alkanes 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
C
C
Cl H
Cl H
C
C
H Cl
Cl H
Cis or (Z) 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

3. Alkenes: Relative Stability
Tetrasubstituted Trisubstituted Geminal Disubstituted Trans Disubstituted
Cis Disubstituted Monosubstituted Unsubstituted
> > >
> >
• 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
HC CH
CH2
H2
C
H2C
HC
HC
C
H
CH
CH
H
C
HC
HC CH2
CH2
HC
HC
CH2
HC
HC
C
H2
CH2
CH2
H2
C
Cyclopropene Cyclobutene Cyclopentene
Cyclohexene Cyclohexatriene (Benzene)

5. Alkenes: Synthesis via Elimination
• Dehydrohalogenation; E2 Elimination Reaction
• E2 Reactions Preferable Over E1 (Rearrangement; SN1 Products)
• Usually Heat These Reactions (Heat Favors Elimination)
H
Br
H
H
H
H
C2H5ONa
H
H
H
H
H
Br
H
H
H
H
O
H
H
H
H

6. Alkenes: Zaitsev’s Rule
• If Multiple Possible Products; Most Stable (Substituted) Forms
• More Substituted: Product and Transition State Lower in Energy
H
Br
CH3
H3C
H
CH2
C2H5ONa
CH3
H3C
H
H
H
Br
CH3
H3C
H
CH3
C2H5ONa
CH3
CH3
H3C
H
31%
69%

7. Alkenes: Forming the Least Substituted
• Bulky Base Favors Least Substituted Product
• Due to Steric Crowding in Transition State (2° Hydrogens)
H
Br
CH3
H3C
H
CH2
CH3
H3C
H
H
H
Br
CH3
H3C
H
CH3 CH3
CH3
H3C
H
72.5%
27.5%
OK
OK

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
H
Br
H
H
H
H
O
Anti Coplanar Conformation
(Hydrogen and Leaving Group)

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
Cl
H
EtO

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??
Cl
H
i
Pr
H
+
i
Pr i
Pr
Me Me
22% 78%
(Zaitsev's Rule)
EtO
EtO

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
Me
i
Pr
i
Pr
Me
100%
Cl
Cl
i
Pr
Me
H
EtO

12. Alkenes: Acid Catalyzed Dehydration
• Have to Pound 1° Alcohols to Dehydrate w/ Acid
• 2° Alcohols Easier, Can Use Milder Conditions
H
H
H
H
OH
H
concd H2SO4
180 o
C
H
H H
H
+ H2O
OH
H
85% H3PO4
165-170 o
C
+ H2O

13. Alkenes: Acid Catalyzed Dehydration
• 3° Alcohols Exceptionally Easy to Dehydrate
• Can Use Dilute Acid, Lower Temperatures
• Relative Ease of Reaction:
3° > 2° > 1°
20% H2SO4
85 o
C
+ H2O
H3C OH
CH3
CH2
H
CH2
CH3
H3C

14. Alkenes: Acid Catalyzed Dehydration
• E1 Elimination Reaction Mechanism (Explains Ease)
H3C OH
CH3
CH2
H
H+
H3C OH2
CH3
CH2
H
CH3
CH3
C
H2
H
+ H2O
CH2
CH3
H3C
-H+
-H2O
Base

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)
H3C
CH3
H H
H
OH H A H3C
CH3
H H
H
OH2
A
H3C
H3C H
H
+ H2O + H-A
• Step One Fast
• Step Two Slow (RDS)
• Proceeds via E2 Due to Primary Carbocation Instability
• Sulfuric and Phosphoric Acids Are Commonly Used Acids

17. Carbocation Rearrangements
H3C
CH3
CH3
H
OH
CH3
85% H3PO4
Heat
CH3
CH3
H3C
H3C
H
H CH3
CH(CH3)2
+
Major Minor
H3C
CH3
CH3
H
OH2
CH3 H3C
CH3
CH3
H
CH3
• 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+
H3C
CH3
CH3
H
CH3
Secondary Carbocation
H3C
CH3 H
CH3
CH3
Tertiary Carbocation
Methanide
Migration
H3C
CH3 H
CH3
C
H
Transition State

19. Alkyne Synthesis: Dehydrohalogenation
H
R
Br
Br
H
R R R
2 eq. NaNH2
• 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
H3C H
NaNH2
NH3
H3C
CH3Br
H3C CH3
H3C H
NaNH2
NH3
H3C
EtBr
H3C Et

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
H3C
H C
C
H3C
Br
H
CH3
H
H3C H
+
H3C
CH3

22. 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)

23. Alkynes: Hydrogenation Reactions
2H2
Pt (catalyst)
Solvent, Pressure
Alkane
Alkyne
• 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
H H
H2/Ni2B
97%
R R
H
R
R
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

25. Alkynes: Hydrogenation to Alkenes
(1) Li, C2H5NH2
(2) NH4Cl
H
H
• 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