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Chapter 11 Study Guide Arenes and Aromaticity Benzene Arenes are simply hydrocarbons that include benzene rings in their structural unit STRUCTURE OF BENZENE Planar with bond angles of 120 Sp2 hybridization o Molecular formula C6H6 o All of the hydrogens are equivalent o Each carbon has four bonds attached to it At first it was thought that the structure is a single structure of alternating single and double bonds However it was later found that the structure of benzene is a resonance structure as neither of the two original Kekule structures correctly described them The circle inside a hexagon represents this resonance structure 11 3 Stability of Benzene alkanes The heat of hydrogenation of benzene and other arenes is higher than alkenes and This is due to its resonance energy or increased stability due to its electron delocalization It is 152 kj mol more stable than expected because of electron delocalization 11 4 Molecular Orbitial View of Bonding in Benzene Remember the structural facts of benzene o Planar o Bond angles 120 o Each carbon bonded to 3 other atoms sp2 hybridization In addition to the 3 sp2 orbitals of each carbon each carbon has a half filled 2p orbital o Overlap of these 2p orbitals creates a pi system encompassing the entire ring o These 6 pi electrons are said to be delocalized over the entire ring There is high electron density above and below the plane of the ring creating the chemical properties we will explore in later reactions FIGURE 11 4 BELOW SHOWS THE DIVISION OF ORBITALS IN BENZENE 3 BONDING MOLECULAR ORBITALS EACH CONTAINING 2 ELECTRONS o These account for the 6 pi electrons 3 ANTI BONDING MOLECULAR ORBITALS CONTAINING NO ELECTRONS 11 6 Nomenclature of Benzene its Derivatives All compounds that contain a benzene ring are aromatic and substituted derivatives of benzene are the largest class of aromatic compounds o These compounds are simply named by attaching the name of the substituent as a prefix to benzene There are many compounds that have been around so long that IUPAC adopted their common name instead of their systematic name it is appropriate to use both names Benzenecarbaldehyde Benzaldehyde Benzenecarboxylic acid Benzoic acid Vinylbenzene Styrene Methyl phenyl ketone Acetophenone Benzenol Phenol Methoxybenzene Anisole Benzenamine Aniline Benzenes with two methyl groups are called xylenes There are three isomers o Ortho o 1 2 disubstituted o Meta m 1 3 disubstituted o Para p 1 4 disubstituted These prefixes can also be used on any other disubstituted benzenes When three or more substituents are present numerical locants must be used instead o The base name of the benzene derivative determines the numbering of the carbons The direction of numbering is chosen to give the next substituted position the lowest number irrespective of what substituent it bears When the benzene ring is used as a substituent the world phenyl stands for C6H5 A benzyl group is C6H5CH2 Biphenyl is the accepted IUPAC name for the compound in which two benzene rings are connected by a single bond 11 7 Polycyclic Aromatic Hydrocarbons Polyclic aromatic hydrocarbons are simply collections of two or more benzene rings fused together Below are the three most common naphthalene anthracene and phenanthrene 11 8 Properties Recall characteristics of alkanes alkenes and alkynes o Non polar o Insoluble with water Physical 11 9 Intro to Reactions of Arenes 2 groups of reactions o One group involves the ring itself as a functional group Reduction and oxidation Electrophilic aromatic substitution o Other group involves aryl group as a substituent and affects the reactivity of the functional unit of which it is attached Carbon atom directly attached to benzene ring is called benzylic carbon 11 10 The Birch Reduction o In the presence of an alcohol a group one metal and liquid ammonia arenes are reduced to nonconjugated dienes o Arene group I metal NH3 alcohol non conjugated diene Remember a diene is simply a hydrocarbon that contains two carbon double bonds Conjugated means that the single and double bonds alternate o Alkyl substituted arenes yield 1 4 disubstituted dienes with the alkyl group as a substituent on the double bond THE MECHANISM The overall reaction 1 An electron is transferred from sodium the reducing agent to the pi system of the aromatic ring The product is an anion radical 2 The anion radical is a strong base so it abstracts a proton from the alcohol 3 The cyclohexadienyl radical is converted to an anion by another atom of sodium through electron transfer 4 Another proton is abstracted from methanol creating 1 4 cyclohexadiene 11 11 Free Radical Halogenation of Alkylbenzenes o The benzylic carbon is important in this reaction The benzylic C H bond is weaker than a normal bond making it relatively easy to abstract benzylic hydrogen leading to high selectivity in this reaction o The halogenation of alkyl benzenes involve the formation of a benzyl radical o For example the chlorination of toluene takes place exclusively at the benzylic carbon o Benzylic bromination is more commonly used than chlorination D 11 12 Oxidation of Alkylbenzenes o An alkyl side chain on a benzene ring is oxidized on being heated with an acid The product is a benzoic acid benzene with carboxylic acid substituent o Arene oxidizing agent H2SO4 KMnO4 Na2Cr2O7 benzoic acid or substituent of benzoic acid SOME EXAMPLES 1 Alkyl side chain being oxidized after being heated with chromic acid 2 Two step reaction of potassium permanganate and chlorotoluene 3 When two alkyl groups are present on the ring both are oxidized 11 17 Polymerization of Styrene o Polystyrene Styrofoam is created from the free radical mechanism using peroxide as an initiator THE MECHANISM 1 Initiation benzoyl peroxide dissacoiates on heating producing two alkoxy radicals 2 Addition free radical produced in step one adds to double bond of styrene Addition occurs in direction that produces a benzylic radical 3 Addition again the benzylic radical in step 2 adds to a molecule of styrene 4 Addition again radical produced in step 3 continues to react with another styrene molecule The process repeats over and over again

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FSU CHM 2211 - Chapter 11

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