Synthesis of Acetylene
Heating coke with lime in an electric furnace to
forms calcium carbide.
Then drip water on the calcium carbide.
Alkynes
C
C
CaC2 +
3 C + CaO
coke
* CaC2 +
CO
lime
H C C H + Ca(OH)2
2 H2O
*This reaction was used to produce light
for miners’ lamps and for the stage.
The Structure of Alkynes
Acidity of Acetylene
and Terminal Alkynes
H
C
C
A triple bond is composed of a σ bond and two π bonds
Acidity of Hydrocarbons
Acetylene
Acetylene is a weak acid, but not nearly
as weak as alkanes or alkenes.
In general, hydrocarbons are
excedingly weak acids
Compound
pK a
Compound
pK a
HF
3.2
HF
3.2
H2O
16
H2O
16
NH3
36
NH3
36
H2C CH2
CH4
45
60
HC
CH
H 2C
CH2
CH4
26
45
60
1
Carbon: Hybridization and Electronegativity
C
10-60
H
H
C
10-45
C
H+ +
H+ +
C
C
:
sp3
:
sp2
C
10-26
C
H
C
H+ +
C
C :
sp
Electrons in an orbital with more s character are closer to the
nucleus and more strongly held.
Sodium Acetylide
The stronger the acid, the weaker its conjugate base
top 252
Solution: Use a stronger base. Sodium amide
is a stronger base than sodium hydroxide.
NaNH
NaC CH + NH3
NaNH2 + HC CH
.. –
H 2N : +
H
C
CH
stronger acid
pKa = 26
..
H 2N
–
H + :C
CH
weaker acid
pKa = 36
Ammonia is a weaker acid than acetylene.
The position of equilibrium lies to the right.
Synthesis Using Acetylide Ions:
Formation of C–C Bond
Preparation of Various Alkynes
by alkylation reactions with
Acetylide or Terminal Alkynes
2
Alkylation of Acetylene and Terminal Alkynes
H—C
Alkylation of Acetylene and Terminal Alkynes
C—H
H—C
R —C
C—H
R—C
C—R
Example: Alkylation of Acetylene
HC
CH
–
C: +
R
X
SN2
HC
NH3
CNa
CH3CH2CH2CH2Br
C—R + : X–
The alkylating agent is an alkyl halide, and
the reaction is nucleophilic substitution.
The nucleophile is sodium acetylide or the
sodium salt of a terminal (monosubstituted
(monosubstituted))
alkyne.
alkyne.
Example: Alkylation of a Terminal Alkyne
(CH3)2CHCH 2C
NaNH 2
H—C
CH
NaNH 2, NH3
(CH3)2CHCH 2C
CNa
CH3Br
HC
C
CH2CH2CH2CH3
(70-77%)
(CH3)2CHCH 2C
C—CH3
(81%)
Example: Dialkylation of Acetylene
H —C
1. NaNH2, NH3
2. CH3CH2Br
CH3CH2—C
Limitation
C —H
C—H
Effective only with primary alkyl halides
Secondary and tertiary alkyl halides
undergo elimination
1. NaNH2, NH3
2. CH3Br
CH3CH2—C
C—CH3
(81%)
3
Reactions of Alkynes
Acidity
Hydrogenation
Metal-Ammonia Reduction
Addition of Hydrogen Halides
Reactions of Alkynes
Addition of Halogens
Hydration
Atomic Force Microscopy of Acetylene
Lawrence Berkeley Laboratory (LBL)
Hydrogenation of Alkynes
H
C
C
H
Excitation of frustrated rotational modes in acetylene
molecules on Pd(111) at T = 30 K
Imaging: acetylene on Pd(111) at 28 K
Molecular Image
Tip cruising altitude ~700 pm
_z = 20 pm
H
C
C
H
Why don’t we see the Pd atoms?
Because the tip needs to be very close
to image the Pd atoms and would knock
the molecule away
Tip
eSurface atomic profile
Tip cruising altitude ~500
pm
_z = 2 pm
TIP
pz
H
+
O π orbital
Calculated image
(Philippe Sautet)
If the tip was made as big as an airplane, it would be
flying at 1 cm from the surface and waving up an down
by 1 micrometer
The STM image is a map of the pi-orbital of
distorted acetylene
M. Salmeron (LBL)
((( ) (
1 cm
( ± 1 _m)
)))
M. Salmeron (LBL)
4
Excitation of translations of C 2H2 molecules:
Measuring the excitation rate
Rotation by electron excitation:
Pd
3
2
x
1
Pd
Pd
((( ) (
2
Pd
Pd
Center of molecule
Tip fixed at position 1:
V = 20 mV
0
2,3
R = 150 MΩ
16
0
50
Δz ~ - 1 Å
current (pA)
Translation by direct contact (orbital overlap):
253 pA
10
1
0
-50
-100
-150
-200
Tip Bias (mV)
-250
-300
R = 10.5 M Ω
Trajectories of molecule pushed by the tip
M. Salmeron (LBL)
Hydrogenation of Alkynes
CR'
+ 2H 2
cat
Partial Hydrogenation
RCH
RCH2CH2R'
catalyst = Pt, Pd, Ni, or Rh
RC
CR'
H2
cat
RCH
CR'
RCH
CHR'
H2
cat
RCH2CH2R'
Example
Lindlar Palladium
RC
CHR'
Alkenes could be used to prepare alkenes if a
catalyst were available that is active enough to
catalyze the hydrogenation of alkynes, but not
active enough for the hydrogenation of alkenes.
alkene is an intermediate
H2
cat
Δz ~ +0.8 Å
100 150 200 250 300 350 400 450
M. Salmeron (LBL)
RC
)))
R = 0.55 G Ω
R = 94 M Ω
Δz ~ -0.2 Å
Δz
8
0.1
1.72 seconds
((( ) (
Tip
-37mV
100
100
)))
24
0
150
50
32
1
Log(Hops/s)
Current (pA)
200
rotations per second
Pd
H2
cat
CH3(CH2)3C
C(CH2)3CH3 + H2
RCH2CH2R'
Lindlar Pd
There is a catalyst that will catalyze the hydrogenation
of alkynes to alkenes, but not that of alkenes to alkanes.
alkanes.
It is called the Lindlar catalyst and consists of
palladium supported on CaCO 3, which has been
poisoned with lead acetate and quinoline.
quinoline.
syn-Hydrogenation
syn-Hydrogenation occurs; cis alkenes are formed.
CH3(CH2)3
(CH2)3CH3
C
C
H
H
(87%)
5
Partial Reduction
Metal-Ammonia Reduction
of Alkynes
Alkynes → trans-Alkenes
trans-Alkenes
RC
CR'
RCH
RCH2CH2R'
CHR'
Another way to convert alkynes to alkenes is
by reduction with sodium (or lithium or potassium)
in ammonia.
trans-Alkenes
trans-Alkenes are formed.
Example
CH3CH2C
CCH2CH3
Na, NH3
CH3CH2
H
C
C
CH2CH3
H
(82%)
6
Mechanism
Metal (Li, Na, K) is reducing agent;
H2 is not involved; proton comes from NH3
four steps
(1) electron transfer
(2) proton transfer
(3) electron transfer
(4) proton transfer
Problem
Problem
Strategy
Suggest an efficient syntheses of (E
(E)- and (Z
(Z)-2heptene from propyne and any necessary organic
or inorganic reagents.
Problem
Strategy
Problem
Synthesis
1. NaNH 2
2. CH3CH2CH2CH2Br
H2, Lindlar Pd
Na, NH3
7
Follows Markovnikov's Rule
Addition of Hydrogen Halides
to Alkynes
CH3(CH2)3C
HBr
CH
CH3(CH2)3C
CH2
Br
(60%)
Alkynes are slightly less reactive than alkenes
Two Molar Equivalents of Hydrogen Halide
CH3CH2C
CCH2CH3
2 HF
HF
CH3CH2
Free-radical Addition of HBr
H
F
C
C
H
F
CH3(CH2)3C
CH
HBr
peroxides
CHBr
CHBr
CH3(CH2)3CH
(79%)
CH2CH3
regioselectivity opposite to Markovnikov's rule
(76%)
Example
Cl
Addition of Halogens to Alkynes
HC
CCH3 + 2 Cl2
Cl2CH
C
CH3
Cl
(63%)
8
Addition is anti
Br
CH3CH2
CH3CH2C
CCH2CH3
Br2
C
Hydration of Alkynes
C
CH2CH3
Br
(90%)
Hydration of Alkynes
expected reaction:
RC
CR'
+ H 2O
H+
RCH
CR'
OH
enol
observed reaction:
RC
CR'
+ H 2O
H+
RCH2CR'
O
ketone
Enols
RCH
CR'
OH
enol
RCH2CR'
O
ketone
enols are regioisomers of ketones,
ketones, and exist
in equilibrium with them
ketoketo-enol equilibration is rapid in acidic media
ketones are more stable than enols and
predominate at equilibrium
9
Mechanism of conversion of enol to ketone
..
:O
H
+
:O
C
Mechanism of conversion of enol to ketone
..
:O
H
H
C
+
:O
H
H
C
H
C
H
H
Mechanism of conversion of enol to ketone
Mechanism of conversion of enol to ketone
H
..
:O
H
H
C
..
:O
H
H
C
+
C
H
C
+
: O:
H
: O:
H
Mechanism of conversion of enol to ketone
H
..
:O
H
C
C
+
H
Useful for symmetrical starting alkynes
to produce a single product.
: O:
H
Unsymmetrical starting alkynes that are
not terminal produce a mixture of
ketones…
ketones…non-regioselectively
non-regioselectively..
10
Regioselectivity
Aldehyde vs. Ketone
Markovnikov's rule followed in formation of enol,
enol,
Useful with terminal alkynes.
O
H2O, H2SO4
CH3(CH2)5C CH
CH3(CH2)5CCH3
HgSO4
(91%)
via
OH
CH3(CH2)5C
CH2
Can you identify and name the function?
11
Example
12