Transcript Document

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
H3C
Br
H3C
CH3
31%
H
CH3
CH3
H
CH3
H3C
CH3
C2H5ONa
H
H3C
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
H3C
Br
H3C
CH3
72.5%
H
CH3
CH3
OK
H
CH3
H3C
CH3
H
H3C
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
+
H2O
+
H2O
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
H3C
+
CH2
OH2
CH2
H
H
-H2O
Base
CH3
CH2
+
H3C
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
H3C
CH3
85% H3PO4
H3C
CH3
H3C
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
H3C
H
H3C
H
NaNH2
NH3
NaNH2
NH3
H3C
CH3Br
EtBr
H3C
H3C
CH3
H3C
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
H3C
CH3
H
C
H
H3C
H
H3C
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