Chapter 7: Alkenes and Alkynes ydrocarbons Containing Double and Triple Bonds Unsaturated Compounds (Less than Maximum 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 C C Cl Cl C Cl C Cis or (Z) Trans or (E) Same Molecular Formula (C 2 Cl 2 2 ) 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 Trans Disubstituted > > Cis Disubstituted Monosubstituted Unsubstituted igher Alkyl Substitution = igher Alkene Stability Note Stability Trends of Disubstituted Alkenes Can Also Observe Cyclic Alkenes
Alkenes: Cyclic Structures C 2 C C C 2 C 2 C C 2 2 C C C 2 C C Cyclopropene Cyclobutene Cyclopentene C 2 C C 2 C C C C C 2 C 2 Cyclohexene Note all of These are Cis Alkenes Can Observe Trans Cycloalkenes; z.b. trans-cycloctene trans-cycloheptene Observable Spectroscopically; Can t Isolate C C C Cyclohexatriene (Benzene)
Alkenes: Synthesis via Elimination C 2 5 ONa Br O Br Dehydrohalogenation; E 2 Elimination Reaction E 2 Reactions Preferable Over E 1 (Rearrangement; SN 1 Products) Usually eat These Reactions (eat Favors Elimination)
Alkenes: Zaitsev s Rule C 2 C 3 C 2 5 ONa 3 C Br 3 C 31% C 3 3 C C 3 C 3 C 3 C 2 5 ONa Br 3 C 69% C 3 If Multiple Possible Products; Most Stable (Substituted) Forms More Substituted: Product and Transition State Lower in Energy
Alkenes: Forming the Least Substituted C 2 C 3 OK 3 C Br 3 C 72.5% C 3 C 3 C 3 OK C3 3 C Br 3 C Bulky Base Favors Least Substituted Product 27.5% C 3 Due to Steric Crowding in Transition State (2 ydrogens)
Alkenes: The Transition State in E 2 O Orientation Allows Proper Orbital Overlap in New π Bond Syn Coplanar Transition State only in Certain Rigid Systems Anti: Staggered; Syn: Eclipsed Anti TS is Favored Br Anti Coplanar Conformation (ydrogen and Leaving Group)
Alkenes: E 2 Reactions of Cyclohexanes EtO Cl Anti Transition State Attainable w/ Axial and Leaving Group Axial/Equatorial and Equatorial/Equatorial Improper Combos Let s Look at igher Substituted Cyclohexanes
Alkenes: E 2 Reactions of Cyclohexanes EtO EtO Me Me i Pr + Cl i Pr i Pr Multiple s Axial to Leaving Group Multiple Products Zaitsev s Rule Governs Product Formation 22% 78% (Zaitsev's Rule) What if NO Anti Coplanar Arrangement in Stable Conformer??
Alkenes: E 2 Reactions of Cyclohexanes Me i Pr Cl Me Cl Me i Pr i Pr EtO All Groups Equatorial in Most Stable Conformation 100% Chair Flip Form has Proper Alignment Reaction Proceeds Through igh Energy Conformation Only ONE Possible Elimination Product In This Case
Alkenes: Acid Catalyzed Dehydration O concd 2SO 4 180 o C + 2 O O 85% 3 PO 4 165-170 o C + 2 O ave to Pound 1 Alcohols to Dehydrate w/ Acid 2 Alcohols Easier, Can Use Milder Conditions
Alkenes: Acid Catalyzed Dehydration C 3 C 2 3 C C 2 O 20% 2 SO 4 85 o C 3 C C 3 + 2 O 3 Alcohols Exceptionally Easy to Dehydrate Can Use Dilute Acid, Lower Temperatures Relative Ease of Reaction: 3 > 2 > 1
Alkenes: Acid Catalyzed Dehydration C 3 C 3 3 C O + 3 C O 2 C 2 C 2-2 O 3 C C 2 C 3 + 2 O - + Base C 2 C 3 E 1 Elimination Reaction Mechanism (Explains Ease) C 3
Alkenes: Acid Catalyzed Dehydration 3 Alcohols Easiest to Dehydrate via E 1 ; 1 ardest Recall Carbocation Stablility: 3 > 2 > 1 Relative Transition State Stability Related to Carbocation Why Are More Substituted Carbocations More Stable?? YPERCONJUGATION (Donating Power of Alkyls) 1 Carbocation Instablility Dehydration of These is E 2
Alkenes: 1 Alcohol Dehydration (E 2 ) C 3 C 3 3 C O A 3 C O 2 A Step One Fast Step Two Slow (RDS) 3 C + 2 O + -A 3 C Proceeds via E 2 Due to Primary Carbocation Instability Sulfuric and Phosphoric Acids Are Commonly Used Acids
Carbocation Rearrangements 3 C C 3 C 3 85% 3 PO 4 eat 3 C C 3 + C 3 C 3 O 3 C C 3 C(C 3 ) 2 Major Minor C 3 C 3 3 C C 3 3 C C 3 C 3 O 2 C 3 A Priori One Expects the Minor Dehydration Product This Dehydration Product is NOT Observed Major Product
Carbocation Rearrangements (2) C 3 C 3 Methanide 3 C C 3 3 C Migration C 3 C 3 C 3 Secondary Carbocation C 3 Tertiary Carbocation 3 C C 3 Methanide Migration Results in More Stable 3 Carbocation This Carbocation Gives Rise to Observed Major Product Can Also Observe YDRIDE ( - ) Shifts More Stable C + C Transition State
Alkyne Synthesis: Dehydrohalogenation 2 eq. NaN 2 R R R R Br Br Compounds w/ alogens on Adjacent Carbons: VICINAL Dihalides (Above Cmpd: Vicinal Dibromide) Entails Consecutive E 2 Elimination Reactions NaN 2 Strong Enough to Effect Both Eliminations in 1 Pot Need 3 Equivalents NaN 2 for Terminal Alkynes
Reactions: Alkylation of Terminal Alkynes 3 C NaN 2 N 3 3 C C 3 Br 3 C C 3 3 C NaN 2 N 3 3 C EtBr 3 C Et NaN 2 ( - N 2 ) to Deprotonate Alkyne (Acid/Base Reaction) Anion Reacts with Alkyl alide (Bromide); Displaces alide Alkyl Group Added to Alkyne Alkyl alide Must be 1 or Me; No Branching at 2 nd (β) Carbon
Reactions: Alkylation of Terminal Alkynes SN 2 Substitution Reactions on 1 alides E 2 Eliminations Occur on Reactions w/ 2, 3 alides Steric Problem; Proton More Accessible than Electrophilic Carbon Atom 3 C C 3 C C Br 3 C 3 C + 3 C C 3
Alkenes: ydrogenation Reactions 2 Pt, Pd, or Ni (catalyst) Solvent, Pressure Alkene Alkane Catalytic ydrogenation is a SYN Addition of 2 SYN Addition: Both Atoms Add to Same Side (Face) of π Bond Catalyst: Lowers Transition State Energy (Activation Energy)
Alkynes: ydrogenation Reactions Alkyne 2 2 Pt (catalyst) Solvent, Pressure Alkane Platinum Catalysts Allow Double Addition of 2 On Alkyne Can Also ydrogenate Once to Generate Alkenes Cis and Trans (E and Z) Stereoisomers are Possible Can Control Stereochemistry with Catalyst Selection
Alkynes: ydrogenation to Alkenes 2 /Ni 2 B 97% R R R R 2, Pd/CaCO 3 Quinoline SYN Additions to Alkynes (Result in cis/z Alkenes) Reaction Takes Place on Surface of Catalyst Examples of a ETEROGENEOUS Catalyst System
Alkynes: ydrogenation to Alkenes (1) Li, C 2 5 N 2 (2) N 4 Cl Dissolving Metal Reduction Reaction ANTI Addition of 2 to Alkyne E (trans) Stereoisomer Ethylamine or Ammonia can be used for Reaction
More On Unsaturation Numbers Unsaturation Number (r + π) Index of Rings and Multiple Bonds r + π = C - ½ + ½ N - ½ alogen + 1 Useful When Generating Structures from Molecular Formula Also Called Degree of ydrogen 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 ydrogenation Reactions (Alkynes to E/Z Alkenes) Unsaturation Numbers; Utility in Structure Determination