Acetic resonance energy

Data for acetic acid and eight other monobasic acids lead to a value of 1.20 v.e. for the resonance energy of the carboxyl group relative to the OH... 

Following previous work, Hagiwara and collaborators [71] recently prepared 5 -terminal acridone-labeled DNAs, using the succinimidyl ester 24 of the acridone acetic acid 23 reported before [69], and evaluated their use as donors for a fluorescence resonance energy transfer (FRET) system in combination with 3 -dabcyl-tagged DNA... 

In other words, the unshared electron pair of the base, acetate ion, is delocalized (spread over both oxygens) by resonance. This electron pair is stabilized and less available for bonding to the proton, which localizes this electron pair in the sigma bond and costs resonance energy. The most common effect of resonance on an acid-base reaction is to delocalize and stabilize the unshared electron pair of the conjugate base, resulting in a stronger acid. 

AhydH was measured in acetic acid. The authors give a short history of resonance energy calculations for azulene and arrive at their own crude (their adjective) estimate of 28 kcal mol 1. Azulene is aromatic because transfer of an electron pair from the seven-membered ring to the five-membered ring causes both rings to be aromatic. It is blue because of this electron transfer, which does not occur in its isomer naphthalene. 

The stereochemistry of sy/i- and a/fr/-13 diols in the polyene macrolide antibiotics and polyketides can be determined from the chemical shifts of the acetal carbons and methyl carbons of the corresponding acetonides. The acetonides of jy/i-l,3-diols [Scheme 3.36] occupy a low energy chair conformation displaying an acetal resonance at b 98.5 and methyl resonances at 6 30.0 (equatorial) and b 19.6 (axial). In both possible chair conformations of the a/i/i-l,3-dioI acetonides. there are severe 13-diaxiaI interactions that are relieved in the corresponding twist-boat. In the twist-boat conformation, the acetal carbon resonates at 6 100.6 whereas both methyl groups resonate at b 24.6 because the methyl groups are in nearly identical environments. The twist-boat conformation is more stable than the corresponding chair conformations by about 2 kcal/mole. 

HuckeFs rule leads one to expect resonance stabilization in the cyclopentadienate and cycloheptatrienyl (tropyllum) ions. With a view to determining the resonance energy of the latter, Turner has measured the heat of hydrogenation of tropylium chloride to cycloheptaiie and hj drogen chloride, in acetic acid solution, ag — 86 23 rt 0 08 kcal/mole. An energetic comparison of the relative stabilities of tropylium chloride and the isomeric benzyl chloride... 

Ethylenediamine tetraacetic acid (or any of its conjugate acetate ions) Forster resonance energy transfer, a form of excitation energy transfer (sometimes referred to with the incorrect term fluorescence resonance energy transfer )... 

DTPA = diethylentetramine-penta-acetate FRET = Forster resonant energy transfer MRI = magnetic resonance imaging NIR = near infra-red PET = positron emission tomography. 

We might be hard pressed to estimate the individual resonance stabilization energies in Eqs. (7.23) and (7.24), but the qualitative apphcation of these ideas is not difficult. Consider once again the styrene-vinyl acetate system ... 

Define styrene to be monomer 1 and vinyl acetate to be monomer 2. The difference in resonance stabilization energy ep. - > 1, since... 

Copolymers of VF and a wide variety of other monomers have been prepared (6,41—48). The high energy of the propagating vinyl fluoride radical strongly influences the course of these polymerizations. VF incorporates well with other monomers that do not produce stable free radicals, such as ethylene and vinyl acetate, but is sparingly incorporated with more stable radicals such as acrylonitrile [107-13-1] and vinyl chloride. An Alfrey-Price value of 0.010 0.005 and an e value of 0.8 0.2 have been determined (49). The low value of is consistent with titde resonance stability and the e value is suggestive of an electron-rich monomer. 

Compare energies for the two alternative conjugate acids of methyl acetate (protonated methyl acetate and methoxy protonated methyl acetate) and dimethylacetamide (N-protonated dimethylacetamide and 0-protonated dimethylacetamide). Which acid in each pair is more stable Draw resonance contributors for the more stable conjugate acid for each system. 

Figure 3.8 Two resonance structures that can be written for acetic acid and two that can be written for acetate ion. According to a resonance explanation of the greater acidity of acetic acid, the equivalent resonance structures for the acetate ion provide it greater resonance stabilization and reduce the positive free-energy change for the ionization.

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

---