Textbook 2.45

[GraceLi_2L]

2.45 asks you to identify what the hybridization is for H2C=CHCO. The answer key says that the pi bond between double bonded carbons is (C 2p, C 2p). I don’t understand why that is, are you not counting regions of electron density in the same way for pi bonds & their hybridizations? If so, what is the right way to count it so you’ll end up with just (C 2p, C 2p)for the pi bond?

[HoneyWalnutShrimp]

So first off Im guessing you mean H2C=CHCHO

regardless, it would be C 2p, C 2p. Here is how I would solve it

1. The carbon's non hybridized form would look like this. Note here than there is 1 p orbital that is empty. Hybridization would only o
2p. 1 1 _
2s 1l

2. Hybridized would look like this (remember its sp2 hybridized here because it has 3 regions of e- density. Note Since carbon has 4 valence electrons but in our model sp2 only has 3 orbitals, that would mean that one of p orbitals still exist and hold electrons. (note that although the p orbital and hybrid have different energy levels, the electron electron repulsion that would occur for the last e- to pair with another electron in the sp2 orbital makes it become an unpaired electron in the empty p orbital.
2p 1
sp2 1 1 1

3. Note that pi bonds require a very specific orientation (perpendicular to the sigma bonds) to form. Hybrid orbitals and s orbitals do not have the necessary orientation in the wave function for the presence of pi bonds. Therefore, the only orbital that could be responsible for the pi bond in the double bond would be from the unhybridized leftover 2p orbital. Thats why the pi bonds would be (C 2p, C 2p)