Acetic acid dimer, structure

Display an electrostatic potential map iot acetic acid. Where are the most electron-rich sites Where are the most electron-poor sites Propose a structure for the dimer of acetic acid based on favorable electrostatic interactions between electron-rich and electron-poor sites. Compare your structure to that for acetic acid dimer. What is another name for the types of interactions that hold the two acetic acid molecules together (See also Chapter 2, Problem 2). 

One after the other, examine structures for octane-1,8-dioic acid, nonane-1,9-dioic acid djud decane-1,10-dioic acid. Is there any difference (in structure or conformation) between the diacids with lower and higher melting points Which, if any, of the acids adopt structures similar to that of acetic acid dimer Account for the variation in melting points of these three compounds. 

A=5.4+0.4GHz B+C=1445.2 0.5MHz of the per-deuterated acetic acid dimer (CD3COOD)2 were extracted. It was not possible to determine the rotational constants B and C independently due to the symmetric prolate top nature of the dimeric structure (Ray s asymmetry parameter k=-0.965). More detailed consideration of the fs DFWM spectra taken in the gas cell and in a supersonic expansion for the acetic acid dimer are under way in our laboratory and will be presented in a forthcoming publication. 

The explanation was provided by the theory of the hydrogen bond. According to this theory, two acetic acid molecules with the generally accepted structural formula could combine by forming two hydrogen bonds with one another, to produce a double molecule (called acetic acid dimer), as shown in the adjacent drawing. 

The structure of acetic acid dimer and other acid dimers has been verified by the electron diffraction and x-ray diffraction techniques. Each hydrogen atom is 1.00 A from one oxygen atom and about 1.60 A from another. 

Fig. 5-3. (a) The structure of acetic acid dimer which has an enthalpy of dissociation of 57.9 kJ mol-1 (A. D. H. Ciague and H. J. Bernstein, Spectrochim. Acta, 1959, 25A, 593). (b) Hydrogen bonding between base pairs such as adenine-thymine or cytosine-guanine. 

Fig. 8. Stereoscopic illustration of the inclusion compound of host 5 (folded conformation) with acetic acid and 2 mol of water. Host-host and host-water hydrogen bonding interactions stabilize the structure. The solvation layers consist of cyclic carboxy dimers of acetic acid surrounded by water species (crystal data a = 7.857, b = 11.379,c = 13.831 A,a = 92.50,/i = 101.21, y = 101.12°, space group Pi taken from Ref. 351)...

Reductive dimerization of the above type is not observed in the 4,4-diacyl triafulvene series88. Instead, l,2-diphenyl-4,4-diacetyl and -4,4-dibenzoyl triafulvene are readily reduced by means of zinc/acetic acid to monomeric products, which are likely to possess structure 474 from their spectral data. 

Acetic acid provides a different situation. The boiling point of acetic acid is 118.2 °C and the heat of vaporization is 24.4kJ mol-1. These values yield an entropy of vaporization of only 62 J mol-1 K-1. In this case, the liquid is associated to produce dimers as described earlier, but those dimers also exist in the vapor. Therefore, structure persists in the vapor so that the entropy of vaporization is much lower than would be the case if a vapor consisting of randomly arranged monomers were produced. It is interesting to note from the examples just described that a property such as the entropy of vaporization can provide insight as to the extent of molecular association. 

Both thermodynamic and kinetic factors need to be considered. Take, for instance, acetic acid. The liquid contains mostly dimer but the crystal contains the catemer and no (polymorphic) dimer crystal has ever been obtained. Various computations (R. S. Payne, R. J. Roberts, R. C. Rowe, R. Docherty, Generation of crystal structures of acetic acid and its halogenated analogs , J. Comput. Chem, 1998, 19,1-20 W. T. M. Mooij, B. P. van Eijck, S. L. Price, P. Verwer, J. Kroon, Crystal structure predictions for acetic acid , J. Comput. Chem., 1998, 19, 459-474) show the relative stability of the dimer. Perhaps the dimer is not formed in the crystal because it is 0-dimensional and as such, not able to propagate so easily to the bulk crystal as say, the 1-dimensional catemer. 

Schonberg et al. reportedly obtained the photodimer in excellent yields when a solution of 138 in diethyl phthalate was heated to 270°C for 30 minutes and then rapidly chilled with ice water (on slow cooling no dimer was obtained see also Mustafa ). These observations could not be substantiated a product obtained in this way (mp 200°C) differed from the photodimer (comparison of IR spectra). The structure of this product is not known. A dimer was also obtained when 1,3-bis(dimethyl-aminophenyl)benzo[c]furan was treated with acetic acid with sulfuric acid it was reconverted to the monomer. . There seem to be no reports concerning the photodimerization of other benzo[cjfurans. As we have found,... 

The mixed-metal dimer [CrMo(02CMe)4] has been prepared in 30% yield by addition of [Mo(CO)6] in acetic acid, acetic anhydride and CH2C12, to a refluxing solution of [Cr2(02CMe)4(H20)2] in acetic acid and acetic anhydride.13 The yellow product is volatile and gives the expected parent ion peak in the mass spectrum. It has a Raman active v(Cr—Mo) mode at 394 cm-1 consistent with quadruple bonding. The crystal structure gives a metal-metal... 

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