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Term Test 2 notes from the textbook

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Mitch Winnik

Chapter 15 Benzene and Aromaticity  Aromatic: class of compounds that contain 6-membered benzene-like rings with 3 double bonds  Alkyl-sub benzenes are sometimes referred to as arenes; named in diff ways: o Smaller than the ring, arene is named as an alkyl-sub benzene o Larger than the ring, phenyl-sub alkane o Benzyl for group 15.3 Aromaticity and the Huckel 4n+2 rule  Benzene is cyclic and conjugated  Benzene is unusually stable  Planar and has the shape of a regular hexagon. All C are sp2 hybridized  Benzene undergoes substitution reactions that retain the cyclic conjugation  Resonance hybrid  Huckel 4n+2 rule: a molecule is aromatic only if it has a planar, monocyclic system of conjugation and contains a total of 4n+2 pi electrons, where n is an integer  Planar, conjugated molecules with 4n pi electrons are said to be antiaromatic because delocalization of their pi electrons would lead to their destabilization 15.4 Aromatic Ions  For aromaticity, a molecule must be cyclic, conjugated and have 4n+2 pi electrons  Cyclopentadiene is one of the most acidic hydrocarbons because the anion formed by loss of H is so stable  Cycloheptatrienyl is extraordinarily stable 15.5 Aromatic heterocycles: pyridine and Pyrrole  Heterocycle: cyclic compound that contains atoms of 2 or more elements in its ring, usually N, O, S Pyridine  Each of the 5 sp2 hybridized C has a p orbital perpendicular to the plane of the ring and each p orbital contains 1 pi electron  N atom is also sp2 hybridized and has 1 electron in a p orbital  N lone pair are in an sp2 orbital in the plane of the ring and are not part of the aromatic pi system Pyrrole and imidazole  5-membered heterocycles, have 6 pi electrons and are aromatic  Each of the 4 sp2-hybridized C contributes 1 pi electron and the sp2-hybridized N atom contributes the two from its lone pair, which occupies a p orbital  N atoms have diff roles depending on the structure of the molecule Chapter 16 – Chemistry of Benzene: Electrophilic Aromatic Substitution  Electrophilic aromatic substitution: an electrophile reacts with an aromatic ring and substitutes for one of the H 16.1 – Electrophlic Aromatic Substitution Reactions: Bromination  For bromination of benzene to take place, a catalyst such as FeBr3 is needed  The catalyst makes the Br2 molecule more electrophilic by polarizing it -> FeBr4 – Br+ species ~ Br+  The polarized Br2 molecule then reacts with the nu benzene ring to yield a nonaromatic carbocation intermediate that is doubly allylic and has 3 resonance forms  The intermediate is much less stable than benzene -> endergonic reaction -> high Ea and slow  The carbocation intermediate loses H+ from the Br-bearing C to give a substitution product  When substitution occurs, the stability of the aromatic ring is retained -> exergonic 16.2 – Other Aromatic Substitutions Aromatic Nitration  Electrophile is nitronium ion NO2+, which is generated from HNO3 by protonation and loss of water Aromatic Sulfonation  Reactive nu is either HSO3+ or neutral SO3, depending on reaction conditions  Readily reversible  Favoured in strong acid but desulfonation is favoured in hot, dilute aqueous acid Halogenation 16.3 Alkylation and Acylation of Aromatic Rings: The Friedel-Crafts Reaction o Alkylation: the introduction of an alkyl group onto the benzene ring; Friedel-Crafts reaction o Treating aromatic compound with an alkyl chloride, RCl, in the presence of AlCl3 to generate a carbocation electrophil, R+. o AlCl3 catalyzes the reaction by helping the alkyl halide to dissociate in much the same way that FeBr3 Several limitations: 1. Only alkyl halides can be used o Aryl and vinylic carbocations are too high in E 2. Don’t succeed on aromatic rings that are substituted either by a strongly electron-withdrawing group or by an amino group 3. Polyalkylation: often difficult to stop the reaction after a single substitution o High yield of monoalkylation product obtained only when a large excess of benzene is used 4. Skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used o Acylated by reaction with a carboxylic acid chloride, RCOCL, in the presence of AlCl3 o Mechanism similar to alkylation o Same limitations o An acyl cation is stabilized by interaction of the vacant orbital on C with lone-pair electrons on the neighbouring O o No rearrangement occurs o acylations never occur more than once on a ring because the product acylbenzene is less reactive than the nonacylated starting material 16.4 Substituent Effects in Substituted Aromatic Rings o A substituent already present on the ring has 2 effects 1. Substituents affect the reactivity of the aromatic ring o Some activate -> more reactive than benzene o Some deactivate -> less reactive than benzene 2. Substituents affect the orientation of the reaction\ o Ortho, meta, para are not usually formed in equal amounts o Nature of the substituent already present on ring determines the position 3 groups: 1. Ortho and para directing activators 2. Ortho and para deactivators o Halogens 3. Meta directing deactivators  Inductive effect: withdrawal or donation of electrons through a sigma bond due to electronegativity o Halogens, OH, carbonyl, cyano, nitro  Resonance effect: withdrawal or donation of electrons through a pi bond due to overlap of a p orbital on the substituent with a p orbital on the aromatic ring o Pi electrons flow from the rings to the substituents, leaving a + charge in the ring o Halogen, OH, alkoxyl (-OR), NH substituents donate electrons to the ring by resonance o Lone pair electrons flow from the substituents to the ring, placing a – charge in the ring  Inductive and resonance effects don’t necessarily act in the same direction; eg. halogens o Stronger of the two dominates 16.5 An Explanation of Substituents Effects  Activating groups: o Donate electrons to the ring -> ring more electron rich, stabilizing the carbocation intermediate, lowering Ea for its formation o Hydroxyl, alkoxyl, amino groups are because of their strong electron-donating resonance effect outweights their weaker electron withdrawing inductive effect o Alkyl groups have a strong electron inductive effect  Deactivating groups: o Withdraw electrons from the ring -> ring more electron poor, destabilizing the intermediate, raising the Ea for its formation o Carbonyl, cyano, and nitro because of both electron withdrawing resonance and inductive effects o Halogens are deactivating because their stronger electron withdrawing inductive effect outweighs their weaker electron donating resonance effects Ortho- and para- directing activators: alkyl groups  ortho and para are more
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