organic chemistry b chapter 11 conjugation and resonance by prof. dr. adel m. awadallah
DESCRIPTION
Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah Islamic University of Gaza. Effect of double bond on reactions (Double bond as a substituent). A) Radical and nucleophilic substitution B) Conjugated diens. Free Radical Halogenation of Alkenes. - PowerPoint PPT PresentationTRANSCRIPT
![Page 1: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/1.jpg)
Organic chemistry B
Chapter 11Conjugation and Resonance
By Prof. Dr.Adel M. Awadallah
Islamic University of Gaza
![Page 2: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/2.jpg)
Effect of double bond on reactions (Double bond as a substituent)
• A) Radical and nucleophilic substitution
• B) Conjugated diens
CH2
X
A B
![Page 3: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/3.jpg)
Free Radical Halogenation of Alkenes
• Reactions at the double bond
![Page 4: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/4.jpg)
Substitution at the Alkane
![Page 5: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/5.jpg)
Free Radical addition does not occur at high temperature Why?
• Because the allyl radical formed leading to substitution is more stable than the secondary one leading to addition
![Page 6: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/6.jpg)
Free Radical substitution in alkenesorientation and reactivity
![Page 7: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/7.jpg)
Bromination of Alkenes at the allylic position by N-
Bromosuccinimide
NBS, h
CCl4
Br
(CH3)3CO - K+
NBS, h
CCl4
Br
N
O
O
Br
N-bromosuccinimide(NBS)
![Page 8: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/8.jpg)
![Page 9: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/9.jpg)
Allylic Rearrangement
![Page 10: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/10.jpg)
Orbital picture of the allyl Radical
![Page 11: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/11.jpg)
Hyperconjugation and stability of Radicals
• Hyperconjugation: Delocalization of electrons involving sigma bond orbitals over three atoms (two carbons and one hydrogen).
• Three possible hyperconjugation structures for the ethyl radical, six for isopropyl and nine for tertiary butyl
![Page 12: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/12.jpg)
The allyl cation as a resonance hybrid
• SN1 Substitution in allylic substrates
![Page 13: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/13.jpg)
•The allyl carbocation is another example of a conjugated system.
•Conjugation stabilizes the allyl carbocation.
![Page 14: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/14.jpg)
Example: SN1 substitution using a weak nucleophile (ethanol)
![Page 15: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/15.jpg)
Example: SN2 substitution using a strong nucleophile (EtO-)
![Page 16: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/16.jpg)
Nucleophilic substitution in vinylic substrates is very difficult
• To solve the problem, you need • a) very good leaving group • b) Electron release in the carbocation being formed
![Page 17: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/17.jpg)
Example
![Page 18: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/18.jpg)
Resonance in Conjugated Dienes
![Page 19: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/19.jpg)
•Having three or more p orbitals on adjacent atoms allows p orbitals to overlap and electrons to delocalize.
•1,4-Pentadiene is an isolated diene .•The bonds in 1,4-pentadiene are too far apart to be conjugated.
![Page 20: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/20.jpg)
![Page 21: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/21.jpg)
•Conjugation occurs whenever p orbitals can overlap on three or more adjacent atoms.
•The four p orbitals on adjacent atoms make a 1,3-diene a conjugated system.
![Page 22: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/22.jpg)
Stability of substituted alkenes due to hyperconjugation
• The greater the number of alkyl groups attached to the doubly bonded carbon atoms, the more stable the alkene
![Page 23: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/23.jpg)
Electrophilic addition to conjugated diene
![Page 24: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/24.jpg)
Mechanism
H H
Br
HH
Br
+ +
Br- Br-
H+
+
![Page 25: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/25.jpg)
Example: suggest a mechanism
![Page 26: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/26.jpg)
1,2 -vs. 1,4-addition. Rate vs. equilibrium• The 1,2-product is the kinetic product (formed
faster)• The 1,4- product is the thermodynamic product
(more stable)
![Page 27: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/27.jpg)
![Page 28: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/28.jpg)
![Page 29: Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah](https://reader033.vdocuments.us/reader033/viewer/2022061418/56813d2f550346895da6f383/html5/thumbnails/29.jpg)
Volcanization (Formation of sulfur bridges between different chaines)