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Mechanisms of organic reactions
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How Organic Reactions Occur Homolytic bond breaking (radical): A-B A + B radicals are formed Heterolytic bond breaking (polar): A-B A + + :B - ions are formed
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Heterolytic Reactions Nucleophile has an electron-rich atom (e.g Cl -, CN -, NH 3 ) and can form a bond by donating a pair of electrons to electron-poor atom Electrophile has an electron-poor atom (e.g H +, CH 3 + ) and can form a bond by accepting a pair of electrons from a nucleophile A + + :B - A:B
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Kinds of Organic Reaction I. Addition reactions Addition reactions - two reactants add together to form a single new product with no atoms „left over“ Elimination reactions Elimination reactions - single reactant splits into two products
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Kinds of Organic Reaction II. Substitution reactions Substitution reactions - two reactants exchange parts to give new products Rearrangement reactions Rearrangement reactions - single reactant undergoes a reorganization of bonds and atoms
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Nuccleophilic Substitution reactions SN1 & SN2 SN1 & SN2
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Uni-/Bi- molecular reactions S N 1 & S N 2 Unimolecular reaction SN1 Unimolecular reaction SN1 only one of the reactant molecules is present in the transition state R - X + Y - R + + X - + Y - R + + Y - R - Y Bimolecular reaction SN2 Bimolecular reaction SN2 both of reactants must be present together in transition state Y - + R - X [Y…R…X] R - Y + X -
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Electrophilic Substitution - an electrophile (E ) reacts with an aromatic ring and substitutes for one of the hydrogens MECHANISM Resonance forms of complex
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Electrophiles Substance that is „electron-loving“ Has an electron-poor atom Lewis acids Lewis acids (AlCl 3, FeCl 3 ) catalyze formation of electrophilic molecules Lewis acid – accepts electron pair Lewis base – donates electron pair
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Aromatic Electrophilic Substitutions I Halogenation MECHANISM X = Cl
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Aromatic Electrophilic Substitutions II. Friedel-Crafts alkylation reactionFriedel-Crafts alkylation reaction Friedel-Crafts acylation reactionFriedel-Crafts acylation reaction AlCl 3 benzene + HCl tert- butylchloride tert- butylbenzene
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Aromatic Electrophilic Substitutions III. Nitration
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Substituent Effects in Substituted Aromatic Rings 1. Ortho- and para-directing substituents increase the electron density inside the ring Y = - NH 2, -OH, -F, -Cl, alkyl 2. Meta-directing substituents decrease the electron density inside the ring Y = - NO 2, -CN, -COOH, -SO 3 H
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2. Nucleophilic Substitution Reactions of Alkyl Halides Reactions of Alkyl Halides Nu : - + CH 3 -Br CH 3 -Nu + Br - Nucleophile = HS - > CN - > I - > HO - > Cl -
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3. Radical Substitution Initiation Initiation Cl 2 2Cl Propagation Propagation Cl + H 3 C-CH 3 H 3 C -CH 2 + HCl CH 3 -CH 2 + Cl 2 H 3 C -CH 2 Cl + Cl Termination TerminationH 3 C -CH 2 + Cl H 3 C -CH 2 Cl UV
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Electrophilic addition Markovnikov´s Rule: Markovnikov´s Rule: In the addition of HX to an alkene, the H attaches to the carbon with more H. cis addition – both groups attach to the same side of the double bond trans addition –groups attach to the opposite side of the double bond
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Elimination Reactions Dehydration Dehydrohalogenation
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Organic halide SN1 Remember the reactivity order Benzylic > allylic > 3° RX> 2° RX> 1° RX
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SN2 Remember the reactivity order 1° RX > 2° RX > 3° RX > allylic >Benzylic
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SN2 example
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E1 Remember the reactivity order Benzylic > allylic > 3° RX> 2° RX> 1° RX
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E1 & SN1
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E2 Remember the reactivity order 1° RX > 2° RX > 3° RX > allylic >Benzylic
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ALCOHOLS & Phenols Forming the phenoxide ion Remember that the negative charge here is localized, therefore this ion more stable than the alkoxide ion (from alcohol)
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Mechanism of the Kolbe-Schmitt Reaction
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Ethers
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Williamson synthesis
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Aldehydes & Ketones IDOFORM REACTION OF THE METHYL KETONE
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Aldol condensation
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Amines HOFMANN rearrangement
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