Hybridization Mcat question of the day

[ozzi22]

Given the structure below, the hybridization, electronic geometry, and observable geometry of the oxygen atom is:

A
sp2, trigonal planar, trigonal planar

B
sp2, trigonal planar, bent

C
sp3, tetrahedral, tetrahedral

D
sp3, tetrahedral, trigonal pyramidal



I thought it would be sp3 since there are no pi bonds on the oxygen atom and Tetrahedral because the two lone pairs and two sigma bonds and thus the answer would be D. Can someone please explain why its B.

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[Cawolf]

The short and fast "rule" is that an atom with at least one lone pair adjacent to an SP2 hybridized atom will adopt a SP2 conformation. So even though the oxygen has four groups, it is SP2 by this rule due to the adjacent SP2 carbon.

The reason is stability through resonance. When the oxygen adopts an SP2 hybridization, one of it's lone pairs becomes aligned with the SP2 carbon pi bond. This creates a conjugated pi system where the lone pair can delocalize, leading to increased stability.

It would also be good to recognize that this is an enol. Enols will generally (with some exceptions) rapidly tautomerize to their keto form. In the keto form the oxygen is SP2 in a carbonyl. Resonance forms do not change hybridization, so the oxygen must be SP2 in all resonance structures.

 

[sazerac]

Another quick and dirty way to zoom in on the answer is to realize the oxygen atom is only bonded to two other atoms (H and C).

Therefore the observable geometry must be either Bent or Linear.

Therefore the answer must be B. You don't even have to think about hybridization and stuff.


Not every MCAT question has a quick and dirty solution, but a surprising number of them do, and if you recognize them you can save yourself a LOT of time. I approached this question as "it's SP3! well, hang on maybe SP2, crap... electron geometry, that's messy... wait! there's only TWO atoms, must be bent or linear... (B)". Took me 7 seconds (yes I timed it), and now I have another minute or two to work on some complicated passage elsewhere.