Honors Organic Chemistry Lecture 9/9/2013

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CHEM 2331H
T.Andrew Taton

Honors Organic Chemistry 9/9/2013  When electronegativity difference is too big, the more electronegative atom just takes the electron  not sharing, transferring  ionic bond  Formal Charge: Difference between number of valence electrons and ―owned‖ electrons o = (# valence electrons) – (#lone pair electrons) – ½ (# bonding electrons) o Ex: Ammonium: Nitrogen has 4 bonds and no lone pairs  formal charge of +1  N normally owns 5…but in NH , o4ns 4  5 – 4 = +1 o Ex: Hydroxide: O has 1 bond and 3 lone pairs  formal charge of -1  Normally owns 6…. But now owns 7….  6 – 7 = -1 o Tip: When # of bonds varies from typical valency, atom is probably charged. o Example:  Resonance forms: for a given molecular structure, different ways of placing electrons o Ex: How would you draw [(CH ) COH3 2 +   What are positive, negative features of these resonance forms?  1 one: carbon doesn’t have filled octet  bad  2 one: oxygen is positive even though it is more electronegative….better to have carbon be positive  What does this mean for electronic distribution in molecule?   consensus structure: graphical average of both structures  Use partial charges rather than exact fractions because haven’t weighted which resonance structures is better….don’t know which one is more dominant  Bond is also between length of single and double bond  1 solid bond and 1 dashed bond  Don’t draw lone pairs in consensus structures  Ex: o Resonance structures are not different molecules….they are 2 forms of same molecules o Resonance structures are related by pushing pairs of elecrons – lone pairs or multiple bonds (not breaking single bonds)—from one location to an adjacent location. o So, wherever there is a
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