Conjugation in Organic Chemistry

Aug 24, 2025

Overview

This lecture introduces the concept of conjugation in organic chemistry, focusing on the stability provided by delocalized electrons in conjugated systems and defining key related terms.

What is Conjugation?

  • Conjugation occurs when there are continuous, overlapping p orbitals across three or more adjacent sp2-hybridized atoms.
  • This overlap allows pi electrons to delocalize, spreading over multiple atoms, which stabilizes the molecule.
  • Conjugated systems are typically lower in energy due to this electron delocalization.

Hybridization and Pi Bonds Review

  • A pi (ฯ€) bond forms from the side-by-side overlap of unhybridized p orbitals on sp2-hybridized carbons.
  • An sp2-hybridized carbon has three bonding areas and one unhybridized p orbital.

Stability Through Resonance

  • Resonance is the movement of electrons within a molecule, allowing charge to be spread out (delocalized).
  • Delocalization through resonance stabilizes molecules by lowering their overall energy.

Identifying Conjugated Structures

  • A structure is conjugated if there are at least three adjacent sp2-hybridized atoms, each with p orbitals that overlap.
  • Simple carbon chains with alternating double bonds (like 1,3-butadiene) are classic examples.

Conjugation Beyond Double Bonds

  • Carbocations (positively charged carbons), carbanions (negatively charged carbons), and radicals (atoms with an unpaired electron) can participate in conjugation if they are sp2-hybridized.
  • Lone pairs and charges can align their orbitals to participate in conjugation, even if not originally sp2-hybridized.

The Allylic Position

  • The allylic position refers to atoms or charges (carbocation, carbanion, or radical) directly adjacent to a double bond but not part of it.
  • Allylic charges/radicals benefit from conjugation and are stabilized by resonance.

Key Terms & Definitions

  • Conjugation โ€” overlapping p orbitals across three or more adjacent atoms, allowing electron delocalization and increased stability.
  • sp2 hybridization โ€” a type of carbon orbital arrangement with three bonds and one unhybridized p orbital.
  • Pi (ฯ€) bond โ€” bond formed by sideways overlap of two p orbitals.
  • Resonance โ€” distribution of electrons across multiple structures, leading to stabilization.
  • Allylic position โ€” position directly next to a double bond, capable of conjugation if containing a charge or radical.

Action Items / Next Steps

  • Complete the worksheet on identifying conjugated structures versus non-conjugated ones.