Formaldehyde electron density

Both aromatic and aliphatic acids work, though aromatic aldehydes are far more common. Increasing electron density of the aromatic aldehyde lowers the yield. Formaldehyde can also be used."... 

Figure 6.13 Relief map of the electron density for methanal (formaldehyde) in the molecular plane. There is a bond critical point between the carbon and the oxygen nuclei, as well as between the carbon nucleus and each hydrogen nucleus. No gradient path or bond critical point can be seen between the two hydrogen nuclei because there is no point at which the gradient of the electron density vanishes. There is no bond between the hydrogen atoms consistent with the conventional picture of the bonding in this molecule.

In the acid-catalyzed formaldehyde condensation, position 6 appeared to be the most reactive since in acid solution the relative electron density is... 

Hydrogen bonding must have an effect on the electron density distribution of a molecule. In principle, this should be observed in the deformation density distributions discussed in Chapter 3. There are, in fact, two methods available. One is purely theoretical, in which the calculated deformation density for a hydrogen-bond dimer or trimer is compared with that of the isolated molecule. The other method compares the experimental deformation density of a hydrogen-bonded molecule in a crystal structure with the theoretical deformation density of the isolated molecule. Formamide has been studied by both methods [298, 380], and there appear to be significant differences in the results which are not well accounted for. Theoretical difference (dimer vs. monomer) deformation density maps have been calculated for the water dimer and the formaldehyde-water complex [312]. When those for the water dimer are decomposed into the components described in Chapter 4, a small increase in the charges on the atoms in the O-H -O bond due to the charge-transfer component is predicted [312]. 

Figure 17.2 compares the electrostatic potential surfaces of ethylene and formaldehyde and vividly demonstrates how oxygen affects the electron distribution in formaldehyde. The electron density in both the o and tt components of the carbon-oxygen double bond is displaced toward oxygen. The carbonyl group is polarized so that carbon is partially positive and oxygen is partially negative. 

A contemporary calculation [168] verified the importance of electrostatics in this complex but argued that this interaction was indeed a tme H-bond based on the topology of the electron density. This conclusion confirmed Novoa et al. [169] who claimed that even the weak interaction between CH4 and formaldehyde constitutes a H-bond, based again on the properties of the electron density and its bond critical points. Similar treatments further buttressed this conclusion for other donor-acceptor combinations [170,171]. Finally, a combination of at initio calculations and statistical analysis of crystal stmctures [172] suggested that while the CH- -O interaction polarizes its partner proton acceptor molecule less than would a traditional OH donor, one should nonetheless categorize both as H-bonds. 

Fig. 1.26. Comparison of electron density of formaldehyde with a spherical atom model (a) total electron density in the molecular plane (b) the electron density difference in the molecular plane (c) the electron density difference in a plane perpendicular to the molecular plane. The contours in (b) and (c) are in steps of 0.2eA. The solid contours are positive and the dashed contours are negative. From S. Shibata and F. Hirota, in Stereochemical Applications of Gas-Phase Electron Diffraction, I. Hargittai and M. Hargittai, eds., VCH Publishers, Weinheim, 1988, Chap. 4.

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