Synthesis of Acetylene
Alkynes
C
C
Heating coke with lime in an electric furnace to
forms calcium carbide.
Then drip water on the calcium carbide.
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
Question
Arrange ethane, ethene, and ethyne in order of
increasing C-C bond length.
A) ethane < ethene < ethyne
B) ethene < ethane < ethyne
C) ethyne < ethene < ethane
D) ethane < ethyne < ethene
A triple bond is composed of a σ bond and two π bonds
Acidity of Hydrocarbons
Acidity of Acetylene
and Terminal Alkynes
H
C
C
In general, hydrocarbons are
very weak acids
Compound
pK a
HF
3.2
H2O
16
NH3
36
H2C CH2
CH4
45
60
1
Acetylene
Question
Acetylene is a weak acid, but not nearly
as weak as alkanes or alkenes.
HC
CH
Compound
pK a
HF
3.2
H2O
16
NH3
36
H 2C
CH2
CH4
26
Which one of the following is the strongest acid?
A) water
B) ammonia
C) 1-butene
D) 1-butyne
45
60
Carbon: Hybridization and Electronegativity
C
10-60
H
H
C
10-45
C
H+ +
H+ +
C
C
:
sp3
:
sp2
C
10-26
C
C
H
H+ +
C
C :
sp
Electrons in an orbital with more s character are closer to the
nucleus and more strongly held.
Question
The stronger the acid, the weaker its conjugate base
Which one of the following statements best explains the
greater acidity of terminal alkynes (RC≡CH) compared
with monosubstituted alkenes (RCH=CH2)?
A) The sp-hybridized carbons of the alkyne are less
electronegative than the sp2 carbons of the alkene.
B) The two π bonds of the alkyne are better able to
stabilize the negative charge of the anion by
resonance.
C) The sp-hybridized carbons of the alkyne are
more electronegative than the sp2 carbons of the
alkene.
D) The question is incorrect - alkenes are more acidic
than alkynes.
top 252
2
Sodium Acetylide
Question
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
weaker acid
pKa = 36
CH
Which of the following bases is strong enough
to completely deprotonate propyne?
A) NH3
B) CH3OH
C) NaNH 2
D) NaOH
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
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
–
C: +
R
X
SN2
H—C
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.
3
Example: Alkylation of Acetylene
HC
CH
NaNH 2
HC
NH3
CNa
CH3CH2CH2CH2Br
HC
C
CH2CH2CH2CH3
Question
Which alkyl halide will react faster with the
acetylide ion (HC≡CNa) in an SN2 reaction?
A) bromopropane
B) 2-bromopropane
C) tert-butyl iodide
D) 1-bromo-2-methylbutane
(70-77%)
Example: Alkylation of a Terminal Alkyne
(CH3)2CHCH 2C
Example: Dialkylation of Acetylene
H—C
CH
C—H
1. NaNH2, NH3
NaNH 2, NH3
2. CH3CH2Br
(CH3)2CHCH 2C
CNa
CH3CH2—C
CH3Br
(CH3)2CHCH 2C
C—CH3
2. CH3Br
CH3CH2—C
Acetylide Ion as a Base
E2 predominates over SN2 when alkyl
halide is secondary or tertiary.
Effective only with primary alkyl halides
Secondary and tertiary alkyl halides
undergo elimination
C—CH3
(81%)
(81%)
Limitation
C—H
1. NaNH2, NH3
H—C
–
C:
H
C
C
X
E2
H—C
C —H +
C
C
+
: X–
4
Question
Consider the reaction of each of the following
with cyclohexyl bromide. For which one is
the ratio of substitution to elimination highest?
A) NaOCH2CH3, ethanol, 60°C
B) NaSCH 2CH3, ethanol-water, 25°C
C) NaNH 2, NH3, -33°C
D) NaC≡CH, NH3, -33°C
Preparation of Alkynes
by "Double Dehydrohalogenation"
H
X
H
H
C
C
C
C
H
X
X
Geminal dihalide
Preparation of Alkynes
by Elimination Reactions
Geminal dihalide → Alkyne
(CH3)3CCH
CCH2—CHCl
CHCl2
1. 3NaNH 2, NH3
2. H2O
X
Vicinal dihalide
(CH3)3CC
CH
The most frequent applications are in preparation
of terminal alkynes.
(56-60%)
Geminal dihalide → Alkyne
Question
(CH3)3CCH
CCH2—CHCl
CHCl2
(CH3)3CCH
CCH
NaNH 2, NH3 (slow)
CHCl
CHCl
(CH3)3CC
CH
(CH3)3CC
CNa
H 2O
NaNH 2, NH3 (slow)
NaNH 2, NH3 (fast)
In addition to NaNH 2, what other base can be
used to convert 1,1-dichlorobutane into
1-butyne?
A) NaOCH3
B) NaOH
C) NaOCH2CH3
D) KOC(CH3)3
5
Vicinal dihalide → Alkyne
CH3(CH2)7CH—CH2Br
Br
1. 3NaNH 2, NH3
2. H2O
CH3(CH2)7C
CH
Question
Which of the following compounds yield 1heptyne on being treated with three moles of
sodium amide (in liquid ammonia as the
solvent) followed by adding water to the
reaction mixture?
A) 1,1,2,2-tetrachloroheptane
B) 1-bromo-2-chloroheptane
C) 1,1,2-trichloropentane
D) all of the above
(54%)
Reactions of Alkynes
Acidity
Hydrogenation
Reactions of Alkynes
Metal-Ammonia Reduction
Addition of Hydrogen Halides
Hydration
Addition of Halogens
Ozonolysis
Atomic Force Microscopy of Acetylene
Lawrence Berkeley Laboratory (LBL)
Hydrogenation of Alkynes
H
C
C
H
6
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
Tip
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
C
H
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)
Hydrogenation of Alkynes
RC
CR'
+ 2H 2
cat
Partial Hydrogenation
RCH
RCH2CH2R'
catalyst = Pt, Pd, Ni, or Rh
RC
CR'
H2
cat
RCH
CR'
RCH
CHR'
RCH2CH2R'
Example
Lindlar Palladium
RC
H2
cat
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
CHR'
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%)
7
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%)
8
Question
How would you accomplish the following
conversion?
A)
B)
C)
D)
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
NaNH 2
H2, Lindlar Pd
Na, NH3
either B or C
Question
Select the most effective way to synthesize ciscis2-pentene from 1-propyne.
A) 1) NaNH2 2) CH3CH2Br 3) H2, Pd
B) 1) NaNH2 2) CH3Br 3) H2, Lindlar Pd
C) 1) NaNH2 2) CH3CH2I 3) H2, Lindlar Pd
D) 1) NaNH2 2) CH3CH2Br 3) Na,NH3
(4) proton transfer
Answer
Select the most effective way to synthesize ciscis2-pentene from 1-propyne.
A) 1) NaNH2 2) CH3CH2Br 3) H2, Pd
B) 1) NaNH2 2) CH3Br 3) H2, Lindlar Pd
C) 1) NaNH2 2) CH3CH2I 3) H2, Lindlar Pd
D) 1) NaNH2 2) CH3CH2Br 3) Na,NH3
Question
Which reagent would accomplish the
transformation of 3-hexyne into trans-3trans-3hexene?
A) H2/Ni
B) H2, Lindlar Pd
C) Na, NH3
D) NaNH 2, NH3
9
Problem
Answer
Which reagent would accomplish the
transformation of 3-hexyne into trans-3trans-3hexene?
A) H2/Ni
B) H2, Lindlar Pd
C) Na, NH3
D) NaNH 2, NH3
Problem
Strategy
Problem
Strategy
Problem
Synthesis
Question
1. NaNH 2
2. CH3CH2CH2CH2Br
H2, Lindlar Pd
Suggest an efficient syntheses of (E
(E)- and (Z
(Z)-2heptene from propyne and any necessary organic
or inorganic reagents.
Na, NH3
Which would be the best sequence of reactions to use in
order to prepare cis-3-nonene
from 1-butyne?
A) 1. NaNH 2 in NH 3; 2. 1-bromopentane; 3. H 2,
Lindlar Pd
B) 1. NaNH 2 in NH 3; 2. 1-bromopentane; 3. Na,
NH3
C) 1. H 2, Lindlar Pd; 2. NaNH 2 in NH3; 3. 1bromopentane
D) 1. Na, NH 3; 2. NaNH 2 in NH 3; 3. 1bromopentane
10
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
CH3(CH2)3CH
CHBr
CHBr
(79%)
CH2CH3
regioselectivity opposite to Markovnikov's rule
(76%)
Hydration of Alkynes
expected reaction:
RC
CR'
+ H 2O
H+
RCH
CR'
OH
enol
observed reaction:
RC
CR'
+ H 2O
H+
RCH2CR'
O
ketone
11
Enols
RCH
CR'
RCH2CR'
OH
O
enol
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
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
C
+
..
:O
H
H
C
C
+
H
: O:
H
: O:
H
12
Mechanism of conversion of enol to ketone
Key Carbocation Intermediate
Carbocation is stabilized by
electron delocalization (resonance).
..
:O
H
C
C
+
..
+O
H
H
C
H
..
:O
H
H
C
H
: O:
C
+
H
C
Regioselectivity
Useful for symmetrical starting alkynes
to produce a single product.
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
Unsymmetrical starting alkynes that are
not terminal produce a mixture of
ketones…
ketones…non-regioselectively
non-regioselectively..
OH
CH3(CH2)5C
CH2
Aldehyde vs. Ketone
Question
What is the product of the acid catalyzed
hydration of 1-hexyne?
A)
B)
C)
D)
13
Example
Cl
Addition of Halogens to Alkynes
CCH3 + 2 Cl2
HC
Cl2CH
C
CH3
Cl
(63%)
Addition is anti
Br
CH3CH2
CH3CH2C
CCH2CH3
Br2
C
C
Ozonolysis of Alkynes
CH2CH3
Br
gives two carboxylic acids by cleavage
of triple bond
(90%)
Example
CH3(CH2)3C
Question
CH
What product is formed when 2-butyne is
subjected to ozonolysis?
ozonolysis?
1. O3
2. H2O
O
CH3(CH2)3COH
O
+
A)
B)
C)
D)
HOC
HOCOH
(51%)
14
Can you identify and name the function?
Alkynes
Synthesis & Functions
15
Example
Question
What is the structure of Compound Y in the
following synthetic sequence?
A)
B)
C)
D)
pentane
cis-2-pentene
cis-2-pentene
trans-2-pentene
trans-2-pentene
2-pentyne
16