chapter17
Post on 12-Sep-2014
4 views
DESCRIPTION
TRANSCRIPT
![Page 1: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/1.jpg)
Chapter 17
Spring 2009
![Page 2: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/2.jpg)
Benzene• Six-membered ring• Is planar• All C-C bond lengths are equal
Kekulé structure
Resonance description of benzeneImplication of
the geometry of the structure above
Shorter isolated double bonds
Longer single bonds Common representation of
benzene
![Page 3: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/3.jpg)
BenzeneBenzene is the parent aromatic molecule.Benzene has the following characteristics that are common among all aromatic compounds•Benzene is cyclic and conjugated•Benzene is unusually stable in comparison to a molecule with three isolated pi bonds•Benzene is planar and has the shape of a regular hexagon. All bond angles are 120º, all carbon atoms are sp2 hybridized, and all carbon-carbon bond lengths are intermediate in length between a single and double bond. •Benzene undergoes substitution reactions rather than electrophilic addition reactions that would destroy its conjugation.
![Page 4: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/4.jpg)
Larger aromatic hydrocarbons
http://www.cem.msu.edu/~reusch/VirtualText/react3.htm
![Page 5: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/5.jpg)
Pictures of Benzene
Electrostatic potential surface
Benzene molecular orbitalIn top picture notice a p
orbital on each C atom.Bottom picture show the overlap of the p orbitals.
![Page 6: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/6.jpg)
Orbital picture of benzene
each carbon atom is sp2 hybridized
The resonance picture indicates why each C-C bond is the same length.
![Page 7: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/7.jpg)
Electrostatic potential surface
Electron-rich portion represents π electron cloud
![Page 8: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/8.jpg)
For a compound to be classified as aromatic, it must fulfill both of the of the following criteria:
• It must have an uninterrupted cyclic cloud of π electrons above and below the plane of the molecule (this is often called the π cloud. Also, the π cloud must be cyclic, uninterrupted, and planar.
• The p cloud must contain an odd number of pairs of π electrons. (4n+2 rule)
![Page 9: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/9.jpg)
Common names for aromatic compounds
![Page 10: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/10.jpg)
IUPAC names
• To name benzene ring with one substituent, name the substituent and add the word benzene.
ethyl benzene
?
![Page 11: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/11.jpg)
If the alkane chain is longer than six carbon atoms name the benzene ring as a phenyl substituent.
![Page 12: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/12.jpg)
Benzyl
Benzyl substituent
![Page 13: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/13.jpg)
Disubstituted benzenes
1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene
IUPAC names
![Page 14: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/14.jpg)
Same substituent
meta-dichlorobenzeneor, 1,3-dichlorobenzene
![Page 15: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/15.jpg)
Different substituents
These two have common root names
![Page 16: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/16.jpg)
Different substituents
These two have common root names
toluene
analine
![Page 17: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/17.jpg)
Different substituents
These two have common root names
meta-chlorotoluene
ortho-chloroanaline
![Page 18: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/18.jpg)
Different substituents
These don’t have a common root name so alphabetize the name of the substituents.
![Page 19: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/19.jpg)
Different substituents
These don’t have a common root name so alphabetize the name of the substituents.
ortho-bromochlorobenzene
para-bromonitrobenzene
![Page 20: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/20.jpg)
Heat of hydrogenation dataThe low heat of hydrogenation means that benzene is especially stable – this extra stability is characteristic of all aromatic compounds.
![Page 21: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/21.jpg)
Reaction of aromatic compounds
• Benzene does not undergo addition reactions typical of other unsaturated hydrocarbons.
![Page 22: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/22.jpg)
What makes a compound aromatic?
• A molecule must be cyclic
• Planar
• Completely conjugated (alternating single-double bonds)
• Have a particular number of π electrons
![Page 23: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/23.jpg)
This molecule is non-aromatic
It is cyclic, completely conjugated, non-planar, and contains 8 π electrons
![Page 24: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/24.jpg)
This molecule is anti-aromatic
Cyclic, planar, completely conjugated, and contain four π electrons.
![Page 25: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/25.jpg)
Aromatic, anti-aromatic, or non-aromatic
• Aromatic: A cyclic planar, completely conjugated compound with 4n+2 π electrons
• Anti-aromatic: A cyclic, planar, completely conjugated compound with 4n π electrons
• Non-aromatic: A compound that lack one (or more) of the following requirements for aromaticity.
![Page 26: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/26.jpg)
Aromatic compounds can have more than one ring.
![Page 27: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/27.jpg)
Aromatic rings can contain a single ring with lots of alternating single-double bonds
![Page 28: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/28.jpg)
atomic orbitals
molecular orbitals
![Page 29: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/29.jpg)
Anti-aromatic
![Page 30: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/30.jpg)
The inscribed polygon method
• Draw the polygon in question inside a circle with its vertices touching the circle and one of the vertices pointing down. Mark the points at which the polygon intersects the circle.
• Draw a line horizontally through the center of the circle and label MOs as bonding, nonbonding, or antibonding.
• Add the electrons, beginning with the lowest energy MO.
![Page 31: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/31.jpg)
Label the cation, radical, and anion as aromatic, antiaromatic, or
nonaromatic.
![Page 32: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/32.jpg)
Heterocyclic aromatic compounds
![Page 33: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/33.jpg)
![Page 34: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/34.jpg)
Nucleic acids
![Page 35: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/35.jpg)
Thymine
![Page 36: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/36.jpg)
![Page 37: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/37.jpg)
Aromatic Question
Thiamin, or vitamin B1, contains a five-membered, nitrogen-sulfur heterocycle called a thiazalium ring. Explain why the thiazolium ring is aromatic.
![Page 38: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/38.jpg)
Cycloheptatrienyl cation, radical and
anion problem
![Page 39: Chapter17](https://reader034.vdocuments.mx/reader034/viewer/2022051815/5412d0b57bef0a6e048b752c/html5/thumbnails/39.jpg)