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Organic Chemistry Wisebridge Learning Systems Organic Chemistry Reaction Mechanisms Pocket-Book WLS www.wisebridgelearning.com © 2006 J S Wetzel LEARNING STRATEGIES CONTENTS ● The key to building intuition is to develop the habit ALKANES of asking how each particular mechanism reflects Thermal Cracking - Pyrolysis . 1 general principles. Look for the concepts behind Combustion . 1 the chemistry to make organic chemistry more co- Free Radical Halogenation. 2 herent and rewarding. ALKENES Electrophilic Addition of HX to Alkenes . 3 ● Acid Catalyzed Hydration of Alkenes . 4 Exothermic reactions tend to follow pathways Electrophilic Addition of Halogens to Alkenes . 5 where like charges can separate or where un- Halohydrin Formation . 6 like charges can come together. When reading Free Radical Addition of HX to Alkenes . 7 organic chemistry mechanisms, keep the elec- Catalytic Hydrogenation of Alkenes. 8 tronegativities of the elements and their valence Oxidation of Alkenes to Vicinal Diols. 9 electron configurations always in your mind. Try Oxidative Cleavage of Alkenes . 10 to nterpret electron movement in terms of energy Ozonolysis of Alkenes . 10 Allylic Halogenation . 11 to make the reactions easier to understand and Oxymercuration-Demercuration . 13 remember. Hydroboration of Alkenes . 14 ALKYNES ● For MCAT preparation, pay special attention to Electrophilic Addition of HX to Alkynes . 15 Hydration of Alkynes. 15 reactions where the product hinges on regio- Free Radical Addition of HX to Alkynes . 16 and stereo-selectivity and reactions involving Electrophilic Halogenation of Alkynes. 16 resonant intermediates, which are special favor- Hydroboration of Alkynes . 17 ites of the test-writers. Catalytic Hydrogenation of Alkynes. 17 Reduction of Alkynes with Alkali Metal/Ammonia . 18 Formation and Use of Acetylide Anion Nucleophiles . 19 Coupling of Alkyl Halides with Gilman Reagents . 20 ALYL HALIDES ALDEHYDES AND KETONES SN2 Mechanism with Alkyl Halides . 21 Reduction of Ketones and Aldehydes . 57 SN1 Mechanism with Alkyl Halides . 22 Reduction of Aryl Alkyl Ketones . 58 E2 Mechanism with Alkyl Halides . 23 Oxidation of Aldehydes and Ketones . 59 E1 Mechanism with Alkyl Halides . 24 Reaction with Grignard Reagents. 60 The Wittig Reaction. 61 ALLYLIC AND CONJUGATED STRUCTURES Acetal Formation. 63 SN1 Mechanism with Allylic Cation Intermediate. 25 The Wolff-Kishner Reaction . 65 1,2 and 1,4 Addition to Conjugated Diene . 27 Reductive Amination . 67 Diels-Alder Reaction . 29 The Cannizzaro Reaction . 69 Acid or Base Catalyzed Enolization . 71 AROMATIC COMPOUNDS Alpha Halogenation . 73 Electrophilic Aromatic Substitution with Halogen . 31 Haloform Reaction of Methyl Ketones. 75 Electrophilic Aromatic Substitution - Nitration. 33 Aldol Condensation. 77 Electrophilic Aromatic Substitution - Sulfonation . 35 Claisen Condensation . 79 Friedel-Crafts Alkylation . 37 Conjugate Nucleophilic Addition . 81 Friedel-Crafts Acylation . 39 Conjugate Addition of Gilman Reagents . 83 Alkylbenzene Oxidation . 43 Alkylbenzene Halogenation . 44 CARBOXYLIC ACIDS AND DERIVATIVES Nucleophilic Aromatic Substitution . 41 Acid Halide Formation . 85 Fischer Esterification . 86 ALCOHOLS AND ETHERS Use of Carboxylate Anion Nucleophile to form Esters . 87 Dehydration of Alcohols . 45 Hydrolysis of Acid Halides . 88 Reaction of Alcohols with HX – Dehydrohalogenation. 47 Reaction of Acyl Halide with Ammonia or Amine. 89 Reaction of Alcohols with Thionyl Chloride . 48 Esterification of Acid Halides . 90 Reaction of Alcohols with Phosphorus Tribromide. 49 Esterification of Acid Anhydrides . 91 Oxidation of Alcohols . 50 Saponification of Esters . 92 Alkoxide Ion Formation from Alcohols . 50 Nitrile Hydrolysis. 93 Reaction of Alcohols to form Ethers. 51 Nitrile Reduction . 94 Williamson Ether Synthesis . 52 Hofmann Rearrangement . 95 Acid Cleavage of Ethers . 53 Epoxidation of Halohydrins . 54 Acid Epoxide Ring Opening . 55 Thermal Cracking - Pyrolysis Alkanes heat CH3(CH2)xCH3 CH3CH3 + CH4 + H2CCH2 + etc High carbon number Lower carbon number petroleum distillate cleavage product mixture A process carried out on petroleum distillates at high temperature and pressure, thermal cracking yields lower carbon number product, probably by means of a radical (homeolytic) mechanism. The thermodynamics are dominated by the entropy change rather than the enthalpy change, especially if the volume is kept constant. Combustion Alkanes CnH2n + 2 + (3n + 1)/2 O2 nCO2 + (n + 1) H2O Hydrocarbon Oxygen Carbon dioxide Water Many organic molecules can undergo combustion, forming carbon dioxide and water in an exothermic reaction. The heat released in the combustion reaction (the enthalpy change) can be used as an indicator of the relative stability of isomers. Combustion is more exothermic for unbranched alkanes, for example, than for their branched isomers, and we can infer that the branched isomer is the more stable. Such comparisons are often used in organic chemistry. For example, ketones are pointed out as more stable than their aldehyde isomers. The more stable the isomer, the lower the heat of combustion. 1 Free Radical Halogenation Alkanes RCH3 + X2 RCH2X + HX Alkane Halogen Alkyl halide Hydrogen halide hv Initiation X2 2 X. Halogen Halide radical Propagation X. + RCH3 RCH2. + HX Halide radical Alkane Alkyl radical X2 + RCH2. RCH2X + X. Halogen Alkyl radical Alkyl halide Halide radical Termination X. + RCH2. RCH2X X. + X. X2 RCH2. + RCH2. RCH2CH2R Because of the relative stability of alkyl radical intermediates, selectivity in free radical halogenation favors tertiary over secondary over primary carbon radicals. Bromination, though, is more selective than chlorination, because the proton extraction step is more endothermic in bromination than chlorination. This follows from Hammond’s postulate, which governs the correlation between proximity in energy and proximity in structure among transition states and intermediates. Halogenation is the classic illustration of Hammond’s postulate. Because the activated complex prior to formation of the alkyl radical intermediate must have more radical character for bromination compared to chlorination, the effect of substitution in stabilizing radicals plays a greater roll with bromination leading to a higher degree of regioselectivity. 2 Electrophilic Addition of HX to Alkenes Alkenes X H R R H + HX CC H CC H Hydrogen H Alkene halide H H Alkyl halide δ- H Br R H δ+ CC + HBr R H H H H CC H Hydrogen Alkene halide H Br H + R R + - H CC Br CC H H Carbocation H H H Intermediate Alkyl halide Because tertiary and secondary carbocations are more stable than primary carbocations, Markovnikov addition is observed in the electrophilic addition of HX to alkenes, so the product formed is the one with the halogen substituent upon the more highly substituted carbon. Also, rearrangement (hydride or methyl shift to form a more stable carbocation) might occur, typical of reactions that have a carbocation intermediate. Remember that electrophilic addition will not be observed in the presence of peroxides. Peroxides initiate anti-Markovnikov addition via free-radical addition. An interesting fact about electrophilic addition of HX to alkenes, is that the more acidic the hydrogen halide, the more electrophilic it will be. HF, for example, only a weak acid, does not react. 3 Acid Catalyzed Hydration of Alkenes Alkenes H O H R H H 0, H+ R CC 2 CC H H Aqueous Alkene H Sulfuric acid H H Alcohol H δ+ H O H H + H H 0, H+ δ+ R R H 2 R H CC + O H CC H CC H H H H H Carbocation H Alkene intermediate H H H O + H O H R R CC CC + H+ H H H H H H Oxonium ion Alcohol Markovnikov’s rule is followed in hydration of alkenes. Therefore, in the alcohol product, the hydroxyl group is located upon the more highly substituted carbon. Watch for rearrangement of the carbocation intermediate, if methyl or hydride shift is probable. Note that this reaction is the reverse of acid catalyzed dehydration of alcohols. 4 Electrophilic Addition of Halogens to Alkenes Alkenes X H R R H + X CC H CC H 2 H Alkene Halogen molecule H X Vicinal dihalide δ- Br Br Br + R + - R H δ CC + Br CC + Br2 R H H H H H CC H H H Alkene + δ+ Br Br Br H - R CC + Br CC CC R H R H Br H H H H H H Cyclic halonium Br δ- Vicinal dihalide ion intermediate In analysis of the addition of halogen to an alkene, the anti stereospecificity of the dihalide product serves as evidence that the mechanism occurs via a cyclic halonium ion intermediate. For problem solving, this anti stereospecificity is especially pertinent in the cases of addition to cyclic alkenes or where the product carbons are chiral. 5 Halohydrin Formation Alkenes H X X R R H 2 CC + HX CC H H H H2O Alkene Halogen in OH H aqueous conditions Halohydrin δ- Br Br Br + R + - R H δ CC + Br CC + Br2 R H H H H H CC H H Halogen H Alkene + + H Br Br Br Br- R - CC CC + Br CC H R H R H + O H H H H H HH Cyclic halonium O HH ion intermediate Br- H Br R H Br CC R H CC + HBr + O H H OH H HH Halohydrin Br- In aqueous solution, electrophilic addition of halogen results in the formation of halohydrin. Water performs the ring opening instead of halide ion, which opens the ring in non aqueous halogenation. Water addition is preferential for the more highly substituted carbon, which receives a bit more of the distributed positive charge in the halonium ion than the other carbon. 6 Free Radical Addition of HX to Alkenes Alkenes H R Hydrogen halide Br H hv + R + HBr C C Br H C C H peroxide H Halide radical Alkene aqueous conditions are H H sufficient to supply peroxide Alkyl halide hv R O OR 2R O Peroxide Alkoxy radical initiator Alkoxy RO + HBr ROH + Br Halide radical Hydrogen radical halide H Br H R R Br + C C C C Halide H H H Alkyl radical Alkene H radical H H R Br Br R + Br C C H + HBr C C Halide Alkyl H radical radical H Hydrogen halide H H Alkyl halide In the presence of a peroxide initiator, hydrogen halide adds to alkene via an anti-Markovnikov, free-radical mechanism.
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