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CHM 234|Conjugated Pi Systems and Pericyclic Reactions

by: Alvin Notetaker

CHM 234|Conjugated Pi Systems and Pericyclic Reactions CHM 234

Marketplace > Arizona State University > Chemistry > CHM 234 > CHM 234 Conjugated Pi Systems and Pericyclic Reactions
Alvin Notetaker
GPA 3.33

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Topic: Conjugated Pi Systems and Pericyclic Reactions
General Organic Chemistry II
Class Notes
Organic Chemistry
25 ?




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This 8 page Class Notes was uploaded by Alvin Notetaker on Tuesday March 8, 2016. The Class Notes belongs to CHM 234 at Arizona State University taught by Pillai in Fall 2015. Since its upload, it has received 11 views. For similar materials see General Organic Chemistry II in Chemistry at Arizona State University.

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Date Created: 03/08/16
Conjugated Pi Systems and Pericyclic Reactions    Classes of Dienes  Dienes​ are compounds with two C=C bond.                    Cumulated                    Conjugated              Isolated    Cumulated dienes (allenes):  ● pi bonds are adjacent  ● pi orbitals are perpendicular  Conjugated dienes:  ● pi bonds are separated by exactly ​one​ single bond  ● pi bond overlap extends over the entire system  Isolated dienes:  ● pi bonds are separated by any distance greater than one​ single bond  ● pi bonds are separated by too great a distance to experience extra overlap    Conjugated Dienes  Preparation  SN​ can be avoided with tert­butoxide (t­BuOK)   1,3­butadiene    Bond Length  2​ In a conjugated diene, the single bond length is formed by theoverlapping sp​  orbital, while in  ethane, the single bond is from theoverlapping sp​3 orbital. This overlapping orbital in the  conjugated diene will produced a shorter length​ than in the ethane.    Stability  Conjugated​  dienes are more stable​ than isolated dienes.  Reason: The electron densities are connected. (Resonance) ​    Conformations of 1,3­Butadiene  Single bond can rotate at room temperature    With a s­trans diene, the half of the overall orbital can rotate to a 90° angle and C2 and C3 does  not overlap.  Due to steric hinderance, the s­trans is more stable.    Molecular Orbital Theory    ● a)sp −s 2 2 2 ● b)σ (sp −sp )  ● c)π (p−p)  AMO ­ have a node (Higher in energy)  BMO ­ no node (Lower in energy)    4π electrons(Butadiene)    6π electrons(Hexatriene)      Highest occupied molecular orbital (HOMO)  Lowest unoccupied molecular orbital (LUMO)      Electrophilic Addition  Addition of HX across 1,3­butadiene    Protonation creates the more stable allylic carbocation.  When it react with a hydrohalide, like HBr, it will produce two product.     1,2­adduct   1,4­adduct  1,2­adduct: ​ the addition of hydrogen and halide will be added on C1 and C2.  1,4­adduct:​  the addition of hydrogen and halide will be added on C1 and C4.    Addition of Br​ 2​across 1,3­Butadiene      Thermodynamic Control vs. Kinetic Control            1,2­Adduct  1,4­Adduct  Low temperature  0°C  71%  29%  High temperature  40°C  15%  85%  The 1,2 product forms faste atlow temperature​  because it predominate at low temperature.  Forming faster is call​inetic product​.  The 1,4 product forms atequilibrium​, thus itmore stable​. The formation at equilibrium is  calledthermodynamic product​ .  Example:      Introduction to Pericyclic Reactions  A concerted process​ and do not involve either ionic or radical intermediates  Cycloaddition reaction 2π bonds forms 2σ bonds, a ring, and changed πbond location.    Electrocyclic reactio​ 1π bond converts into a 1σ bond and forms a ring.    Sigmatropic rearrangements: ​ 1σ bond breaks to from another 1σ.      Pericyclic reactions’ 4 general features  1. The reaction mechanism is ​ concerted​ .  2. Involves a ​ring of electrons moving around a ​closed loop​.  3. The transition state iscyclic.  4. The polarity of the solvent generally has no effect on the reaction rate.    Diels Alder Reaction  In a Diels­Alder reaction, a conjugated diene reacts with a molecule with a carbon­carbon double  bond.  This reaction is called[4+2] cycloaddition ​because of the interaction of the 4π electrons in the  conjugated diene and the 2π  electrons in the alkene or alkyne.    Mechanism for the Diels­Alder Reaction  The reaction is initialed byheat.  pi bond breaks​  and two new C­C sigma​ bonds​  and one new C­C pi bond​  form.  This is aconcerted ​reaction.    Electron withdrawing substituents​  in the dienophile increases the reaction rate.  Electron withdrawing substituents consists of electronegative atom such as oxygen and nitrogen.  (C=O) ; (CN)    Thermodynamic Considerations  Retro Diels­Alder reaction​ : The reverse of a Diels­Alder reaction  This can be achieved at high temperature where the ring decomposes into diene and dienophile.  ● Ring (Diels­Alder reaction) predominate at low and moderate temperature.  ● Diene and dienophile (Retro Diels­Alder reaction) predominate at high temperature  (200°C)      Conformation of the Diene  s­trans conformation is more stable because s­cis causes more steric strain due to the hydrogen  atoms.    Reactivity (most to least)​  1. s­cis (locked)  2. s­cis  3. s­trans  4. s­trans (locked)    Common dienophiles in Diels­Alder reaction  Z=electron­withdrawal group      Example:    A ​carbon­carbon triple bond​  can be used as a dienophile.      The Stereochemistry of the Diels­Alder Reaction: Syn addition  If the substituents is cis in the dienophile​ , then the product will produce a​  cis product​ .  If the substituents is trnas in the dienophile​ , then the product will produce a​  trans product​ .      (Remember: When a substituents is bonded to a C=C, it is on the​  same​  plane as the ring.)    Endo Rule  When the dienophile has a substituent with ​pi electrons​, more of theendo product​  is formed    Endo has a interaction between the developing π bond and the electron­withdrawal group  Exo has no interaction because it placement is too far away. 


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