amine H-bonded complexes (Reaction 2, Scheme 2.27).297... 

C—H bonds. Phenol, mono-ortho, and di- and tri-substituted phenolic rings can be monitored between 814-831, 753-794, 820-855, and 912-917 cm-1, respectively. Para-substituted phenolic rings also absorb in the 820-855-cm 1 region. 

Spectroscopy of the PES for reactions of transition metal (M ) and metal oxide cations (MO ) is particularly interesting due to their rich and complex chemistry. Transition metal M+ can activate C—H bonds in hydrocarbons, including methane, and activate C—C bonds in alkanes [18-20] MO are excellent (and often selective) oxidants, capable of converting methane to methanol [21] and benzene to phenol [22-24]. Transition metal cations tend to be more reactive than the neutrals for two general reasons. First, most neutral transition metal atoms have a ground electronic state, and this... 

Phenol and hexamethylphoshoramide (HMPA) were selected, respectively, as the standard H-bond donor and H-bond acceptor with their values fixed on free energy and enthalpy H-bond scales for phenol, -2.50 for the H-bond donor enthalpy factor (TJ and also for the H-bond donor free energy factor (Ca) for HMPA, 2.50 for the H-bond acceptor enthalpy factor ( J and 4.00 for the H-bond acceptor free energy factor (CJ. 

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

Figure 5.3 illustrates the key features of the Fujita study. In relation to the reference phenol in frame (a), frames (b), and (c) illustrate the effect of H-bonding, and frames (d) and (e) illustrate steric hindrance. Given that the H-bond donor strength of (b) is greater than that of (c), since pKa (b) membrane partitioning, 8, increases in (b) and decreases in (c), relative to (a). Similarly, steric hindrance in (d) produces negative 8, compared to (e). 

Figure 5.3 The effect of hydrogen bonding and steric hindrance on the difference between liposome-water and octanol-water partition coefficients 8 increased H-bond donor strength and decreased steric hindrance favor membrane partitioning in the substituted phenols [381]. [Avdeef, A., Cun Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

Phenolic-OH (H-bonded) 17-11 Can be broad but more usually sharp as proton exchange is slowed by need to break both bonds. Can therefore be more difficult exchange. Warm if necessary. 

Hultgren, V. M. Atkinson, I. M. Beddoes, R. L. Collison, D. Garner, C. D. Helliwell, M. Lindoy, L. F. Tasker, P. A. Formation of folded complexes retaining intramolecular H-bonding in the extraction of nickel(II) by phenolic oxime and aliphatic diamine ligands. Chem. Commun. 2001, 573-574. 

Fig. 10.7 Crystal structure of com pounds 4 stick representation of H-bond (dotted lines) superstructures of (A) phenyl, (B) indole, (C) phenol ureido-silsesquioxanes H atoms were omitted for clarity.

The free radical equilibrium between PINO and hydroxylamines and phenols with known BDE of the O—H bonds was used for to estimate the BDE of the O—H bond of PINO and other hydroxylamines [90], The values of BDE (kJ mol ) are given below. 

Antioxidants that break chains by reactions with peroxyl radicals. These are reductive compounds with relatively weak O—H and N—H bonds (phenols, naphthols, hydro-quinones, aromatic amines, aminophenols, diamines), which readily react with peroxyl radicals forming intermediate radicals of low activity. 

Phenols decrease the intensity of CL 7chi in oxidized hydrocarbons as a result of chain termination by the reaction with peroxyl radicals. Since Icu [R02 ]2 (see Chapter 2), the ratio (/0//)12 was found to be proportional to [ArOH] [7]. The kinetic isotope effect (k0K/k0n 1) proves that the peroxyl radical abstracts a hydrogen atom from the O—H bond of phenol [2,8]. 

Polar solvents block the O—H bond of phenols in the reaction with peroxyl radicals due to the formation of hydrogen bond and decrease the activity of phenols as chain terminating agents [1,9,10]. 

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