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Chapter 7: Alkenes and Alkynes – Properties and Synthesis. Elimination Reactions Ch 7.1–7.4: Olefins, Acetylenes, E–Z System, Relative stability of alkenes, Cycloalkenes Ch 7.5–7.6: Dehydrohalogenation (E2), Zaitsev’s rule, Hofmann rule Syn/Anti coplanar conformation Ch 7.7–7.8: Dehydration of alcohols (E2 / E1), Carbocation stability Molecular rearrangement (1,2 shift) Ch 7.9–7.11: Synthesis of alkynes, vic-dihalide, gem-dihalide Terminal alkynes and their use in synthesis Ch 7.12–7.115: Hydrogenation, Reduction, Syn/Anti addition Dissolving metal reduction, Index of hydrogen deficiency Alkene Diastereomers: Cis-Trans vs. E-Z . . VS. . . . . Overall Relative Stability of Alkenes The cis isomer is less stable due to greater strain from crowding by the adjacent alkyl groups. The greater the number of attached alkyl groups (i.e., the more highly substituted the carbon atoms of the double bond), the greater is the alkene's stability. Dehydrohalogenation: Zaitsev’s Rule . . Dehydrohalogenation: Zaitsev’s Rule . . . . . . . . . . The Stereochemistry of E2 Reactions . . . . . . Acid-Catalyzed Dehydration of Alcohols: E1 Reaction The temperature and concentration of acid required to dehydrate an alcohol depend on the structure of the alcohol substrate: Primary alcohol Tertiary alcohol Secondary alcohol General reactivity order Rearrangement during Dehydration A (80%) . B (20%) . . . . . . Path B B H CH3 H2C C C CH3 CH3 H B Path A The formation of the more stable alkene is the general rule (Zaitsev's rule) in the acid-catalyzed dehydration reactions of alcohols. Hydrogenation of Alkynes to Form cis-Alkenes . . . . Pd/CaCO3 plus . N . . . Poisoned catalyst (P-2 and Lindlar’s catalyst) is required to stop the hydrogenation at an alkene stage. Anti Addition of Hydrogens to Form trans-Alkenes . . . . . Li Dissolving metal reduction . . . Index of Hydrogen Deficiency (Degree of Unsaturatiopn) . Alkane . Saturated Hydrocarbons (CnH2n+2) Cycloalkane (CnH2n) Alkene (CnH2n) Alkyne (CnH2n-2) Unsaturated H2 H2 Aromatic The index of hydrogen deficiency : difference in the number of hydrogen molecules between the corresponding alkane and molecular formula of the compound under consideration. . . IHD = 1 . Cyclohexane (C6H12) IHD = 2 IHD = 2 IHD = 1 Cyclohexene (C6H10) . . IHD = 3 . Chapter 8: Alkenes and Alkynes – Addition Reactions Ch 8.1–8.5: Electrophilic addition reaction, Markovnikov’s rule, Regioselective reaction, hydration Ch 8.6–8.11: Oxymercuration–demercuration, Hydroboration–Oxydation Anti-Markovnikov addition, Steric factors Ch 8.12–8.15: Anti-addition of halogens, Bromonium ion, Ionic mechanism, Stereospecific reaction, Halohydrin Carbene, α-Elimination Ch 8.16–8.21: Oxidation of alkenes, 1,2-Diols(glycols), syn-dihydoxylation Oxidative cleavage, Ozonolysis, Stereoselctive reaction Synthon, Synthetic equivalent Addition of HX to Alkenes: Markovnikov’s Rule Stereochemistry of the Addition of HX to Alkenes . . . . . . . . . . Alcohols from Alkenes: Oxymercuration–Demercuration . . . . . . . . . . Alcohols from Alkenes: Hydroboration–Oxidation . . Syn Addition . . . . . . Stereospecific Reactions . . Reaction 1 S S + (R,R)-isomer Stereospecific Reactions . . . . Reaction 2 S R = (R,S)-isomer (meso) . . Structure and Reactions of Methylene . . . . Oxidation of Alkenes: Syn-1,2-Dihydroxylation . . . . . . Oxidative Cleavage Alkenes: Ozonolysis . . . H Zn HOAc O H + + O Zn(OAc)2 O . . Synthetic Strategy for Multi-step Synthesis . In planning a synthesis we often have to consider four interrelated aspects: 1. 2. 3. 4. Construction of the carbon skeleton Functional group interconversions Control of regiochemistry Control of stereochemistry Retrosynthesis Synthesis Chapter 11. Alcohols and Ethers Ch 11.1–11.10: Structure and nomenclature Synthesis of alcohols and ethers Reactions of alcohols (as an acid, to alkyl halides with PBr3 and SOCl2, to sulfonates) Ch 11.11–11.12: Synthesis of Ethers–Williamson ether synthesis Alkoxymercuration–Demercuration Protection groups (tert-Butyl ether, Silyl ether) Ch 11.13–11.15: Epoxides (their synthesis and reactions) Anti-1,2-dihydroxylation of alkenes via epoxides Crown ethers Synthesis of Alcohols from Alkenes Acid-Catalyzed Hydration of alkenes Oxymercuration-Demercuration Hydroboration-oxidation Alkyl Halide from ROH: with HX, PBr3 and SOCl2 . . . . . . . . . . Synthesis of Ethers: The Williamson Synthesis . . . . . . . . Synthesis of Ethers Alkoxymercuration-Demercuration . . R H O . R R O H NaBH4 H Hg OAc O H H H H AcOHg H . . . Epoxides (Oxiranes) . . . . Anti 1,2-Dihydroxylation of Alkenes via Epoxides . ?? . OH H . OH H trans-1,2-Cyclopentadiol (R,R and S,S) . . . Chapter 12. Alcohols from Carbonyl Compounds Ch 12.1–12.4: Structure and reactivity of carbonyl group Oxidation–reduction reactions (oxidation of alcohols to carbonyl groups and their reduction to alcohols) Ch 12.5–12.8: Organometallic compounds Organolithium and Organomagnesium compounds Grignard reaction. Alcohols from Grignard reagents Ch 12.9: Organocopper reagents Lithium dialkylcuprate Alcohols by Reduction of Carbonyl Compounds . Lithium Aluminum Hydride (LiAlH4) Sodium Borohydride (NaBH4) . Oxidation of Primary Alcohols to Aldehydes . . PCC Jones reagent Oxidation H2CrO4 R R O OH HO Oxidation . . Organometallic Compounds: Grignard Reagent . . Victor Grignard Novel Prize (1912) . . Alcohols from Carbonyls and Grignard Reagents Grignard reagents react with aldehydes to give secondary alcohols Grignard reagents react with ketones to give tertiary alcohols Esters react with two molecules of Grignard reagents to form tert-alcohols Summary: Synthetic Connection between Alcohols and Carbonyls Synthetic Plans Based on Grignard Reaction . . Retrosynthetic Analysis Synthesis Chapter 13. Conjugated Unsaturated System Ch 13.1–15.4: Allylic substitution, Allyl radical, Allylic chlorination Allylic bromination, N-Bromosuccinimide MO of allyl radical and allyl cation Rules for writing resonance structures Ch 13.6–13.8: Polyunsaturated hydrocarbons, 1,3-Butadiene (electron delocalization, conformation, MO) Ch 13.10–13.11 Electrophilic attack on conjugated dienes Kinetic vs. thermodynamic control Diels-Alder reaction (factor favoring for D-A, Stereochemistry of D-A, MO consideration, Endo/Exo-transition state, Intramolecular D-A) Introduction to Conjugated Unsaturated Systems A B A . H . B A X Addition + A H Allylic Substitution . X Systems that have a p orbital on an atom adjacent to a double bond– with delocalized π bonds–are called conjugated unsaturated systems. This general phenomenon is called conjugation. . + Cycloaddition (Diels-Alder) Allylic Bromination with N-Bromosuccinimide . . . . . . The Stability of Allyl Radical: MO Description . . . . energy Node + + . Antibonding MO H H C C H . C H Three isolated p orbitals Node + H Allyl Radical Nonbonding MO + Bonding MO The Stability of Allyl Radical: Resonance Structures . . . . . . . . The Allyl Cation . e- . . . . . energy Node . Antibonding MO H H . C H C C H Three isolated p orbitals Node H Allyl Cation Nonbonding MO Bonding MO The Allyl Anion . + e- - e- . . . . . energy Node . Antibonding MO H H . C H C C H Three isolated p orbitals Node H Allyl Anion Nonbonding MO Bonding MO Molecular Orbital of 1,3-Butadiene . . Node energy Antibonding MO LUMO p orbital Node HOMO Bonding MO . . Kinetic vs. Thermodynamic Control of a Chemical Reaction . . . . . . . . . . . . . . The Diels–Alder Reaction: 1,4-Cycloaddition of Dienes . . . In 1928 Otto Diels and Kurt Alder developed a 1,4-cycloaddition reaction of dienes that has since come to bear their names. The reaction proved to be one of such great versatility and synthetic utility that Diels and Alder were awarded the Nobel Prize in Chemistry in 1950. . . . . . . . . . Factors Favoring the Diels–Alder Reaction . . . . Diels-Alder reaction is favored by the presence of electron-withdrawing groups in the dienophile and by electron-releasing groups in the diene. . . . . . . . . . . Stereochemistry of the Diels–Alder Reaction 1. The Diels-Alder reaction is highly stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in in product. . . . . . . . . . . . . Stereochemistry of the Diels–Alder Reaction 3. The Diels-Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled. . . . . . . . . . . . . Stereochemistry of the Diels–Alder Reaction 3. The Diels-Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled. . . . . LUMO LUMO . HOMO . H H HOMO Secondary Orbital Interaction . O Primary Orbital Interactions O Diene Dienophile O . . . . . . .