Dipole moment carboxylic acids

Furan-2-carboxylic acid dipole moment, 4, 555 ... 

Carboxylic acids are fairly polar and simple ones such as acetic acid propanoic acid and benzoic acid have dipole moments m the range 1 7-1 9 D... 

The structure was established on the basis of substitution at the 3-position (11CB2409), optical investigations (26LA(437)162), dipole moment (3.06 D) (44MI41600) and IR measurements (57MI41600, 66DIS(B)102). The first 2,1-benzisoxazole to be synthesized was 5,6-dimethoxy-2,l-benzisoxazole-4-carboxylic acid in 1881 (1881JPR353). 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

Typical examples are the conversion of the neutral form of an amino acid into its zwitterionic form, the helix-coil transitions in polypeptides and polynucleotides, and other conformational changes in biopolymers. Reactions of higher molecularity in which reactants and products have different dipole moments are subject to the same effect (association of the carboxylic acids to form hydrogen-bonded dimers). Equilibrium involving ions are often more sensitive to the application of an electric field ... 

Structural information of LB films has also been obtained from FTIR studies. In the carboxylate form of the fatty acid, the relative intensities of the vs(C02-) and va(C02-) signals are dependent on the orientation of the chain axis. The dipole moments of the vs(C02-) and va(C02 ) stretches are parallel to and perpendicular to the chain axis, respectively. In transmission mode the electric vector of the IR radiation interacts strongly with dipole moments parallel to the substrate. This means that in transmission mode the vs(C02-) will be most intense, and the va(C02-) the weakest, for films with the chain axis perpendicular to the substrate. The opposite is true for the FTIR-RA mode. There is general consensus that in M-FA films the chain axis is approximately perpendicular to the substrate while the protonated form of the acid after exposure to H2S has a tilt relative to the substrate. Further discussion of FTIR as an investigative tool into the reaction of M2+-FA films with dihydrogen chalcogenides is given in later sections. 

Problem 16.4 Write resonance structures for the COOH group and show how these and orbital hybridization account for (a) polarity and dipole moments (1.7-1.9D) of carboxylic acids (b) their low reactivity toward nucleophilic additions, as compared to carbonyl compounds. ... 

Another estimate seemed to support an inductive contribution to deuterium IEs on the acidity of carboxylic acids.37 This IE on acidity of some carboxylic acids was attributed to an inductive effect arising from the electrostatic interaction of the C-H or C-D dipole with the negative charge of the carboxylate, as expressed in Equation (31). The derivative dpK/dfi was estimated from the effect of a C-Cl dipole on acidity, using the difference in pATas of trichloroacetic acid (0.63) and acetic acid (4.75) and the difference between the dipole moments of t-butyl chloride (2.13 D) and isobutane (-0.13 D). Next Afj, was estimated as 0.0086 D, the difference between the dipole moments of (CH3)3CD and (CH3)3CH. Thus ApK was estimated as 0.005 per D, in excellent agreement with the observed 0.014 for acetic-d3 acid. Moreover, the IE of 0.002 per D in pivalic-J9 acid is consistent with a 2.8-fold falloff factor for inductive effects. Yet those estimates depend crucially on the difference between the dipole moments of isobutane and isobutane-d, which is unusually large, amounting to 6.5% of either s total dipole moment. 

Let us mention that there are still other relations between conformation and dipole moments. Relative stability of conformational isomers was often brought in connection with their dipole moments. For instance, the preferred sp conformation of esters (3a), or of carboxylic acids, was often explained by the lower dipole moment6 compared to the conformation 3b, or in other words by the electrostatic repulsion7-9 of partial dipoles in 3b. Reasoning of this kind is usually based on anticipated rather than measured moments that of 3b cannot be directly measured. Interaction of more distant dipoles is commonly neglected, viz both its effect on the conformation and on the total /z (see, for example, Klinot and coworkers10). This is in fact the fundamental assumption underlying all analyses of dipole moments. 

The extremely stable and general sp conformation of esters and carboxylic acids (3a) is one of the remarkable features of stereochemistry. It was observed uniformly2 in all other molecules with the group O—C=0 and also with their heteroatom analogues with S, Se or Te. In contrast with the rigid conformation, the conjugation in the ester molecule, expressed by the formulas 69, is relatively weak and can be just detected in the dipole moment values164. 

Alkenes (R2 < 0.69), amines (R2 < 0.64), carboxylic acids (R2 < 0.001), and sulfonic acids (R2 < 0.20) show a poor correlation with dipole moment. With dipole moment as descriptor, benzene and halide classes have a predictable trend for linear correlations. The correlation coefficients of multicarbon alkanes or combinations of one-carbon and multicarbon halogenated alkanes are smaller than 0.67. They all have the same trend of correlations in that the rate constants decrease as the dipole moment increases. 

We consider now other approaches to the problem, namely, MO calculations, dipole moment measurements, and pAffl s of carboxylic acids. 

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