sp2
Summary
sp² describes an atom whose one s and two p orbitals mix into three equivalent hybrids, trigonal planar with angles near 120°. The leftover p orbital forms a π bond (or is empty in a carbocation). Testable: three electron groups (σ bonds plus lone pairs), as at a C=C or C=O carbon, usually means sp².
Detail
Meaning: In sp² hybridization, one s and two p orbitals mix into three equivalent hybrid orbitals in one plane, trigonal planar, with bond angles near 120°. The third p orbital stays unhybridized, perpendicular to that plane. An sp² carbon has three σ bonds plus either one π bond (alkenes, carbonyl carbons, benzene carbons) or an empty p orbital (carbocations). With about one-third s character, sp² bonds are shorter and stronger than sp³ bonds, and the carbon is somewhat more electronegative.
On the MCAT: Expect to name hybridization, geometry or bond angle from a structure, or to rank bond lengths or C–H acidity (sp is more acidic than sp², which is more acidic than sp³). Rotation about a C=C is restricted because turning it would break the π overlap, so cis/trans (E/Z) isomers exist. Conjugation and aromaticity need a p orbital on each atom, which sp² atoms supply.
Common confusion: Students count a double bond as two electron groups. Count only σ bonds plus lone pairs; the π bond does not change hybridization, so three groups means sp². An atom with a lone pair can also be sp² when the lone pair is in resonance, as in planar amide nitrogen.
Sources
- Organic Chemistry by Paula Bruice
- Organic Chemistry by Jonathan Clayden
- Organic Chemistry by Francis Carey
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.