1. Like alkenes, but ending -ene turns into –yne.
HC CH
Ethyne
5
4
3
2
1
1-Pentyne
Br
1
2
3
4
5
6
5-Bromo-2-hexyne
-ol -yne> OH
1
2
3 2-Propyn-1-ol
Alkynes: The C C Triple
Bond
Names
Priority:
2. When the alkyne contains also double bonds, it
is called an enyne. However, despite being an
“yne”, numbering begins closest to either group:
1
2
3 4
5
6
3-Hexen-1-yne
1-Penten-4-yne
1
2
3
4
5
When double and triple bond are equidistant from
each terminus: Ene first (alphabetical)
1
2
3
4
5
6
7
1-Hepten-4-yne
6. The Triple Bond is
Energetic
Kcal mol-1
Heat of hydrogenation: More than Two Alkene Bonds
(which would be ~ -60 kcal mol-1)
DH ° HC CH H2C CH2 H3C CH3
229 173 90
8. Hydrogens get more acidic (blue)
Acidity: RC C H + B R C C + HB
- -
: :
pKa ~ 25! cf. CH2 CH2 44, CH3 CH3 50
Why? 50% s-character
K
9. + Li or
Li Li
+- +
+ CH3MgBr MgBr + CH4
pKa 25 pKa 50
H H + Na NH2
+ :
:
H
Na C C::
--
NH3+
NaNH2
Na
+ +
pKa 33
1 equiv.
:-
Synthetic Use of Acidity
-
10. 1H NMR: RC C H δ = 1.7-3.1 !
Recall: RCH CH2 δ = 4.6-6 ppm
12. Long range: RCH2 C C H J = 2-4 Hz
RCH2 C C CH2 R’ J = 2-3 Hz
Coupling:
13.
14. 13C NMR: δ = 65-85 ppm: Also shielded.
Compare alkenes:
120-150ppm.
HC CCH2CH2CH2CH3
14-3169 84
IR spectra: diagnostic peaks for triple bond
and its attached H.
υ (RC CR’) = 2120 cm-1 ; υ (RC C H) = 3300 cm-1~
strong
~
15.
16. Stability of Alkynes:
Heats of Hydrogenation Revisited
CHHC + H2
Special cat.
CH2 CH2
ΔH ° = -44.9
kcal mol-1
CH2 CH2 + H2
Cat.
CH3 CH3
ΔH ° = -32.7
kcal mol-1
First Π bond has more “heat content”, is
also more reactive. Allows for:
R1 C C R2 + A B
A
C C
R2
R1 B
A
C C
B
R1 R2
+
Stereoselective alkene synthesis
17. Internal alkynes are more stable than
terminal ones
+ H2
cat.
+ H2
cat.
ΔH ° = -69.9 kcal mol-1
ΔH ° = -65.1 kcal mol-1
Parallels the behavior of alkenes.
Same reason: hyperconjugation.
18. Preparation
1. Elimination E2 of Dihaloalkanes
C C
H H
X X
C C C C
Br
Br Na
B:- H
X
B:-
NaNH2
excess
NH3 liq.
H+, H2O
work up
75%
20. 2. Alkylation of Alkynyl Metals
SN2 rules
Li
THF
Li
I
∆
90%
Best: RI, THF, ∆. RBr or RCl need
“coordinating” additives: e.g. ;
or HMPA. Remember: Grignards don’t work
for RX, but O.K. for or carbonyls.
H2N NH2
O
22. Reactions
1. Reductions a. Complete hydrogenation
H2, Pt
100%
b. Partial hydrogenation: “Poisoned” Lindlar’s
catalyst: Cis!
Pd-CaCO3, Pb(OCCH3)2, quinoline
O
N
H2, Lindlar
100%
HH
Cis-3-heptene
c. Na reduction: Trans! Via stepwise 2e transfer
+ Na°
NH3 liq.
86%
H
H
Trans-3-
heptene
23. Equilibrates between
cis and trans (more stable) Lipshutz
Mechanism:
Holiday
Na dissolves in liquid ammonia, makes “solvated” electrons
24. 2. Electrophilic additions. Like alkenes.
a. HX:
RR + H+ C C R
R
H
+ X
-
Anti to H;
pushes R
trans
C C
R
H R
X
H+
Markovnikov
C
R
X
RCH2
X
-
RCH2CX2R
Geminal dihalide
CHRC
HX
C C
X
R H
H HX
RCX2CH3
Markovnikov twice
sp
+
sp 2
Internal alkynes
Resonance with X
25. δ 13C = 202.4 ppm
+
ν = 1987 cm -1~
1.22 Å
Linear!
Angew. Chem. 2004, 43, 1543.
27. b. X2: Anti addition, as for alkenes
CH3
Br2
Br CH3
Br
Br2
Br Br
Br Br
c. Cat. HgSO4, H2O hydration, Markovnikov
CRRC
Cat. HgSO4
H2O C C
H
R OH
R Tautomerization
O
H+ or OH
catalyzed
-
Unstable
RCH2CR
No NaBH4
needed
28. O
R
Mechanism of tautomerization
H+ : C
OH
RCH2
+ R
C
O
RCH2 H+
RCH2CR
O
-H+
OH : RCH2CR
O
+H+
C C
H
R O
R
C C
H
R
R-
-
-
C C
H
R OH
R
H+ or OH catalyzed
-
33. A variant with alkynes: Sonogashira reaction
C C
R
H R
X
R’C CH+ C C
R
H R
R’
34. BR2
e. Hydroboration-Oxidation
Use R2BH (R = bulky group) to protect
alkenylborane: R = cyclohexyl
CHRC + B-H
2
C C
R
H
H H2O2, -OH
OH
C C
R
H
H Tautomerization
RCH2CH
O
Aldehyde !
Steric control
35. 1. R2BH
2. H2O2,-OH
1. HBR2
2. H2O2,-OH
Therefore: H
O
but HgSO4
H2O
cat. O
RR RCCH2R
O
R = R : Mixtures
