Home / Organic Chemistry I / 4. Structure & Bonding in Organic Molecules

4. Structure & Bonding in Organic Molecules

Organic chemistry is the chemistry of carbon — and carbon's tricks (four bonds, chains, rings, double bonds) all follow from hybridization, resonance, and formal charge.

Hybridization and Bond Types

sp³: four σ bonds, tetrahedral (~109.5°) — alkanes.
sp²: three σ bonds + one π bond, trigonal planar (~120°) — alkenes, carbonyls.
sp: two σ bonds + two π bonds, linear (180°) — alkynes.

Example: In ethene, each carbon is sp²: the unhybridized p orbitals overlap side-on to form the π bond, locking the molecule flat and preventing rotation.

Resonance

When one Lewis structure can't capture a molecule, several resonance forms contribute to the real, delocalized structure. The acetate ion's negative charge is shared equally by both oxygens — which is why carboxylic acids are far more acidic than alcohols.

Formal Charge

$$\text{Formal charge} = \text{valence e}^- - (\text{nonbonding e}^- + \tfrac{1}{2}\text{bonding e}^-)$$

Example: In hydronium, $\text{H}_3\text{O}^+$: oxygen has 6 valence electrons, 2 nonbonding, 6 bonding shared: $6 - (2 + 3) = +1$.

Acid–Base Chemistry in Organic Molecules

pK_a ranks acidity: carboxylic acids (~5) > phenols (~10) > alcohols (~16) > terminal alkynes (~25) > alkanes (~50). A lower pK_a means a stronger acid — and a more stable conjugate base.

Practice Problems

What is the hybridization of nitrogen in NH₃?
What is the formal charge on carbon in CH₃⁺?
Which is more acidic: phenol (pK_a 10) or ethanol (pK_a 16)?
Draw the two resonance forms of the nitrate ion, NO₃⁻.