Conjugated Dienes & UV Spectroscopy: Stabilizing Effect of Conjugation on Molecules, Study notes of Chemistry

Download Conjugated Dienes & UV Spectroscopy: Stabilizing Effect of Conjugation on Molecules and more Study notes Chemistry in PDF only on Docsity! 1 Chemistry 201 - C Conjugated Dienes and Ultraviolet Spectroscopy This presentation was created by Professor Carl H. Snyder Chemistry Department University of Miami Coral Gables, FL 33124 CSnyder@miami.edu Copyright 2004 by Carl H. Snyder, University of Miami. All rights reserved. Conjugation A conjugated system consists of alternating single and double bonds, with at least two double bonds (separated by a single bond). For a conjugated hydrocarbon, the minimal requirement is C=C-C=C All carbons of a conjugated system in a polyene are sp2 hybridized Heats of Hydrogenation Thermodynamic Stabilization Heats of hydrogenation demonstrate thermodynamic stabilization through conjugation. C-C Bonds of A Conjugated System All C-C single bonds of a conjugated system are σ (sp2-sp2). The increased s character of these bonds (relative to sp3-sp3 bonds) is partly responsible for the thermodynamic stability of a conjugated system. 2 The Conjugated System All carbons of a conjugated system are sp2 hybridized. The C2-C3 bond of CH2=CH-CH=CH2 has double-bond character. C3 of CH2=CH-CH2-CH=CH2 is sp3 hybridized; it interrupts the conjugated system. 1,3-butadiene, a conjugated diene 1,4-pentadiene, a nonconjugated diene π electrons of 1,3-butadiene (left) are spread out among the 4 carbons of the π system. These electrons are delocalized. π electrons of 1,4-pentadiene (right), an unconjugated system, are localized between C- 1 and C-2, and between C-3 and C-4 1,3-butadiene, a conjugated diene 1,4-pentadiene, a nonconjugated diene Delocalization of π Electrons Delocalization of π Electrons The delocalization of π electrons is a stabilizing factor in conjugated systems. Bonding and Antibonding σ Orbitals in H2 Bonding and Antibonding π Orbitals in Ethylene The π and π* Orbitals of Ethylene The node of the π* orbital is a region of zero electron density. π* (antibonding) orbital π (bonding) orbital