Orbital hybridization formaldehyde

The carbon-oxygen double bond in aldehydes and ketones is similar and can be described in either of these two ways. If we adopt the iocalised-orbital description, formaldehyde will have two directed lone pairs in place of two of the C-H bonds in ethylene. In this case the axes of these hybrid orbitals will be in the molecular plane (unlike the oxygen lone pairs in water). Either the components of the double bond or the lone pairs can be transformed back into symmetry forms. The alternative description of the lone pairs would he one er-type along the 0-0 direction and one jr-type with axis perpendicular to the 0-0 bond hut in the molecular plane. It is the latter orbital which has the highest energy, so that an electron is removed from it in. ionisation or excitation to the lowest excited state. 

FIGURE 17.1 Similarities between the orbital hybridization models of bonding in (a) ethylene and ( >) formaldehyde. Both molecules have the same number of electrons, and carbon Is sp -hybrldlzed in both. In formaldehyde, one of the carbons Is replaced by an sp -hybrldlzed oxygen (shown in red). Oxygen has two unshared electron pairs each pair occupies an sp -hybrldlzed orbital. Like the carbon-carbon double bond of ethylene, the carbon-oxygen double bond of formaldehyde Is composed of a two-electron cr component and a two-electron tr component. 

Combine the lone pair orbitals on formaldehyde to achieve what resemble the classic sp hybrid lone pairs. 

For example, in formaldehyde, H2CO, one forms sp hybrids on the C atom on the O atom, either sp hybrids (with one p orbital "reserved" for use in forming the n and 7i orbitals and another p orbital to be used as a non-bonding orbital lying in the plane of the molecule) or sp hybrids (with the remaining p orbital reserved for the n and 7i orbitals) can be used. The H atoms use their 1 s orbitals since hybridization is not feasible for them. The C atom clearly uses its sp2 hybrids to form two CH and one CO a bondingantibonding orbital pairs. 

In the formaldehyde molecule, H2CO, a double bond exists between the carbon and oxygen atoms, (a) What type of hybridization is involved (b) The molecule is found to be planar one bond between the C and O atoms is a cr bond, and the other is a rr bond. With a simple sketch, show the atomic orbital overlap that is responsible for the tt bond. 

The formaldehyde molecule is planar in its ground state (Figure 7-6). We first construct three strong a bonds involving thje carbon, the two hydrogen, and the oxygen atoms. Since the angles in the plane all are approximately 120°, we construct three equivalent orbitals in the xy plane which are directed from carbon toward Hx, H2, and O. For this purpose we hybridize the three carbon atomic orbitals 2s, 2px, and 2py. From three orbitals we can construct three linearly independent hybrid orbitals. 

PROBLEM 7.21 Describe the hybridization of the carbon atom in formaldehyde, H2C=0, and make a rough sketch of the molecule showing the orbitals involved in bonding. 

The geometries found for the complexes of formaldehyde with Brst and second row cations in theoretical studies were analyzed in terms of molecular orbitals. Based on the results of photoelectron spectroscopy, it was argued that the carbonyl group contains two nonequivalent lone pairs an sp-hybridized orbital contains one pair of electrons along the C—O axis and a second, higher energy lone pair in a p-like orbital lies perpendicular to the C=0 axis (Figure 7). ... 

Figure 7.1 presents contour diagrams for the Lewis-type NBOs of formaldehyde (panels a, c, e, g, h), calculated at the B3LYP/6-311-H-G""" level of hybrid density functional theory (DFT). As is immediately apparent, the bonding Gch, and Kcq NBOs exhibit the familiar sigma and pi shapes of idealized textbook depictions. The resemblance to expected textbook forms is further emphasized when each A-B bond is written as a linear combination (LC) of its constituent natural hybrid orbitals (NHOs) and with polarization coefficients and Cg, namely. 

Valence electron sets (lone pairs and electrons in sigma bonds) are housed, at least in part, in hybrid orbitals. This means that an atom surrounded by three electron sets uses three hybrid orbitals, as in formaldehyde. There, the central carbon atom uses hybrid orbitals in forming the C-H single bonds and the sigma portion of the C=0 double bond. The... 

Describe how the bonds in formaldehyde are formed in terms of overlaps of hybrid and unhybridized orbitals. 

Analyze We are asked to describe the bonding in formaldehyde in terms of hybrid orbitals. 

---