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 Same Molecular Formula (C 2 Cl 2 H 2 ) and Connectivity Different Structures  Double Bonds Don’t Rotate For Tri/Tetra Substituted Alkenes; Use (E), (Z) Labels

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

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

Alkenes: Synthesis via Elimination Dehydrohalogenation; E 2 Elimination Reaction E 2 Reactions Preferable Over E 1 (Rearrangement; SN 1 Products) Usually Heat These Reactions (Heat Favors Elimination)

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

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

Alkenes: The Transition State in E 2 Orientation Allows Proper Orbital Overlap in New  Bond Syn Coplanar Transition State only in Certain Rigid Systems Anti: Staggered; Syn: Eclipsed  Anti TS is Favored

Alkenes: E 2 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

Alkenes: E 2 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??

Alkenes: E 2 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

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

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

Alkenes: Acid Catalyzed Dehydration E 1 Elimination Reaction Mechanism (Explains Ease)

Alkenes: Acid Catalyzed Dehydration 3° Alcohols Easiest to Dehydrate via E 1 ; 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 E 2

Alkenes: 1° Alcohol Dehydration (E 2 ) Step One Fast Step Two Slow (RDS) Proceeds via E 2 Due to Primary Carbocation Instability Sulfuric and Phosphoric Acids Are Commonly Used Acids

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

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 +

Alkyne Synthesis: Dehydrohalogenation Compounds w/ Halogens on Adjacent Carbons:  VICINAL Dihalides (Above Cmpd: Vicinal Dibromide) Entails Consecutive E 2 Elimination Reactions NaNH 2 Strong Enough to Effect Both Eliminations in 1 Pot Need 3 Equivalents NaNH 2 for Terminal Alkynes

Reactions: Alkylation of Terminal Alkynes NaNH 2 ( - NH 2 ) 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 2 nd (  ) Carbon

Reactions: Alkylation of Terminal Alkynes SN 2 Substitution Reactions on 1° Halides E 2 Eliminations Occur on Reactions w/ 2°, 3° Halides Steric Problem; Proton More Accessible than Electrophilic Carbon Atom

Alkenes: Hydrogenation Reactions Catalytic Hydrogenation is a SYN Addition of H 2 SYN Addition: Both Atoms Add to Same Side (Face) of  Bond Catalyst: Lowers Transition State Energy (Activation Energy)

Alkynes: Hydrogenation Reactions Platinum Catalysts Allow Double Addition of H 2 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 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 Dissolving Metal Reduction Reaction ANTI Addition of H 2 to Alkyne  E (trans) Stereoisomer Ethylamine or Ammonia can be used for Reaction

More On Unsaturation Numbers Unsaturation Number (r +  ) Index of Rings and Multiple Bonds r +  = 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