planar

Meaning: A planar group of atoms lies in one flat plane. The MCAT links this to sp² hybridization: an sp² atom has three electron domains in a trigonal planar arrangement with bond angles near 120°, and its unhybridized p orbital points perpendicular to that plane and forms π bonds. Planar centers include alkene carbons, carbonyl carbons, carbocations and the ring atoms of benzene. The amide (peptide) bond is also planar because resonance gives the C–N bond partial double-bond character and restricts rotation.

On the MCAT: A nucleophile can attack a planar carbocation or carbonyl carbon from either face, so an SN1 reaction at a stereocenter gives a mixture of enantiomers, often near racemic. Aromaticity requires a planar, cyclic, fully conjugated ring with 4n + 2 π electrons. Protein questions use the planar peptide bond to explain why backbone rotation is limited to the φ and ψ angles.

Common confusion: Planar does not mean the whole molecule is flat. The atoms bonded directly to an sp² center lie in its plane, but sp³ carbons elsewhere in the molecule are tetrahedral. Also, a ring that is not planar cannot keep its p orbitals overlapping: cyclooctatetraene is tub-shaped and is not aromatic.