Organic Chemistry: Quick Reference for Structure Drawing, Hybridization, and Resonance

Skeletal Structure Rules

  • Rule 1: When two lines cross with nothing written on them, that indicates a carbon atom. Endpoints of lines are also carbons. Hydrogens on carbon are not drawn.
  • Rule 2 (Hydrogen count on carbon): For each carbon, Hs = 4 - B, where B is the sum of bond orders to that carbon (single = 1, double = 2, triple = 3). Example: a carbon with two bonds has 2 hydrogens (4 − 2 = 2).
  • Rule 3 (Non-carbon atoms): Write non-carbon atoms explicitly as their element symbols. Hydrogens on heteroatoms are shown explicitly (e.g., NH₂, OH).

Hybridization and Shape

  • sp³ carbon: four single bonds → tetrahedral geometry.
  • Zigzag line representation is used for main chains to reflect ~109° angles.
  • The majority of a molecule drawn is sp³; count bonds to determine shape.

3D Representation: Wedges and Dashes

  • Wedge = toward you; Dash = away from you (relative to the paper).
  • Planar, line drawings show atoms in the plane; wedges/dashes depict out-of-plane positions.
  • For rings or branched stereocenters, use wedges/dashes to indicate 3D arrangement.

Rings and Cyclohexane

  • Linear molecules are drawn in zigzag for clarity; cyclic molecules are drawn to reflect ring closure (e.g., cyclohexane zigzag to close the ring).
  • Substituents on rings may require wedges/dashes to indicate orientation (axial/equatorial concepts arise from 3D perspective).

Double Bonds, Hybridization, and Geometry

  • Alkene carbons are sp² and the geometry around them is trigonal planar (flat).
  • Around double bonds, cis/trans (E/Z) stereochemistry is possible for substituents.
  • Triple bonds are sp and linear; two pi bonds present; bond length is shorter than single/double.

Octet Rule and Valence

  • Carbon can have at most four bonds (counting multiple bonds as multiple bonds). Do not draw more than four bond lines to carbon.
  • If a carbon has more than four bonds (counting bond order), that is incorrect.

Functional Groups and Nomenclature

  • A small portion of exams tests recognition of functional groups; memorize common functional groups.
  • R = generic substituent or group (placeholder for anything attached).
  • Expect basic naming questions (flashcards can help: ~20–25 key groups depending on course scope).

Resonance

  • Resonance: electrons in pi systems can be delocalized; the real structure is a hybrid of resonance forms.
  • Sigma framework (single bonds) remains fixed in resonance forms; only pi electrons move.
  • Use curved arrows to show electron movement: start at electron-rich site, move to electron-deficient site.
  • Examples: benzene’s alternating double bonds; carboxylate forms with delocalized negative charge.
  • Rules for resonance forms:
    • Do not break sigma bonds when drawing resonance forms.
    • Move only electrons in pi systems; do not violate octet.
    • Prefer resonance forms with fewer and more minimized formal charges.

Electron-Pushing Arrows: How to Use Them

  • Arrows start at lone pairs or pi electrons and end at places that will accept electrons.
  • When moving electrons, preserve the total number of electrons and avoid breaking the sigma network.
  • For carboxylates and related systems, resonance can distribute negative/positive charges across oxygens and carbons.

Quick Takeaways for Last-Minute Review

  • Master the three rules: no octet violations, electronegativity considerations for predicting reactions, and resonance stabilization.
  • For any carbon, always check B (bond orders) to count hydrogens correctly: nH=4−B,<br/>B=∑bond orders to Cn_H = 4 - B,<br /> \quad B = \sum \text{bond orders to C}
  • Remember: sp³ = tetrahedral; sp² = trigonal planar (alkenes); sp = linear (alkynes).
  • Wedges/dashes visualize 3D geometry; planarity applies to double bonds.
  • R = generic substituent; know common functional groups (memorize key ones).
  • Resonance forms are a tool to understand stability; the real structure is a weighted hybrid of forms.