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Chapter 17

CHEM 2OB3 Chapter Notes - Chapter 17: Allyl Group, Tautomer, Sigma Bond


Department
Chemistry
Course Code
CHEM 2OB3
Professor
Paul Harrison
Chapter
17

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17: Conjugated Pi Systems and Pericyclic Reactions
Classes of Dienes
Three Types:
1. Cumulative
-Double bonds are connected to the
same C
-Pi bonds are perpendicular
2. Conjugated
-Alternating double bonds
-Includes C=O
-Pi bond extends over entire system
3. Isolated
-1+ single bond between the double bonds
-Pi bonds don’t overlap
Conjugated Dienes
Preparation
-Sterically hindered bases (t-BuOK) allows only emilination reactions to occur
Bond Lengths
-Single bonds in conjugated system are shorter than a typical single bond
A. Single bonds are connected to an sp2 carbon which means it has more of an s charcter (33%) than
sp3 (25%) = electron density closer to nucleus = shorter bond
B. Conjugated system has some double bond character (resonance allowed), and double bonds are
shorter than single bonds = shorter bond length
Stability
-Conjugated systems have lower heats of hydrogenation than isolated dienes = more stable
-Conjugated systems share delocalized electrons over more carbons = more stability
Determine Stability Between Molecules
1. What’s conjugated (stable) and what’s not (unstable)
2. # of C in conjugated system (more carbon = more stable)
3. Substituents on double bond (more substituents connected to double bond = more stable)
Conformations of 1,3-Butadiene
-s signify
-es that the double bond is separated by 1 single bond
-In both, all p orbitals in the system overlap
-S-trans more stable > s-cis
oLess sterically hindered
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Molecular Bond Theory
MO of Butadiene:
-4 overlapping p orbitals = 4 MOs = bonding (0 nodes), non-bonding, non-bonding and antibonding
(3 nodes)
MO of Hexatriene :
-6 overlapping p orbitals with 6 pi elecrons = occupy 3 bonding orbitals
HOMO: highest occupied MO
LUMO: lowest unoccupied MO
Frontier Orbitals: HOMO and LUMO
-Conjugated systems can interact with light, allowing pi electrons from HOMO to be promoted to a
LUMO: excited state
Electrophilic Addition
Addition of HX
-2 products: nucleophile (DB) attacks H, it forms a
cabocation  undergoes C+ rearrangement =
resonance structures  each C+ can equally and
potentially receive the Br-
o1,2 addition and 1,4-addition
*H should be a Br
-1,4-addition: more stable
Addition of Br2
-Similar reaction as above
Thermodynamic Control vs. Kinetic Control
Thermodynamics:
High Temp: 1,4-addition is favoured
Low Temp: 1,2-addition is favoured
-1,4-add is a more stable product beause the double bond
is more substituted than the 1,2-add
Kinetics:
-When double bond attacks HBr and is protonated, the Br- is closer to C2 than C4
-1,2-add forms at a faster rate, but 1,4-add is more predominate
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