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Hybridization

Hybridization. Klein, p. 77. Resonance Lecture Supplement page 15. Review Concept Focus Questions before lecture begins. What Is Resonance?. Example Carbon monoxide. versus. Which accurately represents the molecular structure?

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Hybridization

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  1. Hybridization Klein, p. 77

  2. ResonanceLecture Supplement page 15 Review Concept Focus Questions before lecture begins

  3. What Is Resonance? • Example • Carbon monoxide versus • Which accurately represents the molecular structure? • Open octet on carbon vs. formal charges - both decrease stability • How to analyze structure? • Double bond (longer) versus triple bond (shorter) Carbon carbon single bond C(sp3)-C(sp3) 1.53 – 1.55 Å Carbon carbon double bond C(sp2)-C(sp2) 1.31 – 1.34 Å Carbon carbon triple bond C(sp)-C(sp) 1.17 – 1.20 Å

  4. H2C=O O=C=O carbon monoxide What Is Resonance?The Lewis structure of CO • Laboratory analysis • Double bond or triple bond? CO length 1.21Å CO length 1.20Å CO length 1.13Å shorter than C=O • Dipole moment favors double bond over triple bond. • Conclusion • The Lewis structure that accurately represents CO is... Empirical observation: Actual structure is somewhere in between double and triple bond. (like a “2.5” bond)

  5. What Is Resonance?The Lewis structure of CO Perhaps an equilibrium between two structures that differ in electron distribution?

  6. Curved Arrows A curved arrow... • Indicates electron redistribution • Starts at electron source; points to electron destination: Electron source Electron destination • When arrow starts at bond, bond is broken: • When arrow points to atom or space between atoms, bond is formed:

  7. What Is Resonance?The Lewis structure of CO Perhaps CO is an equilibrium between two structures that differ in electron distribution? ? X • Equilibrium ruled out by empirical evidence.

  8. What Is Resonance?The Lewis structure of CO Empirical CO structure is intermediate between the Lewis structures: Adequate structure Inadequate Inadequate • What is this “between” structure? • Each inadequate structure has some, but not all, actual features of CO • CO cannot be adequately represented by a single Lewis structure • Therefore “blend” or “average” inadequate structures  composite structure

  9. What Is Resonance?A definition Resonance • A molecule cannot be adequately represented by just one Lewis structure • Two or more Lewis structures must be used. Resonance contributors and How do we indicate a molecule has resonance? X Equilibrium Not accurate for CO Resonance Accurate depiction of CO

  10. C-C 1.54 Å C=C 1.32 Å CC 1.18 Å Additional examples of resonance Benzene (C6H6) single bond 1.54 Å double bond 1.32 Å 1 double bond 1.32 Å single bond 1.54 Å 1 6 6 2 2 vs. 3 3 5 5 4 4 “True” structure: • Prediction: Two types of carbon carbon bond lengths should be observed: C-C (1.54 Å) and C=C (1.32 Å) “1.5” bond 1.4 Å • Actual: From X-ray crystallography, we have found that all the carbon carbon bonds in benzene are of the same length! That length, 1.4 Å, is in-between the bond length of C-C and C=C. • What is going on?

  11. C-C 1.54 Å C=C 1.32 Å CC 1.18 Å Additional examples of resonance Acetate ion (CH3COO-) 1 1 single bond 1.36 Å double bond 1.21 Å 2 2 vs. single bond 1.36 Å double bond 1.21 Å “True” structure: • Prediction: Two types of carbon oxygen bond lengths to be observed: C-O (1.36 Å) and C=O (1.21 Å) “1.5” bond 1.26 Å • Actual: From X-ray crystallography, we have found that all the carbon oxygen bonds are of the same length! That length, 1.26 Å, is in-between the bond length of C-O and C=O. -½ -½ • What is going on?

  12. No resonance Has resonance Why Is Resonance Important? • Common occurrence with Chem 14C atoms: C, N, O, F, Cl, Br, I, S, P • Resonance delocalization increases stability: Electrons (charge) spread over larger volume  stability • Resonance influences molecular structure: Causes planarity Resonance importance example: An acid-base reaction Less stable More stable Keq > 1 H2SO4 stronger acid than H2O

  13. Note: Not necessary to draw for both sides of Drawing Resonance Contributors Key: Recognize the four common electron pair shift patterns Pattern 1: A lone pair next to a pi bond What is electron pair shift pattern? Hint: Consider curved arrows Lone pair and pi bond switch Try working through Klein, Section 2.7

  14. Open valence shell Drawing Resonance Contributors Pattern 2: A pi bond next to a positive charge (an open valence shell) What is electron pair shift pattern? Pi bond and open valence shell switch

  15. Drawing Resonance Contributors Pattern 3: A lone pair next to a positive charge (an open valence shell) Pattern 4: A pi bond between two atoms, where one of those atoms is electronegative Pi bond formed Common Pi bond broken Common Sigma bond broken Rarely important What is electron pair shift pattern? Lone pair and open valence shell switch with bond Usually pi bond; rarely sigma bond

  16. Drawing Resonance Contributors Pattern 5: Pi bonds going all the way around a ring What is electron pair shift pattern? Pi bonds switch Common for benzene rings

  17. Which Resonance Contributor Represents Reality? Your friend asks you to describe what a nectarine is because he’s never seen or eaten one… (Klein, Sec 2.1) “True” structure: Peach Plum Nectarine The carbon monoxide case: Neither contributor fully represents CO Resonance hybrid: A weighted average or blend of resonance contributors; the most accurate representation of the electronic structure of a molecule.

  18. Which Resonance Contributor Represents Reality? Once upon a study break... X Fantasy creatures Neither fully represents reality Real creature A unicorn-dragon hybrid? The carbon monoxide case: X Neither contributor fully represents CO Resonance hybrid: A weighted average or blend of resonance contributors; the most accurate representation of the electronic structure of a molecule.

  19. Resonance hybrid Drawing the Resonance Hybrid Example: Draw the resonance hybrid for acetate ion, CH3CO2-. 1. Draw contributors • 2. Draw the features that are the same for all contributors • Sigma and pi bonds, lone pairs, and formal charges 3. Add features that are not the same for all contributors d- • Partial (shared) pi bonds shown as ---- • Partial (shared) charges shown as d+ or d- d- d+

  20. Do All Contributors Have Equal Importance?Is a rhinoceros more unicorn or more dragon? •  contributor “stability” =  resemblance to reality =  contribution to hybrid • Therefore we need contributor preference (“stability”) rules: • as number and/or magnitude of rules violations  = importance of individual contributor = contribution to resonance hybrid

  21. Example: Resonance Contributor Preference Rules Rule #1: The most important contributor has the maximum number of atoms with full valence shells. All valence shells filled More important contributor Open valence shell on carbon Less important contributor In some cases it may not be possible for all atoms to have full valence shells. Rule #1 is more influential than all the other preference rules. Rules #2-6 have no particular order of preference.

  22. Example: Resonance Contributor Preference Rules Rule #2: The most significant contributor has the maximum number of covalent bonds. Four covalent bonds More important contributor Three covalent bonds Less important contributor

  23. Example: Resonance Contributor Preference Rules Rule #3: The most significant contributor has the least number of formal charges. No formal charges More important contributor Two formal charges Less important contributor

  24. Example: Resonance Contributor Preference Rules Rule #4: If a contributor must have formal charge(s), the most important contributors has these charges on the atom(s) that can best accommodate them. • Negative formal charges best on atoms of high electronegativity O- better than C- • Positive formal charges best on atoms of low electronegativity C+ better than O+ • Minimize formal charge magnitude +1 better than +2 Oxygen EN = 3.5 More important contributor Carbon EN = 2.5 Less important contributor

  25. Example: Resonance Contributor Preference Rules Rule #5: Resonance interaction (i.e., pi bond) is strongest between atoms in the same row of the periodic table. • Usually CNOF • Usually outweighs electronegativity considerations (rule #4) F, C both 2nd row More important contributor Even though EN F > EN Cl C 2nd row; Cl 3rd row Less important contributor

  26. Resonance Contributor Preference Rules Rule #6: Other factors (such as aromaticity) that we will encounter later. Violations to the resonance contributor preference rules exist, but are uncommon.

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