ORGANIC CHEMISTRY 171 Section 201. Alkynes Hydrocarbons that contain carbon-carbon triple bonds...
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Transcript of ORGANIC CHEMISTRY 171 Section 201. Alkynes Hydrocarbons that contain carbon-carbon triple bonds...
ORGANIC CHEMISTRY 171
Section 201
Alkynes
Alkynes
• Hydrocarbons that contain carbon-carbon triple bonds
• Acetylene is the simplest alkyne.• Our study of alkynes provides nomenclature,physical
properties,structure and SP-hybridization,preparation and reactions.
3
Alkynes. General Formula
CnH2n-2
C2H2 H—C C—H sp => linear, 180o
acetylene
ethyne
C3H4 CH3CCH
methylacetylene
propyne
NOMENCLATURENaming Alkynes
• General hydrocarbon rules apply with “-yne” as a suffix indicating an alkyne.
• Numbering of chain with triple bond is set so that the smallest number possible include the triple bond.
C C CH2H2C CH2 CH2 CH2 CH3H3C
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3-Nonyne
nomenclature:
common names: “alkylacetylene”
IUPAC: parent chain = longest continuous carbon chain that contains the triple bond.
alkane drop –ane add -yne
prefix locant for the triple bond, etc.
CH3CH2CCCH3 2-pentyne
ethylmethylacetylene
“terminal” alkynes have the triple bond at the end of the chain:
CH3
CH3CH2CCH HCCCHCH2CH3
1-butyne 3-methyl-1-pentyne
ethylacetylene sec-butylacetylene
Substitutive nomenclature: Similar to alkenes, but with the following differences:
1. The ending "-ene" is replaced with "-yne"CH3
CH3
C C CH3
4-Methyl-2-pentyne
2. The double bond has a priority over the triple bond when numbered
CCH CH2 CH CH2
1-Pentene-4-yne
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3. There are no cis-trans-isomers, because the triple bond is linear
Enynes
• An enyne has a double bond and triple bond.• Number for an Enynes starts at the multiple bond
closest to the end (it does not matter wheather it is a double or triple bond)
CC CH2 CH CH2CH2H3C
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1-Hepten-4-yne
Diyines and Triynes
• A compound with two triple bonds is a diyine.– A triyne has three triple bonds.
• Number from chain that ends nearest of triple bond.
CC CH2 C CHCH2H3C
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1,4-Heptdiyne
Alkynes as Substituents
Alkynes as substituents are called “alkynyl”.
H3C CH2 C C
1-butynyl
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Cycloalkynes• The smallest cycloalkyne isolated is
cyclononyne– the C-C-C bond angle about the triple bond is
approximately 156°
Physical Properties Similar to alkanes and alkenes of
comparable molecular weight and carbon skeletone.
0.766174-36
0.746125-79
0.71671-132
0.69040-900.69127-32
(a gas)8-126(a gas)-23-102(a gas)-84-81
Densityat 20°C(g/mL)
Boiling Point(°C)
MeltingPoint(°C)FormulaName
1-Decyne
1-Octyne
1-Hexyne
1-Pentyne2-Butyne
1-ButynePropyne
Ethyne HC CHCH3C CH
CH3C CCH3CH3(CH2 )2C CH
CH3CH2C CH
CH3(CH2 )3C CH
CH3(CH2 )5C CH
CH3(CH2 )7C CH
physical properties:1-Weakly or non-polar and have no H-bonding.
2-Relatively low m.p. or b.p.but they increase as the molecular weight increase.
3-Water insoluble but soluble in organic solvents.
4-Relatively acidic (have relative acidic character).
Acidity• A major difference between the chemistry of
alkynes and that of alkenes and alkanes is the acidity of the hydrogen bonded to a triply bonded carbon.
AcidityAcetylene reacts with sodium amide to form
sodium acetylide
it can also be converted to its metal salt by reaction with sodium hydride or lithium diisopropylamide (LDA)
++
pKa 38pKa 25Weaker
acidWeaker
baseStronger
baseStronger
acid
NH3–NH2HC CH HC C-
Sodium hydride Lithium diisopropylamide (LDA)
Na+H– [(CH3)2CH]2N– Li+
Acidity• Water is a stronger acid than acetylene;
hydroxide ion is not a strong enough base to convert acetylene to its anion
Keq = 10-9.3
pKa 15.7pKa 25
-- ++
Stronger acid
Strongerbase
Weakerbase
Weakeracid
HC CH HC COH H2O
Electronic Structure of Alkynes• Carbon-carbon triple bond result from • sp hybridized orbital on each C forming a sigma
bond at 180º • unhybridized 2pX and 2py orbitals forming a 2
bond• The formed C-C triple bond is shorter and stronger
than single or double• Breaking a bond in acetylene (HC=CH) requires
318 kJ/mole (in ethylene it is 268 kJ/mole)
C C HH18
In other words, in alkynes: 1-One- and two - bonds form a triple bond.
2-The valence shell of the atom of carbon has one s-orbital and three p-orbitals.
3- The carbon is bonded by two-bonds, each of two p-orbitals participates in the formation of this - bond
4-The remaining one s-orbital and one p-orbital are equally involved in the formation of -bonds, giving rise to the sp-hybridization state.
So,the presence of sp-hybridized carbons is characteristic for alkynes.
- and 2-bonds in alkynes
-bondsp
s
C C HH
p
CH CH
-bond -bond
Preparation
Preparation of Alkynes1.Elimination Reactions of Dihalides or
Dihydrohaloalkenes
• Treatment of a 1,2 dihydrohaloalkene with KOH or NaOH (strong Base) produces a two-fold elimination of HX
CCH3C H
Cl CH2 CH3
1) 2 NaNH2
2) H3O+CH3C CH2 CH3
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dehydrohalogenation of vicinal dihalides
H H H | | |— C — C — + KOH — C = C — + KX + H2O | | | X X X
H | — C = C — + NaNH2 — C C — + NaX + NH3
| X
Preparation of Alkynes: Vicinal Dihalides
• Vicinal dihalides are available from addition of bromine or chlorine to an alkene.
• Intermediate is a vinyl halide.
C CH
H Br2
CH2Cl2
C CH
HBr Br
2 KOH
Ethanol+ 2 H2O + 2 KBr
A vicinal dibromide
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RR XXSSNN22
XX––::++CC
––
::H—C H—C C—RC—RH—C H—C ++
2.Alkylation of Acetylene and Terminal Alkynes
The alkylating agent is an alkyl halide, andthe reaction is nucleophilic substitution.The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne.
NaNHNaNH22
NHNH33
CHCH33CHCH22CHCH22CHCH22BrBr
(70-77%)(70-77%)
HCHC CHCH HCHC CCNaNa
HCHC CC CH2CH2CH2CH3
Example: Alkylation of Acetylene
NaNHNaNH22, NH, NH33
CHCH33BrBr
CCHH(CH(CH33))22CHCHCHCH22CC
CCNaNa(CH(CH33))22CHCHCHCH22CC
(81%)(81%)
C—CHC—CH33(CH(CH33))22CHCHCHCH22CC
Example: Alkylation of a Terminal Alkyne
1. NaNH1. NaNH22, NH, NH33
2. 2. CHCH33CHCH22BrBr
(81%)(81%)
H—C H—C C—HC—H
1. NaNH1. NaNH22, NH, NH33
2. 2. CHCH33BrBr
C—HC—HCHCH33CHCH22—C—C
C—C—CHCH33CHCH33CHCH22—C—C
Example: Dialkylation of Acetylene
Reactions of Alkynes
1. Alkyne Adition Reactions
a.Reactions of Alkynes: Addition of HX and X2
• Addition reactions of alkynes are similar to those of alkenes
• Intermediate alkene reacts further with excess reagent• Regiospecificity according to Markovnikov
CHC CH2 CH3
HBr
CH2Cl2CC
H Br
H CH2 CH3
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b.Addition of Bromine and Chlorine
• Initial addition gives trans intermediate.• Product with excess reagent is tetrahalide.
CHC CH2 CH3
Br2
CH2Cl2CC
H Br
Br CH2 CH3
Br2
CH2Cl2C
Br
H
Br
C
Br
Br
C
H
H
C
H
H
H
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c.Addition of HX to Alkynes Involves Vinylic Carbocations
• Addition of H-X to alkyne should produce a vinylic carbocation intermediate– Secondary vinyl carbocations form less readily than
primary alkyl carbocations– Primary vinyl carbocations probably do not form at all
C C H Br C CH
Br- C
H
C+
Br-
H
C C
Br
+
HC CH H Br C CH
Br- C
H
C+
Br-
C C+H
Br
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d.Hydration of Alkynes
• Addition of H-OH as in alkenes– Mercury (II) catalyzes
Markovinikov oriented addition
– Hydroboration-oxidation gives the non-Markovnikov product
CHC CH2 CH3
BH3 THF
H2O2 OH-CC
H H
HO CH2 CH3
Anti- Markinov
CHC CH2 CH3
HgSO4, H2SO4 CCH OH
H CH2 CH3H2O
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e.Mercury(II)-Catalyzed Hydration of Alkynes
• Mercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation
• The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone
36
Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes
• Addition of Hg(II) to alkyne gives a vinylic cation• Water adds and loses a proton• A proton from aqueous acid replaces Hg(II)
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Keto-enol Tautomerism• Isomeric compounds that can rapidily interconvert by the
movement of a proton are called tautomers and the phenomenon is called tautomerism
• Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon
• The keto form is usually so stable compared to the enol that only the keto form can be observed
OH
H
H
Enol Tautomer
Rapid
(Less favored)
O
H
H
Enol Tautomer
(More favored)
H
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f.Hydration of Unsymmetrical Alkynes• If the alkyl groups at either end of the C-C triple bond are not the
same, both products can form and this is not normally useful• If the triple bond is at the first carbon of the chain (then H is what is
attached to one side) this is called a terminal alkyne• Hydration of a terminal always gives the methyl ketone, which is
useful
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g.Hydroboration/Oxidation of Alkynes
• BH3 (borane) adds to alkynes to give a vinylic borane
• Oxidation with H2O2 produces an enol that converts to the ketone or aldehyde
• Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration
40
Comparison of Hydration of Terminal Alkynes
• Hydroboration/oxidation converts terminal alkynes to aldehydes because addition of water is non-Markovnikov
• The product from the mercury(II) catalyzed hydration converts terminal alkynes to methyl ketones
41
2.Reduction of Alkynes
a.Addition of H2 over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction)
b.The addition of the first equivalent of H2 produces an alkene, which is more reactive than the alkyne so the alkene is not observed
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c.Conversion of Alkynes to cis-Alkenes
• Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene
• The two hydrogens add syn (from the same side of the triple bond)
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d.Conversion of Alkynes to trans-Alkenes
• Anhydrous ammonia (NH3) is a liquid below -33 ºC– Alkali metals dissolve in liquid ammonia and function as
reducing agents• Alkynes are reduced to trans alkenes with sodium or lithium
in liquid ammonia• The reaction involves a radical anion intermediate (see Figure
8-4)
44
3.Oxidative Cleavage of Alkynes
• Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acids
• Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide
• Neither process is useful in modern synthesis – were used to elucidate structures because the products indicate the structure of the alkyne precursor
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4.Alkyne Acidity: Formation of Acetylide Anions• Terminal alkynes are weak Brønsted acids (alkenes and
alkanes are much less acidic (pKa ~ 25. See Table 8.1 for comparisons))
• Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion
• The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus (see figure 8-5)
46
Alkylation of Acetylide Anions• Acetylide ions can react as nucleophiles as well as bases
(see Figure 8-6 for mechanism)• Reaction with a primary alkyl halide produces a
hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes
47