Class Note for CHEM 121 at UMass(18)
Class Note for CHEM 121 at UMass(18)
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Date Created: 02/06/15
RESONANCE AND RESONANCE STRUCTURES H H A single Lewis electron dot formula often adequately represents the locations H of bonding electrons in molecules For instance ethyl alcohol is well represented by I I the Lewis formula at the right H H H 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 ofcontributing structures to represent the actual electronic distribution ofa molecule For example consider the sulfur dioxide S02 molecule Structural studies show that both SO bonds in S02 are of equal length and strength This electronic distribution is not explained by either single Lewis structure but is explained when both Lewis formulas are taken into consideration 9 e OS 39 OS e e 0 Both Lewis structures are called resonance structures or resonance contributors Resonance contributors are separated by double headed arrows as shown The 9 actual molecule is NOT an equilibrium mixture of the imaginary resonance structures but rather a single unchanging composite structure called the S resonance hybrid Resonance hybrids are often represented by the quotpartial bondquot or combined notation The resonance hybrid for sulfur dioxide is shown 0 0 to the right 9 Fundamentally the phenomenon of resonance is invoked because of the inability of single Lewis structures to show electron delocalization over the molecule In fact resonance is often called delocalization For instance in S02 the pi electron pair is delocalized or spread over both SO bonds and not localized in a single SO 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 Further since electronic delocalization is stabilizing the existance of resonance implies molecular stabilization Generally the more resonance contributors the more stable the molecule DRAWING RESONANCE STRUCTURES Resonance structures differ only in the location oftheir electrons The atomic nuclei do not move and the bond angles do not change Resonance structures must all contain equal numbers of e electron airs andor un aired electrons One resonance quot h f 4 p p Is S structue can be converted into another by electronic O O O movements denoted by curved arrows Most of the time quot 39 39 e 9 39 quot quot these electronic shifts invlolve a lone pair becoming a pi Resonance amp Page 2 5 November 2008 Resonance Structures bonding pair or a pibonding pair becoming a lone pair Sigma bonds are generally NOT involved in drawing resonance structures For instance the two resonance structures for S02 are interconverted by the electron movements shown Of course no actual electronic movements occur this process is used as an aid in deriving additional resonance structures Another example of this relationship among resonance structures is shown with the three resonance structures of the nitrate ion NO3391 The implications of resonance are l the actual electron distribution is different than would be eXpected based on a single Lewis structure and 2 the energy of the actual molecule is lower than eXpected from a single Lewis structure This energy lowering is due to electron delocalization and is called resonance stabilization The degree of energy lowering or molecular stabilization is related to the number of resonance structures and the relative stabilities of the resonance structures CONTRIBUTION OF RESONANCE CONTRIBUTORS TO THE HYBRID Resonance structures M or may not contribute equally to the resonance hybrid Resonance structures contribute in proportion to their stability that is the more stable the resonance structure the greater its contribution to the hybrid Thus it is important to evaluate the relative energies and hence relative importance of various resonance contributors In this regard there are several situations to consider These are detailed below with examples EVALUATING THE RELATIVE ENERGY OF RESONANCE CONTRIBUTORS 1 Equivalent resonance contributors have equal energies and therefore make an equal contribution to the hybrid The larger the number of equivalent resonance structures the more stable the molecule For instance consider the allyl carbocationstabilized by the eXistence of equivalent resonance structures We previously saw the nitrate anion stabilkized by three equivalent resonance structures CH CH CHZV CH2 3H2 CH2 9 resonance contributors of equal structure and contribution to the hybrid Resonance amp Page 3 5 November 2008 Resonance Structures 2 Contributors which have no or minimum charge separation have a lower energy than structures with large charge separation In considering charge separation vs expanded octets in atoms in the 3ml period or higher minimizing charge separation has a higher priority see J Chem Educ 2001 787 981983 9 Il l C quot39 quot C e major minor quot4 20 IO 1 1 lt 1 o P O lt gt io P o 1 1 01 o quotD quot1 minor major 3 Contributors with more complete octets have a lower energy than those with an incomplete octet OHS O CHZ 4 CH3 OCH2 minor contributor major contributor 4 Contributors with more bonds are more important than those with fewer bonds 9 lt gt 9 m ajor minor 5 Contributors with formal negative charges on more electronegative atoms or formal positive charge on less electronegative atoms have less energy then those with formal negative charges ofless electronegative atoms or formal positive charges on more electronegative atoms Both structures generally make a contribution to the hybrid although unequal 9 9 HR C ax n C 1 C o e major minor very minor Resonance amp Page 4 5 November 2008 Resonance Structures 6 Contributors containing the following features are of suf ciently high energy so as to not contribute to the hybrid and are unimportant a contain an atom with less than on actet except Be and B b contain an expanded octet for period 2 elements c contain like formal charges on adjacent atoms D L Adams 5 November 2008
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