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CHE 8A: Week 9 Notes

by: Mackenzie Hayes

CHE 8A: Week 9 Notes CHE 8A

Mackenzie Hayes

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These notes cover everything said in lecture this week
organic chemistry - brief
Sarah Lievens
Class Notes
organic, Chemistry
25 ?




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This 7 page Class Notes was uploaded by Mackenzie Hayes on Friday May 27, 2016. The Class Notes belongs to CHE 8A at University of California - Davis taught by Sarah Lievens in Summer 2015. Since its upload, it has received 6 views. For similar materials see organic chemistry - brief in Biological Sciences at University of California - Davis.

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Date Created: 05/27/16
CHe 8A: Week 9 Notes (5/23) ● Resonance and Delocalized e​ - -​ ● Most of the time → we picture e​ as localized (in a particular spot) ○ On one atom → lone pair ○ Between 2 atoms → bond (sigma or pi) ● Real pictures of molecules show e​ -spread over whole molecule ○ Molecular Orbital Theory ○ Delocalized → spread out ○ Usually hybridization is good enough → localized on a central atom ■ Sometimes we get extended systems → e​ -over 3 or more atoms (delocalized) ● Can use resonance and resonance structures to approximate delocalized e​ - ● Classic Delocalized System: ○ Benzene : C=C=C (ring) ■ 2 possible lewis dot structures (dependent on double bonds) ■ Pairs are just bonding to each other (not everyone) ● 3 “pairs” of C ○ Each structure shows a possible Lewis Dot Structure ○ → Blend → end up with a “dotted” bond ■ Resonance hybrid ● 6 in a circle ■ Kind of everywhere and kind of not ■ The bond is constantly switching between where it can be in resonance structures so really everyone is holding hands - ● Even circle of e​ ● Resonance structures are imaginary: ○ They are extremes ○ The REAL structure is the resonance hybrid ■ The blended average of the resonances ● *Like a labradoodle puppy* ○ It isn’t one or another on different days, it is always a mixture of both ● Can draw resonance + resonance structures for delocalized e​ - ○ Who can delocalize/resonate? ■ 3 or more adjacent p orbitals will resonate ● Pi bonds ● Lone pair next to pi bond ● Charges next to pi bonds ○ (+) = empty p orbitals ○ (-) = lone pair ■ Cannot move ​ atoms​ → cannot move sigma bonds (single bonds) -​ ● Can use e​ pushing arrows to show movement between resonance structures ○ Pi bonds next to pi bonds: ● Oxygen is really greedy, it will steal e​ + ○ May steal and end up with C​ ○ Carbons may share to help each other out ■ *can be polar or non-polar but the total charge is constant* ○ Pi bond next to a lone pair or a (-)charge : ■ C=O can steal e​ -but Nitrogen can fill in -​ ■ Alkene: carbon moves the extra e​ over with an arrow ● 2 carbons end up sharing for resonance hybrid ○ Pi bond next to (+) or empty orbital : -​ ■ e​ move like filling in a hole ■ Constantly creates new holes ● Cannot resonate OVER a sigma bond ○ Cannot ever form 5 bonds to carbon ○ Cannot break sigma bonds in resonance ● Some resonance structures are better than others ○ Lower in energy ○ 1. Octets are good ■ Can resonate a ketone all by itself (just unstable) ● While there is resonance, since it is unstable it is uncommon ○ 2. Fewer charges are good ■ Less formal charge ■ Amines → ones with double bond O is preferred ○ 3. If must have formal charge → (-) on electronegative atoms, (+) on not-so-electronegative atoms ■ Never (+)F ■ Carbon has (-) → gives it to greedy oxygen to carry ● Major struct: (-) on O ● Minor struct: (-) on C ○ 4. Can be pretty much the same ● Delocalization → stabilizes molecules -​ ○ Spread out e​ and charges, share w/ each other (friends) ● Conjugated pi bonds = 2 or more adjacent pi bonds ○ Lower in energy than non-conjugate ○ Easier to form and harder to break ● Resonance can stabilize Carbocations → can share empty orb (+) charge ○ Primary is less stable than a secondary carbocation ○ 1​o​with resonance ~ 3​ o​ o​ ○ 2​ with resonance ~ 3with resonance ○ There’s not a more “substituted” side but ■ One side has resonance ● 2 pi bonds on the same carbon are not adjacent (they are “on the same”) ● Resonance helps pKa’s ○ Helps (-) → stabilizes conjugate base (5/25) ● Reactions with Dienes ○ Non-conjugated diene: ■ Act independently (2 separate alkenes) ■ It’s like one cookie per person in line, some don’t get in line and some get in line multiple times ■ Follows Markovnikov’s rule ○ Conjugated diene: ■ E​-are delocalized, act as a single unit + ■ Markovnikov’s rule - want more stable C​ ■ End up with not exactly the same intermediates: ● Upper - secondary carbocation ○ Faster and easier to form products ● Lower - primary carbocation ○ Primary = less stable ○ Higher activation E, slower to form products ■ Not exactly the same products: both will form ● Kinetic Product​: Terminal alkene ○ Less stable and less exothermic ● Thermodynamic Product​ : internal alkene ○ More stable and more exothermic ■ Does depend on branches and exact carbons ● Low E → kinetic product → most stable C​+ o​ ○ Cold, 0​ C ● High E → thermodynamic → most stable product ○ Hot ● Independent of which product is preferred ○ Another way to describe conjugate systems ● Haloalkanes ○ Are sp3 hybridized → no pi bond → polar ○ Carbon can act as an electophile ○ Halogen can act as as a good “leaving” group ■ Leaves with a pair of electrons ○ 2 major classifications of reactions: ■ Substitutions → replace the leaving group with a nucleophile ● 2 things become 2 different things ● 2 types: Sn1, Sn2 ■ Elimination → lose H-X and form a pi bond ● 1 thing becomes 2 things ● S​N​ → Bimolecular Nucleophilic Substitution ○ Start with nucleophile and haloalkane (electrophile + leaving group) -​ ○ O​ will go looking for a new friend ■ It will latch onto C but there are too many things on C ■ Leaving group will leave, and end up with: ○ Products of alcohol and Halogen ○ Bimolecular → rate depends on both molecules (haloalkane and Nu​ )​ ■ Rate = K [R-X][Nu​-] ○ Observe: ■ 1. Rate law depends on both concentration of alkane and nucleophile→ both part of RDS **** o​ o​ ■ 2. CH​3​X > CH3​H​2​X > 2​R-X >> 3​R-X ● Adding alkyl groups slows rxn → hinder RDS ■ 3. stereochemistry gets inverted if stereocenter is present ○ Can extrapolate a mechanism → what’s happening ■ Electrons pour into the backside of the molecule as bromine leaves with the front side ■ End up back at sp3 but facing the other direction ■ Process called a backside attack process ○ RDS is only step that has Nu​coming in ■ Backside attack is slow w/ lots of alkyl groups ■ → steric hindrance to Nu​ -approach ■ Stereochemistry is inverted ● Nucleophile approaches 180​ ofrom Leaving group ● Sn2 is faster w/ less hindered substrates ○ Also faster with a better leaving group ■ Stronger conjugate acid → better the leaving group ● Willing to lose H​/ willing to leave C -​ - ○ Better with a good Nu​ → something that really wants to share e​ ■ Bases donate e​ -to H​​→ really about ΔH ■ Nu​ -donate e​-to E​→ really about rate / activation E ■ Good nucleophile: ● 1. Has extra electrons, negative charge ● 2. pKa / more EN → less willing to share ○ Left on periodic table → better Nu​- ● 3. Size → bigger = more willing to share ○ More polarizable / e​-drift more easily to E​ ○ Down the periodic table = better ● S​ 1: Can do a different substitution N​ ○ Poor leaving group ○ Unimolecular Nuclear Substitution


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