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by: Brady Spinka


Brady Spinka
GPA 3.98

Jonathan Sessler

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Jonathan Sessler
Class Notes
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This 29 page Class Notes was uploaded by Brady Spinka on Monday September 7, 2015. The Class Notes belongs to CH 310N at University of Texas at Austin taught by Jonathan Sessler in Fall. Since its upload, it has received 146 views. For similar materials see /class/181865/ch-310n-university-of-texas-at-austin in Chemistry at University of Texas at Austin.




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Date Created: 09/07/15
Lecture 8 Reactivity of Simple Benzene Derivatives CH 20 review Electrophilic Aromatic Substitution CH 21 new Recommended Problems CH s 20 21 204 206 2072010 2016 2017 20192032 2043 20462048 212 214 215 217219 2111 21142122 21242126 21302143 Exam 1 will cover spectroscopy MS IR NMR UVVis and Aromaticity Chapts 1214 part of 23 20 and 21 Disubstituted Benzenes When two substituents are on benzene three constitutional isomers are possible Ortho 0 or 12benzene Meta m or 13benzene Para p or 14benzene H3C CH2 N02 1639 1chloro3nitrobenzene mchloronitrobenzene 12 dinitrobenzene odinitrobenzene Br 1bromo4ethylbenzene p bromoethylbenzene When one of the two substituents imparts a special name then the compound is named as a derivative of the parent compound NH2 H 0 CH3 839 1 CH3 CI 2 bromobenzaldehyde 4Chl0roaniline obromobenzaldehyde p chloroanlllne mxylene When three or more substituents are present on a ring their locations are speci ed by numerals If any of the substituents imparts a special name then the molecule is named as a derivative of that special molecule If there is no special name then the substituents are named in alphabetical order With the lowest numbering system possible CH3 OH N02 6 1 N02 Br 6 1 Br 6 1 2 2 2 5 5 5 4CI 3 4 3 4 3 Br B CH r H30 2 4chloro2 nitrotoluene 246 tribromophenol 2 bromo1ethy 4 nitrobenzene Polyaromatic hydrocarbons PAH contain two or more benzene rings each of which shair two carbon ring atoms The more common ones naphthalene anthracene and phenanthrene were distilled from coal tar Naphthalene is most commonly known as the ingredient in moth balls Found in car exhaust and cigarette smoke are the lesser known PAHs pyrene benzoapyrene and coronene Benzoapyrene is of particular interest because it is a very potent carcinogen and mutagen 00 099 naphthalene anthracene phenanthrene pyrene benzoapyrene coronene Phenols o The functional group of a phenol is an OH group bonded to a benzene ring CH3 Phenol 3Methylphenol mCresol Phenom Compounds that incorporate an OH functionality directly on a benzene are named as phenols or have special names themselves OH OH OH OH OH CH3 OH OH PhenOI cateChOI resorcinol mcresol 12 benzenedlol 13benzenedlol 3methyl phenol hydroquinone 14benzenediol Phenols are found in coal tar as well as the cresols 0mp Phenol is a low melting solid that is soluble in water It has antiseptic properties and was used in the 19th century in surgery Its medical use is now limited as derivatives with fewer side effects have become available Acidity of Phenols Phenols and alcohols both contain hydroxyl groups However they are classi ed as separate functional groups Why Answer Phenols have different properties than alcohols most noteworthy is their acidity pKa difference of 106 OH 0 H20 j H3O pKa 995 H H C2 c2 H o pK 15 9 H3O OH H20 H3C O 3 a Solutions of alcohols in water are neutral whereas a solution of 01 M phenol is slightly acidic pH 54 Why are phenols more acidic o o O o oob4o J J Resonance The charge is delocalized around the ring This gives a qualitative understanding as to Why phenols are more acidic than alcohols but for quantitative comparison pKas must be determined experimentally Ring substituents especially halogens and nitro groups have marked effects on the acidity of phenol by a combination or resonance and inductive effects Both mcresol and pcresol are weaker acids than phenol with pKa 1001 and 1017 respectively OH OH QCW mcresol pores In uence of substituents on the acidity of phenol Alkyl groups decrease the acidity of phenol Where halogens increase the acidity of phenol through inductive effects 0 OH OH Q pKa885 pKa 1017 Electron donating alkyl group destabilizes c CH3 this resonance structure CH3 m chlorophenol pcresol OH 0 Electron withdrawing halogen groups X F Cl Br stabilize the delocalized negative charge X Fluorine is most electronegative of the halogens and therefore has the greatest in uence on the acidity of halophenols This trend follows electronegativity Chlorine has less of an effect uorine and bromine even less of an effect than chlorine Nitro groups increase acidity by both inductive and resonance effects OH OH OH i N02 N02 pl 3135 mnitrophenol a 39 pKa 828 p nitrophenol pKa 715 pNitrophenol is more acidic than mnitrophenol even though it is farther away from the OH group Nitro groups in the ortho and para positions make an additional resonance contribution to the stability of the negative charge in the ring The extreme is picric acid which is more acidic than phosphoric acid OH O O O2N No2 Delocalization of negative charge onto 39 oxygen futher increases the resonance stabilization of the phenoxide ion No2 P N INT 0 O O O picric acid 246nitrophenol pKa 038 Acidities of Phenols Part of the acidstrengthening effect of N02 is due to its electronWithdrawing inductive effect In addition NO2 substituents in the ortho and para positions help to delocalize the negative charge by resonance This is a BIG effect and must be appreciated for such 0 3390 delocalization of negative charge onto oxygen o N 00 O J Acidities of Phenols An Important Observation Nitro groups increase the acidity of phenols by both an electronWithdrawing inductive effect and a resonance effect However the resonance holds only for the ortho and parasubstituted systems OH OH OH Q Elm lt3 N02 Phenol mNitrophenol pNitrophenol pKa 995 pKa 828 pKa 715 Preparation of Aryl Ethers Aryl ethers can be prepared from phenoxide salts and alkyl halides The opposite case is not true aryl halides are not good leaving groups Often phase transfer catalysts remember crown ethers are used in these reactions as they are made using two phases The organic phase contains the alkyl halide and soon the product While the aqueous phase contains the base and the phenol The phase transfer catalyst transports the phenoxide into the organic layer 6X2 W OH Cl NaOH H20 CH2C2 Bu4NBr39 Anisole can be formed from phenol plus a methylating agent CH3I or dimethyl sulfate this latter compound is a very strong alkylating reagent and should be handled with much care CH3 OH NaOH H20 S CHZCIZ S H3c o s o CH3 Nao s oNa o Bu4NBr o anisole dimethyl sulfate Aryl ethers can be cleaved by hydrohalic acids HX to form alkyl halides and phenol OV OH Oxidation of Phenol to Quinones Phenols Hre subliect toooxidation by a rietylt2or astro 0 oxidation agents i art m t fjelectron dona OHsltgro O Oxidation of phenb lmm otassium dig g gggate gives 14 benzoquinone pquinone Quinones are cyclohexa i d iones by de nition and 12quinones are led ortho While 13quinones K C O are called meta 0 2 r2 7 sto4 Quinones can also becobtained by mi tion of catechol and 14 benzenediol 0 The most important opertysgaf quoHones is that they are readily reduced to benzenedir p 1non an be readily reduced by sodium dithionate in neutral or alkal ie solution 14benzoquinone pqumone 14benzenediol hydroquinone Special OxidationReduction Reactions Involving Hydroquinones and Quinones OH O OX gt red OX K2Cr207 02 etc OH O hydroquinone pquinone red NaZSZO4aq dithionite OH O OH O OX gt catechol oquinone This socallcd redOX reductionoxidation chemistry is critical to life 0 OH H300 d H300 re 2H 2639 4 H300 H H300 0X 0 H OH 11 COCHZyme Q Coenzyme Q oxidized quinone fOI39m reduced hydroquinone form Reactions at Benzyl Carbons CH3 Benzylic Reactions 0 Benzylic radicals and cations are easily formed because of the resonance stabilization of these intermediates the benzyl radicals and cations are hybrids of four contributing resonance structures Be sure you can draw them all Benzylic Bromination o Bromination is by a radical mechanismkn0w it Br ON gt CC14 A Regioselective reaction l Slow l Br 0 gt 12 Br2 O N O O N O I Br NBS N bromosuccinimide Propagation Steps Food For Thought Is there a Difference 0 Front of playing card a Back of playing card LBS CC14 Benzylic Oxidation o Benzene is unaffected by strong oxidizing agents such as HzCrO4 KzCrZO7 V205 and KMnO4 halogen and nitro substituents are also unaffected by these reagents alkyl groups with at least one hydrogen on the benzylic carbon are oxidized to a carboxyl group OZN 2chloro4nitro 2chloro4nitro toluene benzoic acid Benzylic Oxidation If there is more than one alkyl group on the benzene ring with a proton on it each is oxidized to a C02H group t CH C CE 3 chrzo7 0 oH gt CHzCH3 H2504 c OH O O K2CI 207 A O OH H2804 OH H3C CH3 Oxidation of Alkyl Side Chains More Examples CH2CHZCH3 oxidized CH2 HZCrO4 KMnO4 etc COZH products gt from alkyl chain COZH V205 02 heat ZCO gt 2 COZH Recall These oxidations require at least one H on benzylic carbon atom Hydrogenolysis o Hydrogenolysis Bond cleavage by H2 First Observation Benzylic alcohols and ketones are reduced fully under conditions of catalytic hydrogenation OH H2 PdC gt O H DJK 01 J H2 PdC Second Observation the key one Benzyl Ethers and Esters are Cleaved by Hydrogenolysis OCHZCH3 H2 Pd on 0 CH2 gt CH3 EtOH ethanol 0 O H2 Pd on C CH2 gt CH3 HOAC acetic acid While written here to emphasize fate of benzyl portion the key point is that the benzyl group functions a protecting group for the alcohol and acid moieties see bottom of p 785 in B amp F for example Summary of Properties and Reactivity Isomers Phenols Benzyl position Benzyl alcohols Quinones Kobe reaction Ortho meta para Acidic relative to alcohols modi ed by type of substituents and their position Radical bromination oxidation of side chains stabilized carbocations Protecting groups for alcohols amp carboxylic acids Redox partners of hydroquinones and catechols mild oxidants and important natural products Skip this for now


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