Organic Chemistry

sp3 hybridization — the carbon mixes one s and three p orbitals into four equivalent sp3 orbitals arranged in a tetrahedral geometry with ~109.5° bond angles.

sp2 hybridization — one s and two p orbitals mix into three sp2 orbitals in a trigonal planar arrangement (~120°); the remaining unhybridized p orbital forms the pi bond.

sp hybridization — one s and one p orbital mix into two sp orbitals arranged linearly (180°); the two remaining p orbitals form two pi bonds.

s-character rises: sp3 is 25%, sp2 is 33%, sp is 50%. Higher s-character holds electrons closer to the nucleus, making bonds shorter and the carbon more electronegative.

A sigma bond results from head-on (end-to-end) overlap of orbitals along the internuclear axis; a pi bond results from side-to-side overlap of parallel p orbitals above and below that axis.

The sigma bond is stronger because head-on overlap is more effective than side-on overlap. Rotation about a sigma bond preserves overlap, but rotating a pi bond would break the parallel p-orbital overlap, so pi bonds (and thus double bonds) restrict rotation.