Lecture_21

Lecture_21 - Lecture 21 Chapter 7: Delocalized ...

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Unformatted text preview: Lecture 21 Chapter 7: Delocalized Electrons Delocalized Electrons •  Localized vs. Delocalized Benzene has Delocalized Electrons •  How did they deduce the structure of benzene? –  They knew molecular formula was C6H6 which could be: –  They did two separate reacFons using benzene: Kekule’s Proposal Deducing the Structure of Benzene •  Eventually hydrogenaFon gave cyclohexane •  Some Fme later, x- ray diffracFon was used –  All C/C bonds of benzene were found to be 1.39 Å –  C- C bond avg is 1.54 Å –  C=C bond avg is 1.33 Å •  This means electrons must be delocalized The Bonding in Benzene Resonance Contributors •  Dashes or circles don’t show the # of electrons –  Chemists like to visualize localized electrons –  Resonance structure is used to depict delocalized electrons as localized –  The things with dashes and circles are called “Resonance Hybrids” How to Draw Resonance Contributors 1.  Draw a Lewis Structure. Electrons move, not atoms 2.  Only pi and lone pairs move, sigma never 3.  Total # electrons doesn’t change Resonance Contributors •  Allyl caFon resonance contributors •  Resonance Hybrid: •  Sp3 Carbons cannot accept pi electrons: Resonance Contributors •  1,3- hexadienyl caFon •  Amide •  Moving pi electrons towards sp carbon Predicted StabiliFes of Contributors •  The greater predicted stability of a resonance contributor, the greater chance the real structure is close to that. •  What about carboxylate?? Predicted StabiliFes •  Look at 3- buten- 2- one –  Neg charges belong on electronegaFve atoms –  PosiFve charges on less electronegaFve Predicted StabiliFes •  Things that decrease stability: 1.  Incomplete octet 2.  Neg charge not on most electronegaFve atom 3.  Charge SeparaFon DelocalizaFon Gives Stability 1.  Greater stability of contributor, more it contributes to resonance hybrid 2.  Greater # of relaFvely stable contributors the greater the delocalizaFon energy 3.  More nearly equivalent contributors, greater the delocalizaFon energy Example: Carboxylic acid vs. carboxylate? Isolated vs Conjugated •  Which is more stable? Benzylic and Allylic CaFons •  What are their stabiliFes? Which of the following is the most stable diene? Which of the following is the most stable diene? •  III is the most stable. It is conjugated. How many allylic hydrogen atoms are present in the following molecule? •  •  •  •  •  A) 2 B) 3 C) 4 D) 5 E) zero How many allylic hydrogen atoms are present in the following molecule? •  •  •  •  •  A) 2 B) 3 C) 4 D) 5 E) zero DelocalizaFon and pKa •  The more stable the base, the stronger the acid –  AceFc acid vs. ethanol •  Increase in delocalizaFon energy upon loss of an H DelocalizaFon and pKa •  Which is more acidic? A B DelocalizaFon and pKa •  Which is more acidic? A B DelocalizaFon and pKa •  Which is the stronger acid? A B DelocalizaFon and pKa •  Which is the stronger acid? A B DelocalizaFon and pKa •  Which is the stronger acid? A B DelocalizaFon and pKa •  Which is the stronger acid? A B ...
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