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Chapter 7: Alkenes and Alkynes • Hydrocarbons Containing Double and Triple Bonds • Unsaturated Compounds (Less than Maximum H Atoms) • Alkenes also Referred to as Olefins • Properties Similar to those of Corresponding Alkanes • Slightly Soluble in Water • Dissolve Readily in Nonpolar or Low Polarity Solvents • Densities of Alkenes and Alkynes Less than Water Isomerism: Cis/Trans Cl Cl H H Cl C C C C H Cis or (Z) Cl H Trans or (E) • Same Molecular Formula (C2Cl2H2) and Connectivity • Different Structures Double Bonds Don’t Rotate • For Tri/Tetra Substituted Alkenes; Use (E), (Z) Labels Alkenes: Relative Stability Tetrasubstituted > > > Trisubstituted Geminal Disubstituted > Cis Disubstituted Trans Disubstituted > Monosubstituted Unsubstituted • Higher Alkyl Substitution = Higher Alkene Stability • Note Stability Trends of Disubstituted Alkenes • Can Also Observe Cyclic Alkenes Alkenes: Cyclic Structures HC HC CH2 HC CH2 H2 C H2C CH2 HC Cyclopropene HC Cyclobutene H2 C CH2 CH Cyclopentene H C HC CH2 HC CH HC CH2 HC CH C H2 C H Cyclohexene Cyclohexatriene (Benzene) • Note all of These are Cis Alkenes • Can Observe Trans Cycloalkenes; z.b. trans-Cycloctene • trans-Cycloheptene Observable Spectroscopically; Can’t Isolate Alkenes: Synthesis via Elimination H H H H H H H C2H5ONa H H Br H O H H H H H H H H Br • Dehydrohalogenation; E2 Elimination Reaction • E2 Reactions Preferable Over E1 (Rearrangement; SN1 Products) • Usually Heat These Reactions (Heat Favors Elimination) Alkenes: Zaitsev’s Rule H H CH2 CH3 H C2H5ONa H H3 C Br H3 C CH3 31% H CH3 CH3 H CH3 H3 C CH3 C2H5ONa H H3 C Br 69% • If Multiple Possible Products; Most Stable (Substituted) Forms • More Substituted: Product and Transition State Lower in Energy Alkenes: Forming the Least Substituted H H CH2 CH3 OK H H H3 C Br H3 C CH3 72.5% H CH3 CH3 OK H CH3 H3 C CH3 H H3 C Br 27.5% • Bulky Base Favors Least Substituted Product • Due to Steric Crowding in Transition State (2° Hydrogens) Alkenes: The Transition State in E2 H O H H H H Br Anti Coplanar Conformation (Hydrogen and Leaving Group) • Orientation Allows Proper Orbital Overlap in New p Bond • Syn Coplanar Transition State only in Certain Rigid Systems • Anti: Staggered; Syn: Eclipsed Anti TS is Favored Alkenes: E2 Reactions of Cyclohexanes EtO H Cl • Anti Transition State Attainable w/ Axial H and Leaving Group • Axial/Equatorial and Equatorial/Equatorial Improper Combos • Let’s Look at Higher Substituted Cyclohexanes Alkenes: E2 Reactions of Cyclohexanes EtO Me EtO Me H H i + Pr Cl i i Pr 22% Pr 78% (Zaitsev's Rule) • Multiple H’s Axial to Leaving Group Multiple Products • Zaitsev’s Rule Governs Product Formation • What if NO Anti Coplanar Arrangement in Stable Conformer?? Alkenes: E2 Reactions of Cyclohexanes Me Me Cl i Me Cl Pr H i Pr i EtO • All Groups Equatorial in Most Stable Conformation • Chair Flip Form has Proper Alignment • Reaction Proceeds Through High Energy Conformation • Only ONE Possible Elimination Product In This Case Pr 100% Alkenes: Acid Catalyzed Dehydration H H H H H H concd H2SO4 o 180 C H H OH + H2 O + H2 O H OH 85% H3PO4 165-170 oC H • Have to Pound 1° Alcohols to Dehydrate w/ Acid • 2° Alcohols Easier, Can Use Milder Conditions Alkenes: Acid Catalyzed Dehydration CH3 H3C CH2 OH 20% H2SO4 + o 85 C CH2 H3C CH3 H • 3° Alcohols Exceptionally Easy to Dehydrate • Can Use Dilute Acid, Lower Temperatures • Relative Ease of Reaction: 3° > 2° > 1° H2O Alkenes: Acid Catalyzed Dehydration CH3 H3 C CH3 OH H H 3C + CH2 OH2 CH2 H H -H2O Base CH3 CH2 + H 3C CH3 H2O -H+ H C H2 CH3 • E1 Elimination Reaction Mechanism (Explains Ease) Alkenes: Acid Catalyzed Dehydration • 3° Alcohols Easiest to Dehydrate via E1; 1° Hardest • Recall Carbocation Stablility: 3° > 2° > 1° • Relative Transition State Stability Related to Carbocation • Why Are More Substituted Carbocations More Stable?? HYPERCONJUGATION (Donating Power of Alkyls) • 1° Carbocation Instablility Dehydration of These is E2 Alkenes: 1° Alcohol Dehydration (E2) CH3 H CH3 H H3C OH H H H3C A OH2 H H H A • Step One Fast • Step Two Slow (RDS) H3C H + H3C H2O + H • Proceeds via E2 Due to Primary Carbocation Instability • Sulfuric and Phosphoric Acids Are Commonly Used Acids H-A Carbocation Rearrangements CH3 H H3 C CH3 85% H3PO4 H3 C CH3 H3 C CH3 Major CH3 CH3 OH2 H CH(CH3)2 Minor CH3 H CH3 H H3C CH3 + Heat CH3 OH H H3C CH3 CH3 • A Priori One Expects the Minor Dehydration Product • This Dehydration Product is NOT Observed Major Product Carbocation Rearrangements (2) CH3 H CH3 H H3C CH3 Methanide Migration H3C CH3 CH3 CH3 Secondary Carbocation H3C Tertiary Carbocation CH3 H CH3 C H Transition State • Methanide Migration Results in More Stable 3° Carbocation • This Carbocation Gives Rise to Observed Major Product • Can Also Observe HYDRIDE (H-) Shifts More Stable C+ Alkyne Synthesis: Dehydrohalogenation H H R R Br 2 eq. NaNH2 R R Br • Compounds w/ Halogens on Adjacent Carbons: VICINAL Dihalides (Above Cmpd: Vicinal Dibromide) • Entails Consecutive E2 Elimination Reactions • NaNH2 Strong Enough to Effect Both Eliminations in 1 Pot • Need 3 Equivalents NaNH2 for Terminal Alkynes Reactions: Alkylation of Terminal Alkynes H 3C H3 C H H NaNH2 NH3 NaNH2 NH3 H3C CH3Br EtBr H 3C H3 C CH3 H3 C Et • NaNH2 (-NH2) to Deprotonate Alkyne (Acid/Base Reaction) • Anion Reacts with Alkyl Halide (Bromide); Displaces Halide • Alkyl Group Added to Alkyne • Alkyl Halide Must be 1° or Me; No Branching at 2nd (b) Carbon Reactions: Alkylation of Terminal Alkynes • SN2 Substitution Reactions on 1° Halides • E2 Eliminations Occur on Reactions w/ 2°, 3° Halides • Steric Problem; Proton More Accessible than Electrophilic Carbon Atom H3 C CH3 H C H H 3C H H 3C C H3C H + Br CH3 Alkenes: Hydrogenation Reactions H2 Pt, Pd, or Ni (catalyst) Solvent, Pressure Alkene Alkane • Catalytic Hydrogenation is a SYN Addition of H2 • SYN Addition: Both Atoms Add to Same Side (Face) of p Bond • Catalyst: Lowers Transition State Energy (Activation Energy) Alkynes: Hydrogenation Reactions 2H2 Pt (catalyst) Solvent, Pressure Alkyne Alkane • Platinum Catalysts Allow Double Addition of H2 On Alkyne • Can Also Hydrogenate Once to Generate Alkenes • Cis and Trans (E and Z) Stereoisomers are Possible • Can Control Stereochemistry with Catalyst Selection Alkynes: Hydrogenation to Alkenes H2/Ni2B H H 97% R R R R H H H2, Pd/CaCO3 Quinoline • SYN Additions to Alkynes (Result in cis/Z Alkenes) • Reaction Takes Place on Surface of Catalyst • Examples of a HETEROGENEOUS Catalyst System Alkynes: Hydrogenation to Alkenes H (1) Li, C2H5NH2 (2) NH4Cl H • Dissolving Metal Reduction Reaction • ANTI Addition of H2 to Alkyne E (trans) Stereoisomer • Ethylamine or Ammonia can be used for Reaction More On Unsaturation Numbers • Unsaturation Number (r + p) Index of Rings and Multiple Bonds • r + p = C - ½ H + ½ N - ½ Halogen + 1 • Useful When Generating Structures from Molecular Formula • Also Called Degree of Hydrogen Deficiency; Number of Double Bond Equivalencies • Often Combined with Spectroscopic Data when Making Unknown Structure Determinations Chapter 7: Key Concepts • E2 Eliminations w/ Large and Small Bases • E1 Elimination Reactions • Zaitsev’s Rule • Carbocation Rearrangement • Dehydration and Dehydrohalogenation Reactions • Synthesis of Alkynes • Hydrogenation Reactions (Alkynes to E/Z Alkenes) • Unsaturation Numbers; Utility in Structure Determination