Matriculation Chemistry Introduction to Organic Compound part 4.pdf
Organic Chemistry Chapter 4 Part I
-
Upload
patience-bernard -
Category
Documents
-
view
32 -
download
3
description
Transcript of Organic Chemistry Chapter 4 Part I
![Page 1: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/1.jpg)
Aromatic Compounds- Benzene & Its Family -
Nanoplasmonic Research Group
Organic Chem-istry
Chapter 4Part I
![Page 2: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/2.jpg)
There are TWO FACTS about BEN-ZENE beyond our expectation
1. Reaction Behaviors Unlike Other Alkenes
2. Unexpected Stabilization Energy
![Page 3: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/3.jpg)
Benzene has 3 double bonds, so we would expect addition reaction as with other
alkenes, BUT
![Page 4: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/4.jpg)
Then, What could happen to BEN-ZENE ???
![Page 5: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/5.jpg)
It is Unbelievable, but TRUE
![Page 6: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/6.jpg)
What about Next ?- Unexpected Stabilization Energy -
![Page 7: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/7.jpg)
A Measure of Alkene Stability
- Supplementary slide for the previous one -
![Page 8: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/8.jpg)
The Point is that BENZENE is Unexpected STA-
BLE !!
WHY???
Aromaticity
![Page 9: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/9.jpg)
• A chemical property in which a conjugated ring of unsatu-rated bonds, lone pairs, or empty orbitals exhibit a stabi-lization stronger than would be expected by the stabiliza-tion of conjugation alone
Aromaticity
1. Follow Huckel’s rule, having 4n+2 electrons in the delocalized cloud2. Are able to be planar and are cyclic3. Every atom in the circle is able to participate in delocalizing the electrons by having a p or-bital or an unshared pair of electrons
Criteria for Aromaticity
![Page 10: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/10.jpg)
Huckel’s Rule
• Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+2 pi electrons (n is 0,1,2,3,4)
• For n=1: 4n+2 = 6; benzene is stable and electrons are delocalized
![Page 11: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/11.jpg)
Aromatic Cation & Anion
![Page 12: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/12.jpg)
• Planar, cyclic molecules with 4n pi electrons are much ‘LESS’ stable than expected (anti-aromatic)
• They will distort out of plane and behave like ordinary alkenes
• 4- and 8-electron compounds are not delo-calized (single and double bonds)
Compounds With 4n pi Electrons Are Not Aromatic (May be Antiaromatic)
cyclobutadiene cyclooctatetraene
Then, What happens to compounds that donot abide by Huckel’rule
![Page 13: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/13.jpg)
What is principle underlie aro-maticity ?
- Molecular Orbital (MO)!!! -
* Six overlapping p orbitals must form six molecular orbitals* Three will be bonding, three antibonding* As energy of MO increases, the number of nodes increases* System symmetric so 2 pairs of degenerate or-bitals
![Page 14: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/14.jpg)
Cyclobutadiene and Cyclooctate-traene
* Following Hund’s rule two electrons are in sep-arate orbitals because they are at same energy* If these compounds adopted a coplanar geome-try – two of the molecular orbitals would each have a single unpaired electron – very unstable
![Page 15: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/15.jpg)
What if heteroatoms exist in the ring ?
Pyridine & Pyrrole
Since lone pair electrons are in the aro-matic ring, protonation destroys aromatic-ity, making pyrrole a very weak base
Pyridine is a relatively weak base com-pared to normal amines but protonation does not affect aromaticity
![Page 16: Organic Chemistry Chapter 4 Part I](https://reader035.fdocuments.us/reader035/viewer/2022062301/568138af550346895da06e39/html5/thumbnails/16.jpg)
How to name aromatic compounds ?Please refer to the text, page 123
Isomeric Structure: ortho- (o-), meta- (m-), para- (p-)
As a substituent, phenyl-, benzyl