Chapter Menu Hydrocarbons Aromatic Hydrocarbons Exit Click a hyperlink or folder tab to view the corresponding slides.

Section 21-5 Aromatic Hydrocarbons Compare and contrast the properties of aromatic and aliphatic hydrocarbons. hybrid orbitals: equivalent atomic orbitals that form during bonding by the rearrangement of valence electrons aromatic compound aliphatic compound Explain what a carcinogen is, and list some examples. Aromatic hydrocarbons are unusually stable compounds with ring structures in which electrons are shared by many atoms.

Section 21-5 The Structure of Benzene Even by the middle of the 19th century, hydrocarbon ring structures remained unknown. Michael Faraday first isolated benzene in Chemists knew the formula was C 6 H 6 and proposed several possible models which would have all be very reactive, but benzene is unusually stable.

Section 21-5 The Structure of Benzene (cont.) German chemist Friederich Kekulé claimed to have had a dream in which he realized the flat, hexagonal shape ring structure for benzene. Kekulé’s structure explained some of benzene's properties, but not its lack of reactivity.

Section 21-5 The Structure of Benzene (cont.) Linus Pauling’s hybrid orbital theory explained benzenes lack of reactivity. The double bonds in benzene are not fixed, but rather the electrons are delocalized and shared among all six carbon atoms.

Section 21-5 Aromatic Compounds Organic compounds that contain benzene rings as part of their structure are called aromatic compounds. aromatic compounds Aromatic was originally used because many benzene related compounds were found in pleasant smelling oils that came from plants and plant parts.

Section 21-5 Aromatic Compounds (cont.) Aliphatic compounds are the alkane, alkene, and alkyne hydrocarbons, coming from the Greek word for fat because they were obtained by heating animal fat.Aliphatic compounds

Section 21-5 Aromatic Compounds (cont.) Substituted benzene compounds are named in the same way as cyclic alkanes.

Section 21-5 Aromatic Compounds (cont.) Many aromatic compounds were commonly used as industrial and laboratory solvents. Health risks linked to aromatics include respiratory ailments, liver problems, and damage to the nervous system. Some aromatic compounds cause cancer.

Aromatic Hydrocarbons Benzene’s structure was first proposed 150 years ago –A cyclic structure for benzene, C 6 H 6 –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 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 11.6 Aromatic Hydrocarbons

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 11.6 Aromatic Hydrocarbons

Pi Cloud Formation in Benzene The current model of bonding in benzene 11.6 Aromatic Hydrocarbons

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 11.6 Aromatic Hydrocarbons

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 11.6 Aromatic Hydrocarbons

IUPAC Names of Substituted Benzenes 1-bromo-2-ethylbenzene o-bromoethylbenzene 3-nitrotoluene m-nitrotoluene 1,4-dichlorobenzene p-dichlorobenzene

Historical Nomenclature Some members of the benzene family have unique names acquired before the IUPAC system was adopted that are still frequently used today 11.6 Aromatic Hydrocarbons

Benzene As a Substituent When the benzene ring is a substituent on a chain (C 6 H 5 ), it is called a phenyl group –Note the difference between Phenyl Phenol (a functional group) 4-phenyl-1-pentene 11.6 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 11.6 Aromatic Hydrocarbons

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: 1.Halogenation 2.Nitration 3.Sulfonation 11.6 Aromatic Hydrocarbons

Benzene Halogenation Halogenation places a Br or Cl on the ring –The reagent used is typically Br 2 or Cl 2 –Fe or FeCl 3 are used as catalysts 11.6 Aromatic Hydrocarbons

Benzene Nitration Nitration places the nitro group on the ring Sulfuric acid is needed as a catalyst 11.6 Aromatic Hydrocarbons

Benzene Sulfonation Sulfonation places an SO 3 H group on the ring –Concentrated sulfuric acid is required as a catalyst –This is also a substitution reaction 11.6 Aromatic Hydrocarbons

11.7 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 are similar to benzene in stability and chemical behavior Many are significant biologically Found in DNA and RNA Found in hemoglobin and chlorophyll 11.7 Heterocyclic Aromatic Compounds

Diagrammatic Summary of Reactions

A.A B.B C.C D.D Section 21-5 Section 21.5 Assessment Which is NOT true of benzene? A.It is an aromatic compound. B.It has a flat hexagonal shape. C.The double bonds make it unstable. D.It has delocalized electrons.

Study Guide 5 Section 21.5 Aromatic Hydrocarbons Key Concepts Aromatic hydrocarbons contain benzene rings as part of their molecular structures. The electrons in aromatic hydrocarbons are shared evenly over the entire benzene ring.

A.A B.B C.C D.D Chapter Assessment 5 Where were aromatic compounds originally obtained from? A.fossil fuels B.plant oils, plant parts, and spices C.animal fats D.minerals

A.A B.B C.C D.D STP 5 What structural characteristic do all aromatic compounds share? A.They are composed of cyclohexane. B.They have a triple bond. C.They contain a benzene ring. D.They contain a cyclic alkane.