Chem 120 10.2 AROMATIC COMPOUNDS

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Chem 120 10.2 AROMATIC COMPOUNDS

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Slide 1: 

Chapter 10.2 AROMATIC COMPOUNDS

Aromatic Hydrocarbons : 

Aromatic Hydrocarbons Benzene’s structure was first proposed 150 years ago A cyclic structure for benzene, C6H6 Something special about benzene Although his structures showed double bonds, the molecule did not react as if it had any unsaturation Originally named aromatic compounds for the pleasant smell of resins from tropical trees (early source) Now aromatic hydrocarbons are characterized by a much higher degree of chemical stability than predicted by their chemical composition Most common group of aromatic compounds is based on the 6-member aromatic ring, benzene

Benzene Structure : 

Benzene Structure The benzene ring consists of: Six carbon atoms Joined in a planar hexagonal arrangement Each carbon is bonded to one hydrogen atom Two equivalent structures proposed by Kekulé are recognized today as resonance structures The real benzene molecule is a hybrid with each resonance structure contributing to the true structure

Benzene Structure – Modern : 

Benzene Structure – Modern Modern concept of benzene structure is based on overlapping orbitals Each carbon is bonded to two others by sharing a pair of electrons These same carbon atoms also each share a pair of electrons with a hydrogen atom Remaining 6 electrons are located in p orbitals that are perpendicular to the plane of the carbon ring These p orbitals overlap laterally Form a cloud of electrons above and below the ring

Pi Cloud Formation in Benzene : 

Pi Cloud Formation in Benzene The current model of bonding in benzene

IUPAC Names: Benzenes : 

IUPAC Names: Benzenes Most simple aromatic compounds are named as derivatives of benzene For monosubstituted benzenes, name the group and add “benzene” chlorobenzene ethylbenzene nitrobenzene

IUPAC Names: Benzenes : 

IUPAC Names: Benzenes For disubstituted benzenes, name the groups in alphabetical order The first named group is at position 1 If a “special group” is present, it must be number 1 on the ring An older system of naming indicates groups using ortho (o) = 1,2 on the ring meta (m) = 1,3 on the ring para (p) = 1,4 on the ring

Historical Nomenclature : 

Historical Nomenclature Some members of the benzene family have unique names acquired before the IUPAC system was adopted that are still frequently used today

Slide 9: 

Naming Derivatives of Benzene Example 11.10 Name the following compounds using the IUPAC Nomenclature System. 2-chlorotoluene 4-nitrophenol 3-ethylaniline

Slide 10: 

Naming Derivatives of Benzene Example 11.11 Name the following compounds using the common system of nomenclature. ortho-chlorotoluene para-nitrophenol meta-ethylaniline

Benzene As a Substituent : 

Benzene As a Substituent When the benzene ring is a substituent on a chain (C6H5), it is called a phenyl group Note the difference between Phenyl Phenol (a functional group) 4-phenyl-1-pentene

Polynuclear Aromatic Hydrocarbons : 

Polynuclear Aromatic Hydrocarbons Polynuclear aromatic hydrocarbons (PAH) are composed of two or more aromatic rings joined together Many have been shown to cause cancer

Reactions of Benzene : 

Reactions of Benzene Benzene does not readily undergo addition reactions Benzene typically undergoes aromatic substitution reactions: An atom or group substitutes for an H on the ring All benzene reactions we consider require a catalyst The reactions are: Halogenation Nitration Sulfonation

Benzene Halogenation : 

Benzene Halogenation Halogenation places a Br or Cl on the ring The reagent used is typically Br2 or Cl2 Fe or FeCl3 are used as catalysts

Benzene Nitration : 

Benzene Nitration Nitration places the nitro group on the ring Sulfuric acid is needed as a catalyst

Benzene Sulfonation : 

Benzene Sulfonation Sulfonation places an SO3H group on the ring Concentrated sulfuric acid is required as a catalyst This is also a substitution reaction

Heterocyclic Aromatic Compounds : 

Heterocyclic Aromatic Compounds Rings with at least one atom other than carbon as part of the structure of the aromatic ring This hetero atom is typically O, N, S The ring also has delocalized electrons The total number of atoms in the ring is typically either: A six membered ring Some have a five membered ring

Heterocyclic Aromatics : 

Heterocyclic Aromatics Heterocyclic aromatics are similar to benzene in stability and chemical behavior Many are significant biologically