Formaldehyde bond angles

In structure I (numbered 1 in the IRC output), we find a formaldehyde-like structure, although the O-C-H bond angles are distorted from the equilibrium geometry. However, we can identify the minimum along this side of the path as formaldehyde. 

In structure H, on the other hand, the same O-C-H bond angle is increasing. Continuing down this path will eventually result in formaldehyde as the hydrogen moves away from the oxygen and toward its final position on the opposite side of the carbon atom. 

Unlike formaldehyde, acrolein retains its planar structure in the first excited state. Moving to the first excited state principally affects the C-C-H bond angle, decreasing it almost 5°, and the C-C-C bond angle, increasing it about the same amount. The C-O bond also stretches slightly. 

More subtle change can occur to a molecule s structure following photo-excitation. For example, the bond angle in methanal (formaldehyde) increases, so the molecule is flat in the ground state but bent by 30° in the first excited state see Figure 9.14. 

Whereas methane, CH4, is tetrahedral, ethene, C2H4, is not. According to the best available physical measurements, all six atoms of ethene lie in a single plane and the H—C—H bond angles are 117.3°. Methanal (formaldehyde) also is a planar molecule with an H—C—H bond angle of 118°. 

Olah et al.603 have observed the formation of cation 309 (protonated fluorometha-nol) upon treatment of formaldehyde in HF-SbF5 [Eq. (3.81)]. When Minkwitz et al.605 attempted to isolate salts of the ion, however, the hydroxymethyl(methylidene) oxonium ion 310 was obtained [Eq. (3.81)]. Crystal structure analysis of the hexafluoroarsenate salt shows that cations and anions are connected by short H -F distances, forming a three-dimensional network. The bond lengths of the C-0=C fragment (1.226 and 1.470 A) are longer than those in formaldehyde (1.208 A) and dimethyl ether (1.410 A). The C—O—C bond angle is 121.2°. 

For a given nucleophile the equilibrium lies farther on the product side the smaller the substituents R1 and R2 of the carbonyl compound are (Figure 9.1). Large substituents R1 and R2 inhibit the formation of addition products. This is because they come closer to each other in the addition product, where the bonds to R1 and R2 are closer than in the carbonyl compound, where these substituents are separated by a bond angle of about 120°. Formaldehyde is the sterically least hindered carbonyl compound. In H20 this aldehyde is present completely as dihydroxymethane, and anhydrous formaldehyde is exists completely as polymer. In contrast, acetone is so sterically hindered that it does not hydrate, oligomerize, or polymerize at all. 

Formaldehyde is an extremely reactive aldehyde as it has no substituents to hinder attack—it is so reactive that it is rather prone to polymerization (Chapter 52). And it is quite happy to move from sp2 to sp3 hybridization because there is very little increased steric hindrance between the two hydrogen atoms as the bond angle changes from 120° to 109° (p. 139). This is why our aqueous solution of formaldehyde contains essentially no CH2O—it is completely hydrated. A mechanism for the hydration reaction is shown below. Notice how a proton has to be transferred from one oxygen atom to the other, mediated by water molecules. 

Most of the chemical formulas in this text are drawn to depict the geometric arrangement of atoms, crucial to chemical bonding and reactivity, as accurately as possible. For example, the carbon atom of methane is sp 3 hybridized and tetrahedral, with H-C-H angles of 109.5 degrees while the carhon atom in formaldehyde is sp 2 hybridized with bond angles of 120 degrees. 

Make models of methane (CH4), formaldehyde (CH20), and hydrogen cyanide (HCN). Observe the geometries and bond angles at each carbon. 

Make models of methanol (CH40) and formaldehyde (CH20). Note the geometries and bond angles of both the carbons and oxygens in these molecules. 

Two notable aspects of the carbonyl group are its geometry and its polarity. The carbonyl group and the atoms directly attached to it lie in the same plane. Formaldehyde, for example, is planar. The bond angles involving the carbonyl group of aldehydes and ketones are close to 120°. 

The effects of correlation (Table 8) exhibit the same qualitative behavior we have noted for water. (One should note that the rationalization provided for the behavior of the bond angle in water does not apply to formaldehyde. 

Effect of Double Bonds How do bond angles deviate from the ideal angles when the suiTOunding atoms and electron groups are not identical Consider formaldehyde (CH2O), a substance with many uses, including the manufacture of Formica countertops, the production of methanol, and the preservation of cadavers. Its... 

H-C-H angles of 109.5 degrees while the carbon atom in formaldehyde is sp hybridized with bond angles of 120 degrees. 

Note that in the sp hybrid the bond angles need not be equal (120°), except in cases in which all three atoms bonded to the central atom are the same. In formaldehyde, for example, the H—C—H angle is 126° and the other two are 117°. 

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