Resonance and Resonance Structures in Organic Chemistry - Prof. David Adams, Study notes of Chemistry

Download Resonance and Resonance Structures in Organic Chemistry - Prof. David Adams and more Study notes Chemistry in PDF only on Docsity! CHEM111 General Chemistry 1 A single Lewis electron dot formula often adequately represents the locations of bonding electrons in molecules. For instance, methane and ethyl alcohol are correctly represented by the Lewis formula at the right. All electron pairs in these molecules are localized between the atoms that share them There are, however, molecules whose electronic distribution is inadequately represented by a single Lewis formula. In these cases two or more Lewis formulas may be necessary to satisfactorily represent the molecule. Resonance is a phenomenon invoked when more than one Lewis formula is needed to adequately describe the electronic distribution in a molecule. Resonance is the use of contributing structures to represent electron delocalization (dispersal) in a molecule. For example, consider the sulfur dioxide, SO2, molecule. Structural studies show that both S-O bonds in SO2 are of the same length and the same strength. This electronic distribution is not explained by either single Lewis structure, but is explained when both Lewis formulas are taken into consideration. Both Lewis structures are called resonance structures or resonance contributors. Resonance contributors are separated by double headed arrows as shown. The actual molecule is NOT an equilibrium mixture of the imaginary resonance structures, but rather a single unchanging composite structure called the resonance hybrid. Resonance hybrids are often represented by the "partial- bond" or combined notation. The resonance hybrid for sulfur dioxide is shown to the right. Fundamentally, the phenomenon of resonance is invoked because of the inability of single Lewis structures to show electron delocalization within the molecule. In fact, resonance is often called delocalization. For instance, in SO2, two electron pairs are delocalized or spread over the entire O-S-O three atom bonding system and not localized in a single S-O bond. Thus, the existence of electron delocalization in a molecule means that more than a single resonance structure must be drawn to fully describe the electronic distribution within the molecule. Furthermore, since electronic delocalization is stabilizing, the existance of resonance implies molecular stabilization. Generally, the more resonance contributors, the more stable the molecule. RESONANCE AND RESONANCE STRUCTURES S O O S OO S O O C H H H H C C O H H H HH H methane ethyl alcohol