Resonance carboxylate

It is to be expected from chemical evidence that the replacement of hydrogen by an aliphatic radical would have some further inhibiting effect on the carboxyl resonance. It is found, however, that to within the experimental error of about 0.2 v.e. the resonance energy is the same for methyl and ethyl esters as for carboxylic acids. 

Curved-arrow notation is also a very useful device with which to generate resonance structures. In this application it is truly a bookkeeping system. Since individual canonical forms do not exist but are only thought of as resonance contributors to the description of a real molecule, the use of curved-arrow notation to convert one canonical form to another is without physical significance. Nevertheless it provides a useful tool to keep track of electrons and bonds in canonical structures. For example, the structures of carboxylate resonance contributors can be interconverted as follows ... 

We can t really draw an accurate representation of the carboxylate resonance hybrid using Kekule structures, but an orbital picture of acetate if" makes it clear that the carbon-oxygen bonds are equivalent and that ea is intermediate between a single and a double bond (Figure 20.2). The p orbital on the carboxylate carbon atom overlaps equally well with p orbitals on both oxygens, and the four p electrons are delocalized throughout the three-atom ir electron system. 

Fig. 18.16. Experimental data for a PMMA/PVDF 60 40 blend, obtained for the carboxyl resonance of PMMA. The solid line is a calculated depolarisation curve. The dashed line represents the offset from 30% of isolated PMMA. [Figure reproduced with permission from Ref. 80.]...

In a continuation of work on membranes, the 1 3C nmr spectra of membranes from Acholeplasma laidlawii grown on 13C-enrichcd palmitic acid were recorded (Metcalfe et al., 1972 1973a Metcalfe, 1972). The enhanced carboxyl resonance was the only peak observed under conditions where no natural abundance 13 C signals were measurable and the temperature exceeded the thermal transition temperature of the lipids in the membranes. The ability to reduce the 13 C nmr spectra of membranes to a few sharp resonances by incorporating 13 C-enriched lipids biosynthetically allows the T, -values to be obtained. 

It is seen that it is not a carboxylic acid instead of the carboxyl resonance between adjacent single and double bonds to oxygen atoms (Section 13-5), the acidic character results from resonance between the structures... 

Band 3, 3-93y. (2548 cm. ). This absorption is characteristic of carboxylic acids and is due to the 0—H stretching absorption in the reson-ance-stabUised dimer. (Carboxylic acids generally exist as dimers in the solid state and in all but very dilute solutions.)... 

Unsymmetrically substituted dipyrromethanes are obtained from n-unsubstitued pyrroles and fl(-(bromomethyl)pyiToIes in hot acetic acid within a few minutes. These reaction conditions are relatively mild and the o-unsubstituted pyrrole may even bear an electron withdrawing carboxylic ester function. It is still sufficiently nucleophilic to substitute bromine or acetoxy groups on an a-pyrrolic methyl group. Hetero atoms in this position are extremely reactive leaving groups since the a-pyrrolylmethenium( = azafulvenium ) cation formed as an intermediate is highly resonance-stabilized. 

For many years resonance m carboxylate 10ns was emphasized when explaining the acidity of carboxylic acids Recently however it has been suggested that the indue tive effect of the carbonyl group may be more important It seems clear that even though their relative contributions may be a matter of debate both play major roles... 

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

Resonance description of electron delocalization in carboxylate anion... 

The negatively charged oxygen substituent is a powerful electron donor to the carbonyl group Resonance m carboxylate anions is more effective than resonance m carboxylic acids acyl chlorides anhydrides thioesters esters and amides... 

C NMR The C NMR spectra of carboxylic acid derivatives like the spectra of carboxylic acids themselves are characterized by a low field resonance for the carbonyl... 

The role of IR spectroscopy in the early penicillin structure studies has been described (B-49MI51103) and the results of more recent work have been summarized (B-72MI51101). The most noteworthy aspect of a penicillin IR spectrum is the stretching frequency of the /3-lactam carbonyl, which comes at approximately 1780 cm" This is in contrast to a linear tertiary amide which absorbs at approximately 1650 cm and a /3-lactam which is not fused to another ring (e.g. benzyldethiopenicillin), which absorbs at approximately 1740 cm (the exact absorption frequency will, of course, depend upon the specific compound and technique of spectrum determination). The /3-lactam carbonyl absorptions of penicillin sulfoxides and sulfones occur at approximately 1805 and 1810 cm respectively. The high absorption frequency of the penicillin /3-lactam carbonyl is interpreted in terms of the increased double bond character of that bond as a consequence of decreased amide resonance, as discussed in the X-ray crystallographic section. Other aspects of the penicillin IR spectrum, e.g. the side chain amide absorptions at approximately 1680 and 1510 cm and the carboxylate absorption at approximately 1610 cm are as expected. 

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