Organic Chemistry

What hybridization does a carbon atom adopt when it forms four single (sigma) bonds, and what is the geometry?

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.

What hybridization does a carbon adopt when part of one double bond, and what is its geometry?

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.

What hybridization does a carbon in a triple bond (or two cumulated double bonds) adopt, and what is its geometry?

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.

How does the s-character of a hybrid orbital change going from sp3 to sp2 to sp, and what is the effect on bond length?

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.

How does a sigma bond differ from a pi bond in terms of orbital overlap?

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.