Chapter 7: Alkenes and Alkynes

1. 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

2. Isomerism: Cis/Trans • Same Molecular Formula (C2Cl2H2) and Connectivity • Different Structures  Double Bonds Don’t Rotate • For Tri/Tetra Substituted Alkenes; Use (E), (Z) Labels

3. Alkenes: Relative Stability • Higher Alkyl Substitution = Higher Alkene Stability • Note Stability Trends of Disubstituted Alkenes • Can Also Observe Cyclic Alkenes

4. Alkenes: Cyclic Structures • Note all of These are Cis Alkenes • Can Observe Trans Cycloalkenes; z.b. trans-Cycloctene • trans-Cycloheptene Observable Spectroscopically; Can’t Isolate

5. Alkenes: Synthesis via Elimination • Dehydrohalogenation; E2 Elimination Reaction • E2 Reactions Preferable Over E1 (Rearrangement; SN1 Products) • Usually Heat These Reactions (Heat Favors Elimination)

6. Alkenes: Zaitsev’s Rule • If Multiple Possible Products; Most Stable (Substituted) Forms • More Substituted: Product and Transition State Lower in Energy

7. Alkenes: Forming the Least Substituted • Bulky Base Favors Least Substituted Product • Due to Steric Crowding in Transition State (2° Hydrogens)

8. Alkenes: The Transition State in E2 • 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

9. Alkenes: E2 Reactions of Cyclohexanes • Anti Transition State Attainable w/ Axial H and Leaving Group • Axial/Equatorial and Equatorial/Equatorial Improper Combos • Let’s Look at Higher Substituted Cyclohexanes

10. Alkenes: E2 Reactions of Cyclohexanes • Multiple H’s Axial to Leaving Group  Multiple Products • Zaitsev’s Rule Governs Product Formation • What if NO Anti Coplanar Arrangement in Stable Conformer??

11. Alkenes: E2 Reactions of Cyclohexanes • 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

12. Alkenes: Acid Catalyzed Dehydration • Have to Pound 1° Alcohols to Dehydrate w/ Acid • 2° Alcohols Easier, Can Use Milder Conditions

13. Alkenes: Acid Catalyzed Dehydration • 3° Alcohols Exceptionally Easy to Dehydrate • Can Use Dilute Acid, Lower Temperatures • Relative Ease of Reaction: • 3° > 2° > 1°

14. Alkenes: Acid Catalyzed Dehydration • E1 Elimination Reaction Mechanism (Explains Ease)

15. 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

16. Alkenes: 1° Alcohol Dehydration (E2) • StepOne Fast • Step Two Slow (RDS) • Proceeds via E2 Due to Primary Carbocation Instability • Sulfuric and Phosphoric Acids Are Commonly Used Acids

17. Carbocation Rearrangements • A Priori One Expects the Minor Dehydration Product • This Dehydration Product is NOT Observed Major Product

18. Carbocation Rearrangements (2) • 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+

19. Alkyne Synthesis: Dehydrohalogenation • 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

20. Reactions: Alkylation of Terminal Alkynes • 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

21. 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

22. Alkenes: Hydrogenation Reactions • 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)

23. Alkynes: Hydrogenation Reactions • 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

24. Alkynes: Hydrogenation to Alkenes • SYN Additions to Alkynes (Result in cis/Z Alkenes) • Reaction Takes Place on Surface of Catalyst • Examples of a HETEROGENEOUS Catalyst System

25. Alkynes: Hydrogenation to Alkenes • Dissolving Metal Reduction Reaction • ANTI Addition of H2 to Alkyne  E (trans) Stereoisomer • Ethylamine or Ammonia can be used for Reaction

26. 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

27. 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