Chapter 9: Alkynes, from Carey Organic Chemsitry
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Transcript of Chapter 9: Alkynes, from Carey Organic Chemsitry
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Chapter 9: AlkynesChapter 9: Alkynes
9.1. Sources of Alkynes. Acetylene9.1. Sources of Alkynes. Acetylene
Industrial preparation of acetylene isIndustrial preparation of acetylene isby dehydrogenation of ethyleneby dehydrogenation of ethylene
CHCH CHCH 800800CC HH CC CHCH HH22++
11501150CC
++22 22
CostCost of of ener ener makesmakes acet leneacet lene aa moremore
Chem 211 B. R. Kaafarani 1
expensiveexpensive industrialindustrial chemicalchemical thanthan ethyleneethylene..
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9.2. Nomenclature
Acetylene and ethyne are both acceptable IUPAC
names for HCHC CHCH
Higher alkynes are named in much the same
-
of -ene.
HCHC CCHCCH33PropynePropyne
HCHC CCHCCH22CHCH3311--ButyneButyne
(CH(CH33))33CCCC CCHCCH33
Chem 211 B. R. Kaafarani 24,44,4--DimethylDimethyl--22--pentynepentyne
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..Physical Properties of AlkynesPhysical Properties of Alkynes
The physical properties of alkynes are similar toThe physical properties of alkynes are similar to
those of alkanes and alkenes.those of alkanes and alkenes.
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9.4. Structure and Bonding in9.4. Structure and Bonding in
Linear geometry for acetyleneLinear geometry for acetylene
CC CCHH HH
120 pm120 pm
106 pm106 pm 106 pm106 pm
CC CCCHCH33 HHpmpm
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Cyclic Alkynes
Cyclononyne is the
smallest cycloalkyne stableenough to be stored at
room temperature for a
reasonable length of time.
Cyclooctyne polymerizes
on standing.
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Bonding in acetylene is based on
-
x oge er y r ze e s or a
and one of the three 2p orbitals
22pp 22pp
22ss
22ss Each carbon has two half-
Chem 211 B. R. Kaafarani 6
form s bonds.
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aandnd BondsBonds in Acetylenein Acetylene
Each carbon is connected to a hydrogen by a bond.
The two carbons are connected to each other by a
bond and two
bonds.
Two bonds in acetylene at
Chem 211 B. R. Kaafarani 7
r g t ang es to eac ot er.
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TheThe regionregion ofof highesthighest
negativenegative chargecharge lieslies
TheThe regionregion ofof highesthighest
negativenegative chargecharge encirclesencircles
aboveabove andand belowbelow thethemolecularmolecular planeplane inin
thethe moleculemolecule aroundaround itsitscentercenter inin acetyleneacetylene..
Chem 211 B. R. Kaafarani 8
..
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Table 9.1. Comparison of ethane, ethylene, and acetylene
CC distanceCC distance 153 pm153 pm 134 pm134 pm 120 pm120 pm
Ethane Ethylene Acetylene
CH distanceCH distance
111 pm111 pm
110 pm110 pm
106 pm106 pm
CC BDECC BDE
..
368 kJ/mol368 kJ/mol
..
611 kJ/mol611 kJ/mol 820 kJ/mol820 kJ/mol
CH BDECH BDE 410 kJ/mol410 kJ/mol
s 3s 3
452 kJ/mol452 kJ/mol
s 2s 2
536 kJ/mol536 kJ/mol
ssHybridization of CHybridization of C
% scharacter% scharacter 25%25% 33%33% 50%50%
Chem 211 B. R. Kaafarani 9
pKapKa 6262 4545 2626
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9.5. Acidity of Acetylene and
Terminal Alkynes
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In general, hydrocarbons are
CompoundCompound ppKKaa
HFHF 3.23.2
22
NHNH 3636HCHC CHCH 2626
4545HH22CC CHCH22
CHCH44 6060
Acet lene is a weak acid but not nearl as weak asAcet lene is a weak acid but not nearl as weak as
Chem 211 B. R. Kaafarani 11
alkanes or alkenes.alkanes or alkenes.
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Electronegativity of carbon
--6060
CC HH HH++
++ CC : sp3
HH++ ++
1010--4545
CC CC CC CC
:
sp2
1010--2626
++ : sp
Chem 211 B. R. Kaafarani 12
closer to the nucleus and more strongly held.closer to the nucleus and more strongly held.
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Objective:Objective:
Prepare a solution containing sodium acetylide.Prepare a solution containing sodium acetylide.
NaCNaC CHCH
Will treatment of acetylene with NaOH be effective?Will treatment of acetylene with NaOH be effective?
HH22OONaOHNaOH ++ HCHC CHCH NaCNaC CHCH ++
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No H droxide is not a stron enou h base todeprotonate acetylene.
HH22OONaOHNaOH++
HCHC CHCH NaCNaC CHCH++
HOHO
....
:: HH CC CHCH HOHO HH
....
++ ++ CC CHCH::
....weaker acidweaker acid
KK = 26= 26
stronger acidstronger acid
KK = 16= 16
In acid-base reactions, the equilibrium lies to
Chem 211 B. R. Kaafarani 14
e s e o e wea er ac .
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Solution: Use a stronger base. Sodium amideis a
stronger base than sodium hydroxide.
NHNH33NaNHNaNH22 ++ HCHC CHCH NaCNaC CHCH ++
.... ....++ ++
stronger acidstronger acid weaker acidweaker acid
pp aa == aa ==
Ammonia is a weaker acid than acet lene.
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The position of equilibrium lies to the right.
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9.6. Preparation of Alkynes by Alkylationo ce y ene an erm na ynes
There are two main methods for thepreparation of alkynes:
Carbon-carbon bond formation:
a y at on o acety ene an term na a ynes
Functional-group transformations:
e m na on
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Alkylation of acetylene and
RRCC CCHH
RRCC CCRR
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Alkylation of acetylene and terminal
RR XXSS
NN22
XX::++CC
::HHCC CCRRHHCC ++
The alkylating agent is an alkyl halide, and thereaction is nucleophilic substitution.
The nucleophile is sodium acetylide or the sodium
salt of a terminal (monosubstituted) alkyne.
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Example: Alkylation of acetylene
NaNHNaNH22
NHNH33
CHCH33CHCH22CHCH22CHCH22BrBr
HCHC CC CHCH22CHCH22CHCH22CHCH33
(70(70--77%)77%)
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Example: Alkylation of a terminal alkyne
CCHH(CH(CH33))22CHCHCHCH22CC
NaNHNaNH22, NH, NH33
CCNaNa(CH(CH33))22CHCHCHCH22CC
CHCH33BrBr
CCCHCH33(CH(CH33))22CHCHCHCH22CC
Chem 211 B. R. Kaafarani 20(81%)(81%)
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Example: Dialkylation of acetylene
HHCC CCHH
1. NaNH1. NaNH22, NH, NH
33
2.2. CHCH33CHCH22BrBr
CCHHCHCH33CHCH22CC LIMITATION!1. NaNH1. NaNH22, NH, NH33
2.2. CHCH33BrBr
Effective only with
primary alkyl halides.
CCCHCH33CHCH33CHCH22CCtertiary alkyl halides
undergo elimination.
Chem 211 B. R. Kaafarani 21
(81%)(81%)
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E2 predominates over SN2 when
HH CCCC::HHCC
CC XX
++CCHHCC HH CC CC XX::++
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9.7. Preparation of Alkynes by Elimination
Double Dehydrohalogenation
XXHH HHHH
CC CC CC CC
XXHH XX XX
Chem 211 B. R. Kaafarani 23
of terminal alkynes.
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Geminal dihalide Alkyne
1. 3NaNH1. 3NaNH22, NH, NH33
2. H2. H22OO
CHCH CCCC
(56(56--60%)60%)
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Geminal dihalide Alkyne
(CH(CH33))33CCCCHH22CHCHClCl22aa 22,, 33
(CH(CH33))33CCCCHH CHCHClCl
s ows ow
NaNHNaNH22, NH, NH33
(CH(CH )) CCCC CHCH
(slow)(slow)
NaNHNaNH22, NH, NH33HH22OO (fast)(fast)
33 33
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Vicinal dihalide Alkyne
CHCH33(CH(CH22))77CCHHCCHH22BrBr
BrBr
1. 3NaNH1. 3NaNH22, NH, NH33
2. H2. H22OO
(54%)(54%)
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9.9. Hydrogenation of Alkynes
''catcat
'' ++22 22
==
alkene is an intermediatealkene is an intermediate
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Heats of Hydrogenation
CHCH33CHCH22CC CHCH CHCH33CC CCCHCH33
292 kJ/mol292 kJ/mol 275 kJ/mol275 kJ/mol
Alkyl groups stabilize triple bonds in the same
.
bonds are more stable than terminal ones.
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Partial Hydrogenation
RCHRCH22CHCH22R'R'catcatRCRC CR'CR' catcat RCHRCH CHR'CHR'
Alkynes could be used to prepare alkenes if a
catalyst were available that is active enough tocatalyze the hydrogenation of alkynes, but not active
enough for the hydrogenation of alkenes.
Chem 211 B. R. Kaafarani 29
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Lindlar Palladium
HHHHRCHRCH22CHCH22R'R'
catcatRCRC CR'CR'
catcatRCHRCH CHR'CHR'
There is a catalyst that will catalyze the
hydrogenation of alkynes to alkenes, but not that ofalkenes to alkanes. It is called the Lindlar catalyst and
consists of palladium supported on CaCO3, which has
.syn-Hydrogenation occurs; cisalkenes are formed.
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++ HH22CHCH33(CH(CH22))33CC C(CHC(CH22))33CHCH33
Lindlar PdLindlar Pd Lindlar Pd: Pd
on CaCO3;
Pb(CH3CO2)2
33 22 33 22 33 33
CCCC
added to it.
HH HH
Quinoline
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9.10. Metal-Ammonia Reduction of Alkynes
AlkynesAlkynestranstrans--AlkenesAlkenes
'''' ''
Partial ReductionPartial Reduction
22 22
Another way to convert alkynes to alkenes is by
ammonia.
trans-Alkenes are formed.
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Example
CHCH33CHCH22CC CCHCCH22CHCH33
Na, NNa, NHH33
CHCH33CHCH22 HH
CCCC
CHCH22CHCH33HH
82%82%
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Mechanism
Metal (Li, Na, K) is reducing agent; H2 is not involved.
(1) Electron transfer
ro on rans er
(3) Electron transfer(4) Proton transfer
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Mechanism
MM++
alkyne to give an anion radical.
MM ..++R'R'RR CC CC RR R'R'CC.... ..
CC
Step (2) Transfer of a proton from the solvent (liquid
''.... .. ..
''
RR.
....
HH ....
Chem 211 B. R. Kaafarani 35
HH NHNH22 NHNH22
::
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Step (3): Transfer of an electron from the metal to
..RR MM
++
RR....
.
MM..++R'R'CC CC
''
CCCC
Step (4) Transfer of a proton from the solvent (liquid
HH NHNH22
....
....
ammon a o e car an on.
RRCC CC::
CCCC
RR 22::
Chem 211 B. R. Kaafarani 36R'R'HH R'R'HH
S ff f (E) (Z)
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Suggest efficient syntheses of (E)- and (Z)-2- heptene
from propyne and any necessary organic or inorganic
reagents.
1. NaNH1. NaNH222. CH2. CH CHCH CHCH CHCH Br Br
a,a, 33HH22, Lindlar Pd, Lindlar Pd
Chem 211 B. R. Kaafarani 37
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9.11. Addition of Hydrogen Halides to
''
HBrHBr
CHCH33(CH(CH22))33CC CHCH CHCH33(CH(CH22))33CC CHCH22
(60%)(60%)
Alkynes are slightlyAlkynes are slightly lesslessreactive than alkenes.reactive than alkenes.
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Termolecular Transition State
....rrHH ::
....
CHCHRCRC
....BrBrHH ::
....
== 22
Chem 211 B. R. Kaafarani 39
R ti ith t l f h d
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Reaction with two moles of a hydrogen
2 H2 HFF
CC CCCHCH CHCH CHCH CHCH
FFHH
Chem 211 B. R. Kaafarani 40
(76%)(76%)
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Free-radical addition of HBr occurs
w en perox es are presen
HBrHBrCHCH33(CH(CH22))33CC CHCH CHCH33(CH(CH22))33CCHH CHCHBrBr
(79%)(79%)
Regioselectivity opposite to Markovnikov's rule.
Chem 211 B. R. Kaafarani 41
9 12 H d ti f Alk
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9.12. Hydration of Alkynes
Expected reaction:Expected reaction:++
RCRC CR'CR' HH22OO++ RCHRCH CR'CR'
enolenol
RCHRCH22CR'CR'HH++
RCRC CR'CR' HH22OO++
OO
Chem 211 B. R. Kaafarani 42
e onee one
Tautomerism
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Tautomerism
RCHRCH CR'CR' RCHRCH22CR'CR'
enolenol
OHOH OO
Ketone (keto form)Ketone (keto form)
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
e uilibrium.
Tautomers are constitutional isomers that equilibrate by
Chem 211 B. R. Kaafarani 43
.
tautomerism.
Mechanism of conversion of enol to ketone
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....Mechanism of conversion of enol to ketone
OO HH
CC CC
::
HH
OO HH....
::HH++OO::
HH CC CCHH++
HH
HH
HH....OO::
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++
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Key carbocation intermediate is stabilized
by electron delocalization (resonance)
OO HH....
:: OO....
HH++
CC CCHH
++
CC CCHH
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Example
CHCH CHCH CC C CHC CH CHCH
HH22O, HO, H++
viavia
HgHg2+2+OHOH
OO
CHCH33(CH(CH22))22CHCH22C(CHC(CH22))22CHCH33
Chem 211 B. R. Kaafarani 46
Markovnikov's rule followed in
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Markovnikov s rule followed in
OO
HH22O, HO, H22SOSO44
H SOH SOCHCH33(CH(CH22))55CCHCCH33CHCH33(CH(CH22))55CC CHCH
(91%)(91%)
viavia
CHCH33(CH(CH22))55CC CHCH22
Chem 211 B. R. Kaafarani 47
9 13 Additi f H l t Alk
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9.13. Addition of Halogens to Alkynes
Example
ClCl
+ 2+ 2 ClCl22 CCClCl22CHCH CHCH33HCHC CCHCCH33
(63%)(63%)
Chem 211 B. R. Kaafarani 48
Addition isAddition is antianti
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Addition isAddition is antianti
CHCH CHCH BrBr
BrBr22CHCH33CHCH22CC CCHCCH22CHCH33 CC CC
CHCH22CHCH33BrBr
(90%)(90%)
be isolated when 1:1molar ratio is used.
Chem 211 B. R. Kaafarani 49
9 14 Ozonolysis of Alkynes
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9.14. Ozonolysis of Alkynes
Gives two carboxylic acids by cleavage of triple bondGives two carboxylic acids by cleavage of triple bond
CHCH33(CH(CH22))33CC CCHH
1. O1. O33
Example
.. 22OO OO
++CHCH33(CH(CH22))33CCOHOH HOHOCCOHOH
Chem 211 B. R. Kaafarani 50