Osmium Tetroxide Oxidation of Alkenes Vicinal Syn Dihydroxylation with Osmium Tetroxide Mechanism Example Ozonolysis Mechanism Diels Alder Reaction Concerted and Stereospecific Endo Rule Typical dienes Typical dienophiles Cyclooctatetraene Formation Nickel catalyzed Cyclotetramerization of Ethyne Non aromatic Cyclooctatetraene Forms an Aromatic Dianion 16 Annulene Antiaromatic 16 Annulene can be Reduced to an Aromatic Dianion Antiaromatic 16 Annulene can be Oxidized to an Aromatic Dication Hydrogenation of Benzene Heats of hydrogenation provide a measure of benzene s unusual stability Experimental values for cyclohexene and 1 3 cyclohexadiene allow us to estimate the heat of hydrogenation for the hypothetical 1 3 5 cyclohexatriene Comparison with the experimental H for benzene gives a value of approximately 29 6 kcal mol 1 for the aromatic resonance energy Electrophilic Aromatic Substitution General Mechanism Electrophilic Aromatic Substitution Halogenation Benzene is normally unreactive to halogens because they are not electronegative enough to disrupt benzene s aromaticity Fluorination of benzene is very exothermic explosive Iodination of benzene is endothermic and does not occur Chlorination and Bromination do occur o To become powerful enough electrophiles to disrupt the aromaticity of benzene chlorines bromines must first be activated by Lewis acid catalysts such as ferric halides FeX3 aluminum halides AlX3 o The activated halogen complex can then attack the benzene molecule Bromination Mechanism Activation of Bromine by the Lewis acid FeBr3 Electrophilic Attack on Benzene by Activated Bromine Bromobenzene Formation Proceeds analogously to Bromination except this time the Lewis acid catalyst used is AlCl3 Activation of Chlorine by the Lewis Acid AlCl3 Electrophilic Attack on Benzene by Activated Chlorine Chlorination Mechanism Chlorobenzene Formation Electrophilic Aromatic Substitution Nitration Electrophilic Aromatic Substitution Sulfonation Electrophilic Aromatic Substitution Benzenesulfonyl Chlorides EAS Friedel Crafts Alkylation Intramolecular Alcohols as Substrates Alkenes as Substrates EAS Friedel Crafts Acylation Acyl Halides Anhydrides Electrophilic Substitution of Substituted Benzenes Ortho and Para Directing Groups Electrophilic Substitution of Substituted Benzenes Meta Directing Groups Interconversion of Nitro and Amino Groups Interconversion of Acyl and Alkyl Blocking by Sulfonation Moderation of Strong Activators by Protection Electrophilic Aromatic Substitution of Naphthalene Synthesis of Aldehydes Ketones Oxidation of Alcohols 2 Alcohols 1 Alcohols Allylic Oxidation Synthesis of Aldehydes Ketones Hydration of Alkynes Hydration of the carbon carbon triple bond yields enols that tautomerize to carbonyl compounds In the presence of mercuric ion addition of water follows Markovnikov s rule to furnish ketones Mechanism Reactions of Aldehydes Ketones Reduction by Hydrides Aldehydes Ketones Selectivity Reactions of Aldehydes Ketones Addition of Organometallic Compounds Formaldehyde 1 Alcohol Aldehyde 2 Alcohol Ketone 3 Alcohol Reactions of Aldehydes Ketones Addition of Water and Alcohols Hemiacetals Carbonyl Hydrates Geminal Diols Hemiacetals Intramolecular Addition Cyclic Hemiacetals Reactions of Aldehydes Ketones Acid Catalyzed Addition of Alcohols Acetals Cyclic Acetals Reactions of Aldehydes Ketones Wittig Reaction Mechanism 1 Phosphonium Salt Synthesis 2 Ylide Formation 3 The Wittig Reaction Some substituents migrate more easily than others The migratory aptitude of various groups suggests that the migrating carbon possesses carbocationic character in the transition state Reactions of Aldehydes Ketones Baeyer Villager Oxidation Mechanism Acidity Basicity of Carboxylic Acids Acidity of Carboxylic Acids Salt Formation Carboxylic acids are relatively strong acids Carboxylic acids have much lower pKa values than do alcohols The lowered pKa values are due to the electron withdrawing effect of the positively polarized carbonyl carbon and the resonance stabilization of the carboxylate group Two of the three resonance forms of the carboxylate ion are equivalent leading to a symmetrical ion with equal carbon oxygen bond lengths 1 26 midway between a carbon oxygen double bond 1 20 and a carbon oxygen single bond 1 34 Electron withdrawing substituents increase the acidity of carboxylic acids The inductive effect of electron withdrawing groups close to the carboxy group causes an increase in acidity Three electron withdrawing groups on the carbon sometimes results in acidity near that of some inorganic acids The dioic acids have two pKa values In ethanedioic and propanedioic acids the first pKa is lowered by the electron withdrawing effect of the second In higher dioic acids both pKa values are close to monocarboxylic acids Carboxylate salts of carboxylic acids can be prepared by treatment of the acid with a base such as NaOH Na2CO3 or NaHCO3 These salts are much more water soluble than the corresponding acids Carboxylate salts are named by specifying the metal and then replacing ic acid with ate Acidity Basicity of Carboxylic Acids Basicity of Carboxylic Acids Carboxylic acids may be protonated on the carbonyl oxygen The carbonyl oxygen of a carboxylic acid may be protonated by strong acids to give alkyloxonium ions The carbonyl oxygen is more basic than the OH group of alcohols due to resonance stabilization of the alkyloxonium ion Note the protonation reaction is not particularly strong Preparation of Carboxylic Acids Industrial Synthesis of Carboxylic Acids Formic acid and acetic acid are manufactured on a large scale industrially Formic Acid Synthesis Acetic Acid Synthesis Via Oxidation of Ethene Wacker Process O Via Carbonylation of Methanol Monsanto Process Other important industrial carboxylic acids include the two dicarboxylic acids Hexanedioic acid 1 4 benzenedicarboxylic acid Nylon Plastics Preparation of Carboxylic Acids Oxidation of primary alcohols and of aldehydes furnishes carboxylic acids Primary alcohols oxidize first to aldehydes which then may further oxidize to carboxylic acids Oxidation of Primary Alcohols Oxidation of Aldehydes Oxidants include CrO2 KMnO4 and HNO3 Nitric acid is often chosen as the oxidant because it is one of the cheapest strong oxidants Preparation of Carboxylic Acids Carbonation of Organometallic Reagents Organometallic reagents react with carbon dioxide to give carboxylic acids Carbonation or reaction of an organometallic
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