1. Chapter 13: Alkynes: The C C Triple Bond
Naming
Like alkenes, but ending -ene turns into –yne. 4 6 1 3 5 4 2 2
HC CH 5 1 Br 3
Ethyne
1-Pentyne
5-Bromo-2-hexyne 1
Priority: 2
2-Propyn-1-ol
-ol -yne
> 3 OH

2. When the alkyne contains also double bonds, it is called an enyne
When the alkyne contains also double bonds, it is called an enyne. However, despite being an “yne”, numbering begins closest to either group: 2 1 3 4 3 2 6 1 5 4 5
3-Hexen-1-yne 6
1-Hepten-4-yne 7
When double and triple bond are equidistant from each terminus: ene first (alphabetical) 2 1 3
1-Penten-4-yne 4 5

3. 2-Propynyl (or propargyl) 2-Propenyl (or allyl) Ethynyl
Substituents: 1 1 2 2
2-Propynyl (or propargyl)
Remember:
2-Propenyl (or allyl)
Ethynyl
Ethynylcyclohexane but
3-Cyclobutyl-1-hexyne
Smaller R is a substituent to larger R

4. Structure
Two perpendicular π bonds; sp hybrids
R C C R
Ethyne

5. The Triple Bond Is Energetic
Acetylene torch: ~3,500 C
Heat of hydrogenation: more than two alkene  bonds
(which would be ~ -60 kcal mol-1)
Internal triple bond is more stable

6. Alkynes Are Relatively Acidic- -
RC C H B R C C HB : :
pKa ~ 25!
Why? 50% s-character
Hydrogens get more acidic (blue)

7. Synthetic Use of Acidity+ Li - + + Li Li
pKa 25
pKa 50 +
CH3MgBr
MgBr +
CH4 - - + - - + + :
NaNH2
H H + Na NH2 : H : Na
C C : : Na
1 equiv +
NH3
pKa 33

8. Preparation Of Alkynes
Elimination E2 of dihaloalkanes X X - H - B: B: C C C C C C X H H
H+, H2O work up
NaNH2 excess Br
NH3 liq. Br Na
75%

9. Application in synthesis:
RCH CHR R C C R Br Br
Br2 Br Br
NaNH2
NH3 liq
1,5-Hexadiyne

10. 2. Alkylation of Alkynyl Metals SN2 rules applyLi Li I
THF ∆
90%
Best: RI, THF, ∆. RBr or RCl need “coordinating” additives: e.g ; or HMPA solvent. Remember: Grignards don’t work for coupling with RX (but O.K. for or carbonyls).
NH2
H2N O

11. Li + MgBr + CH3MgBr OH O + LiNH2 (1 equiv) OH HO OH + 2 CH3MgBr BrMg
CH2 O
+ CH3MgBr OH O
+ LiNH2 (1 equiv) Li OH O HO OH
CH3CH
+ 2 CH3MgBr
BrMg
MgBr O Li OH
CH3 + Li

12. Reactions Of Alkynes 1. Reductions a. Complete hydrogenation H2, Pt
100%
b. Partial hydrogenation with “poisoned” Lindlar’s catalyst: syn addition gives cis product: O
Lindlar’s catalyst:
Pd-CaCO3, Pb(OCCH3)2, quinoline N
H2, Lindlar H H
cis-3-Heptene
100%
c. Na reduction: anti addition gives trans product:
NH3 liq. H
+ Na
trans-3-Heptene
86% H
Metal
Via stepwise 2e transfer

13. Mechanism Of The Na Reduction Of Alkynes
9/12/2018
Mechanism Of The Na Reduction Of Alkynes
Na dissolves in liquid ammonia, makes “solvated” electrons
1:55 Billy Holiday Swing Brother Swing
Equilibrates between
cis and trans (more stable)
Holiday
Lip
© Univesity of California

14. 2. Electrophilic additions
a. HX:
linear
Internal alkynes + - R X
H+ + R R C C R
Anti to H; pushes R trans H
sp 2 sp
Resonance with X R X X - + X H+ C C
RCH2 C
RCH2CX2R
Markovnikov H R R
Geminal dihalide
Terminal alkynes X H HX HX RC CH C C
RCX2CH3 R H
Markovnikov twice

15. + Examples: Synthetic application: Vicinal Geminal Br Br HBr I I HI ICl Cl
HCl
Synthetic application:
Br2
NaNH2
HBr Br Br Br
NH3 Br
Vicinal
Geminal

16. b. X2: Anti addition, as for alkenesBr
CH3
Br2
CH3 Br Br Br
Br2 Br Br
c. Cat. HgSO4, H2O hydration, Markovnikov H O R
Cat. HgSO4 H
Tautomerization RC CR
RC‒CR C C
H2O -
H+ or OH catalyzed R OH
No NaBH4 needed H
Unstable

17. Mechanism Of Tautomerization- C C
H+ or OH catalyzed R OH O R R
-H+ +
H+ :
RCH2CR
RCH2 C
RCH2 C H + OH O O H R H - R -
+H+
OH : C C C C
RCH2CR - R O R O

18. + d. Radical HBr: Anti-Markovnikov addition O RC CH C CH2 RCCH3
Examples: O
cat.
HgSO4 HO RC CH C
CH2
RCCH3
H2O R
A methyl ketone
d. Radical HBr: Anti-Markovnikov addition Br
HBr
ROOR Br Br H Br
HBr
-Br +
Mixtures H

19. An anti-Markovnikov hydration
e. Hydroboration-Oxidation
Use R2BH (R = bulky group: cyclohexyl) to ensure single hydroboration R H
H2O2, -OH + RC CH
B‒H C C 2 H
BR2
Steric control O R H
Tautomerization C C
RCH2CH H OH
Aldehyde !
An anti-Markovnikov hydration

20. Therefore: H O O But: O O And: R R’ RCCH2R’ + RCH2CR’ Mixtures
1. HBR H2O2,-OH H O
cat.
HgSO4, H2O O
But:
1. R2BH H2O2,-OH O O
And: R R’
RCCH2R’ +
RCH2CR’
Mixtures

21. But organolithium and Grignards reagents:
Haloalkenes
No SN2:
No SN1:
But organolithium and Grignards reagents:

22. Richard F. Heck (b. 1931) Nobel Prize 2010
The Heck Reaction
Alkenyl halides can be coupled with alkenes to give 1,3-dienes
Examples:
Richard F. Heck (b. 1931) Nobel Prize 2010

23. Mechanism Of The Heck Reaction
Pd catalyst is typically Pd(OCCH3)2 O
Additional ligands around Pd are omitted