Structure of the Carbon-Oxygen Double Bond

PROBLEM 16.1 Make an orbital interaction diagram for each different kind of bond in the molecule formaldehyde, shown in Rgure 16.2. 

The structures of some known carbonyl compounds show that this picture is a reasonable one. Bond angles are close to the 120° required by pure sp hybridization, and of course we would not expect a molecule of less than perfect trigonal symmetry to have exactly sp hybridization. Bond lengths are similar to those in simple alkenes, although the carbon-oxygen double bond is a bit shorter and stronger than its carbon-carbon coimterpart (Fig. 16.3). 

Formaldehyde is the smallest and simplest aldehyde. It can be found in substantial concentrations in new buildings as commonly used wood products age. 

FIGURE 16.4 A graphical construction of It and It for the carbonyl group. The It orbital is constructed from more than 50% of the oxygen 2p orbital, and the It orbital from more than 50% of the carbon 2p orbital. 

So the carbonyl it and it orbitals bear a general resemblance to the it and It orbitals of the alkenes, but are less symmetrical. The two electrons naturally occupy the lower energy bonding molecular orbital. In the It molecular orbital, there is 

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