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Chapter 11 Study GuideChapter 11 Study GuideArenes and Aromaticity: Benzene• Arenes are simply hydrocarbons that include benzene rings in their structural unitSTRUCTURE OF BENZENE• Planar with bond angles of 120°• Sp2 hybridizationo Molecular formula C6H6o All of the hydrogens are equivalento 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 structure11. 3 Stability of Benzene• The heat of hydrogenation of benzene and other arenes is higher than alkenes and alkanes. • 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 delocalization11.4 Molecular Orbitial View of Bonding in Benzene• Remember the structural facts of benzeneo Planaro Bond angles 120o 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 orbitalo Overlap of these 2p orbitals creates a pi system encompassing the entire ringo 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 ELECTRONS11.6 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 BenzaldehydeBenzenecarboxylic acid Benzoic acidVinylbenzene StyreneMethyl phenyl ketone AcetophenoneBenzenol PhenolMethoxybenzene AnisoleBenzenamine Aniline• Benzenes with two methyl groups are called xylenes. There are three isomers:o Ortho (o): 1, 2 disubstitutedo 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 insteado 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 phenanthrene11.8 Physical Properties • Recall characteristics of alkanes, alkenes and alkyneso Non-polaro Insoluble with water11.9 Intro to Reactions of Arenes• 2 groups of reactionso One group involves the ring itself as a functional group Reduction and oxidation Electrophilic aromatic substitutiono 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 carbon11.10 The Birch Reductiono In the presence of an alcohol, a group one metal, and liquid ammonia, arenes are reduced to nonconjugated dieneso 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 bondTHE MECHANISMThe 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 alcohol3. 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 cyclohexadiene11.11 Free Radical Halogenation of Alkylbenzeneso 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 radicalo For example, the chlorination of toluene takes place exclusively at the benzylic carbonDo Benzylic bromination is more commonly used than chlorination11.12 Oxidation of Alkylbenzeneso 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 acidSOME EXAMPLES:1. Alkyl side chain being oxidized after being heated with chromic acid2. 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 Styreneo Polystyrene (Styrofoam) is created from the free-radical mechanism using peroxide as an initiatorTHE MECHANISM1. Initiation: benzoyl peroxide dissacoiates on heating producing two alkoxy radicals2. Addition: free radical produced in step one adds to double bond of styrene. Addition occurs in direction that produces a benzylic radical3. 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

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FSU CHM 2211 - Chapter 11

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