Transcript Document

Alkynes
C
C
Synthesis of Acetylene
Heating coke with lime in an electric furnace to
forms calcium carbide.
Then drip water on the calcium carbide.
3 C + CaO
coke
* CaC2 +
CaC2 +
CO
lime
2 H2O
H C C H + Ca(OH)2
*This reaction was used to produce light
for miners’ lamps and for the stage.
The Structure of Alkynes
A triple bond is composed of a s bond and two p bonds
Acidity of Acetylene
and Terminal Alkynes
H
C
C
Acidity of Hydrocarbons
In general, hydrocarbons are
excedingly weak acids
Compound
pKa
HF
3.2
H2 O
16
NH3
36
H2C
CH2
CH4
45
60
Acetylene
Acetylene is a weak acid, but not nearly
as weak as alkanes or alkenes.
HC
CH
Compound
pKa
HF
3.2
H2 O
16
NH3
36
H2C
CH2
CH4
45
60
26
Carbon: Hybridization and Electronegativity
10-60
C
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.
The stronger the acid, the weaker its conjugate base
top 252
Sodium Acetylide
Solution: Use a stronger base. Sodium amide
is a stronger base than sodium hydroxide.
NaNH2 + HC CH
NaC CH + NH3
.. –
H2N : +
H
C
CH
stronger acid
pKa = 26
..
H2N
–
H + :C
weaker acid
pKa = 36
Ammonia is a weaker acid than acetylene.
The position of equilibrium lies to the right.
CH
Preparation of Various Alkynes
by alkylation reactions with
Acetylide or Terminal Alkynes
Synthesis Using Acetylide Ions:
Formation of C–C Bond
Alkylation of Acetylene and Terminal Alkynes
H—C
C—H
R—C
C—H
R—C
C—R
Alkylation of Acetylene and Terminal Alkynes
H—C
–
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)
alkyne.
Example: Alkylation of Acetylene
HC
CH
NaNH2
HC
NH3
CNa
CH3CH2CH2CH2Br
HC
C
CH2CH2CH2CH3
(70-77%)
Example: Alkylation of a Terminal Alkyne
(CH3)2CHCH2C
CH
NaNH2, NH3
(CH3)2CHCH2C
CNa
CH3Br
(CH3)2CHCH2C
(81%)
C—CH3
Example: Dialkylation of Acetylene
H—C
C—H
1. NaNH2, NH3
2. CH3CH2Br
CH3CH2—C
C—H
1. NaNH2, NH3
2. CH3Br
CH3CH2—C
C—CH3
(81%)
Limitation
Effective only with primary alkyl halides
Secondary and tertiary alkyl halides
undergo elimination
Reactions of Alkynes
Reactions of Alkynes
Acidity
Hydrogenation
Metal-Ammonia Reduction
Addition of Hydrogen Halides
Hydration
Hydrogenation of Alkynes
Hydrogenation of Alkynes
RC
CR'
+
2H2
cat
RCH2CH2R'
catalyst = Pt, Pd, Ni, or Rh
alkene is an intermediate
Partial Hydrogenation
RC
CR'
H2
cat
RCH
CHR'
H2
cat
RCH2CH2R'
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.
Lindlar Palladium
RC
CR'
H2
cat
RCH
CHR'
H2
cat
RCH2CH2R'
There is a catalyst that will catalyze the hydrogenation
of alkynes to alkenes, but not that of alkenes to alkanes.
It is called the Lindlar catalyst and consists of
palladium supported on CaCO3, which has been
poisoned with lead acetate and quinoline.
syn-Hydrogenation occurs; cis alkenes are formed.
Example
CH3(CH2)3C
C(CH2)3CH3 + H2
Lindlar Pd
CH3(CH2)3
(CH2)3CH3
C
C
H
H
(87%)
Metal-Ammonia Reduction
of Alkynes
Alkynes  trans-Alkenes
Partial Reduction
RC
CR'
RCH
CHR'
RCH2CH2R'
Another way to convert alkynes to alkenes is
by reduction with sodium (or lithium or potassium)
in ammonia.
trans-Alkenes are formed.
Example
CH3CH2C
CCH2CH3
Na, NH3
CH3CH2
H
C
C
CH2CH3
H
(82%)
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
Suggest an efficient syntheses of (E)- and (Z)-2heptene from propyne and any necessary organic
or inorganic reagents.
Problem
Strategy
Problem
Strategy
Problem
Synthesis
1. NaNH2
2. CH3CH2CH2CH2Br
H2, Lindlar Pd
Na, NH3
Addition of Hydrogen Halides
to Alkynes
Follows Markovnikov's Rule
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
CH3CH2
H
F
C
C
H
F
(76%)
CH2CH3
Free-radical Addition of HBr
CH3(CH2)3C
CH
HBr
peroxides
CH3(CH2)3CH
(79%)
regioselectivity opposite to Markovnikov's rule
CHBr
Hydration of Alkynes
Hydration of Alkynes
expected reaction:
RC
CR'
+ H2O
H+
RCH
CR'
OH
enol
observed reaction:
RC
CR'
+ H2O
H+
RCH2CR'
O
ketone
Enols
RCH
CR'
OH
enol
RCH2CR'
O
ketone
enols are regioisomers of ketones, and exist
in equilibrium with them
keto-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
H
C
H
C
H
Mechanism of conversion of enol to ketone
..
:O
H
+
:O
H
C
H
C
H
Mechanism of conversion of enol to ketone
..
:O
H
: O:
H
H
C
C
+
H
Mechanism of conversion of enol to ketone
H
..
:O
H
C
C
+
H
: O:
H
Mechanism of conversion of enol to ketone
H
..
:O
H
C
C
+
H
: O:
H
Useful for symmetrical starting alkynes
to produce a single product.
Unsymmetrical starting alkynes produce
a mixture of ketones… not so useful.
Aldehyde vs. Ketone
Can you identify and name the function?
Example