211 LSW ALKYNES F04
Alkynes Doubly unsaturated C≡C functional group Terminal alkynes, R-C≡C-H
1) 2) 3) 4)
Nomenclature: Functional group suffix = Disubstituted alkynes, R-C≡C-R' Monosubstituted alkynes, R-C≡C-H
Stability:
Substituted alkynes
The "extra" π bond in an alkyne than the alkene π bond by kJ/mol or kcal/mol
∆Hh 1-hexyne = kJ/mol or kcal/mol ∆Hh 1-hexene = kJ/mol or kcal/mol
C≡C - C=C = kJ/mol or kcal/mol C=C - C-C = kJ/mol or kcal/mol
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Rank 1-hexyne (terminal) and 3-hexyne(internal) for each of the following properties:
Heat of hydrogenation
Heat of formation
Heat of combustion
Stability
Structure: Hybridization:
Shape:
C CY X C CY X
Physical Properties:
Polarity: Volatililty: Melting points: Boiling points: Aqueous solubility: Organic solubility: Acidity: pKa =
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Reactivity: Alkynes act as nucleophiles. Addition reactions:
Terminal alkynes: R-C≡C-H
C CR HB:
C CR + H-B Acetylide ion:
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Reactions:
Alkyne Preparation
C C
XX
H H
NaNH2
C C
XH
H X
NaNH2
C C
C C
+ 2 HX
+ 2 HX
Hydrogen-ation
C CH H
H2 / CatalystC C
Metal Reduction C C
H
H
Na / NH3(l)C C
Hydrogen Halides
C CXH
C C + 1 HX
C C
XHC C
xs HX
H X
Hydration C COHH
C C + H2OH+
Hg2+ C COH
H
Halogen-ation
C CX2 C C
X
X
Ozonolysis
C COHHO
C C + O3 Then H2OOO +
Deproton-ation C C HR C CR
B:+ H-B
Alkylation C C H C CR1. NaNH2
2. R'XR'R
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Preparation of Alkynes
C C
XX
H H
NaNH2
C C
XH
H X
NaNH2
C C
C C
+ 2 HX
+ 2 HX
Reaction Type:
Overall reaction: Leaving groups: Requires:
Regioselectivity: Stereospecificity:
Mechanism:
LG
H + LGB:
Leaving Group Base: Selectivity:
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What is the alkyne product from the reactions of the following with NaNH2 : 1,2-dibromopropane 2,3-dibromooctane 1,2-dibromohexane 2,2-dibromohexane Synthesis:
Try it: 2-methyl-4-octene 2-methyl-4-octyne
E2 mechanism (For both alkenes and alkynes)
Leaving Groups: -LG Base: B
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Stereochemistry E2 reactions
180º
0º Synperiplanar
Regioselectivity:
Zaitsev's rule: Zaitsev product: Hoffman product: Stereoselectivity:
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Addition Reactions of Alkynes Two factors influence the relative reactivity of alkynes compared to alkenes:
Hydrogenation of Alkynes
C CH H
H2 / CatalystC C
Reaction type: Overall reaction: Regioselectivity: Stereoselectivity: Requirements:
Alkanes: Alkenes:
Lindlar's catalyst: Pd / CaCO3 / quinoline Mechanism: CATALYTIC HYDROGENATION Step 1: Step 2: Step 3: Step 4:
H HH H H H
H H H H
H HH H
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Dissolving Metal Reduction of Alkynes
C CH
H
Na / NH3(l)C C
Reaction type: Overall reaction: Reactivity order: Regioselectivity: Stereoselectivity:
Requirements: Mechanism: Step 1: Step 2: Step 3: Step 4:
C C CH3CH3
Na
C CCH3
CH3
Na
NH2 HC C
CH3
CH3H
Na
C C
CH3
CH3H
C C
CH3
CH3H
NaNH2
H
NH2 H
+ + +
Na+
Synthesis:
How would you prepare trans 2-pentene from cis 2-pentene?
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Reaction of Alkynes with Hydrogen Halides C C
XHC C + 1 HX
C C
XHC C
xs HX
H X
Reaction type: Overall reaction: Reactivity order: Regioselectivity:
C CXH
CH3HC C HCH3 + 1 HX C C
HX
CH3H+
Stereoselectivity: Requirements:
C CXH
CH3X
C C HCH3 HX
C CHX
CH3 H+H
HX H
CH h
C CXH
CH3HC C H3 + 1 HX C C
HX
CH3H+
υ Mechanism:
C C HCH3H+
CH3
Br- Br
CH3H H
Br
CH3 H
HH
HH
+Br
-
CH3
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Hydration of Alkynes
C CH+
Hg2+ C COHH
C C
OH
H+ H2O
Reaction type: Overall reaction: Regioselectivity: Stereoselectivity: Requirements: Mechanism:
C C HCH3H+
CH3
H2O
H
+ O
CH3H
H
H H+
H2O O
CH3H
H
HO
CH3H
H
H +H+ H2O O
CH3H
H
Tautomerization: enol and keto:
HNN N
NNH
N
O
O NHH
H
T A
HNN N
NHNH
N
O
O
T GOH
OH
H
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Halogenation of Alkynes
C CX2 C C
X
X Reaction type: Overall reaction: Reactivity order: Regioselectivity: Stereoselectivity:
Requirements: Mechanism:
Br Br
Br
Br
Br
+ Br_
+
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Ozonolysis of Alkynes
C COHHO
C C + O3 Then H2OOO +
Reaction type: Overall reaction: Requirements: Mechanism: What would be the products of the ozonolysis: ethyne ? 1-butyne ? 2-butyne ? cyclooctyne ?
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Acidity of Terminal Alkynes
C C HR C CRB:
+ H-B Terminal alkynes:
C C H C CH C C H
Stability of the conjugate base (i.e. the carbanion).
C C C C C C
s character: (sp = %, sp2 = % and sp3 = %) Increased s =
= Conjugate base of alkyne =
= Mechanism:
C C HR C CRB:
+ H-B Could you use a NaOH or NaOEt for this reaction ? pKa H2O = pKa CH3CH2OH = pKa NH3 =
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Alkylation of Alkynes: Acetylide ion:
C C HR C CR1. NaNH2
2. R'XR'
Reaction type: Overall reaction: Reactivity order:
Alkyne: Alkyl halide: Halide: Regioselectivity: Stereoselectivity: Requirements:
Mechanism:
HCH3 NH2 CH3
RCH2BrCH2RCH3+
_
+ NH3
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Examples: Ethylene (acetylene):
monosubstituted (R-C≡C-H):
symmetrical (R = R'): R-C≡C-R':
unsymmetrical (R ≠ R') R-C≡C-R':
Problems to try:
1. What is the product of the reactions of CH3-C≡C- with: (a) 2-bromopropane (b) ethanol (c) 1-iodooctane (d) ethyl tosylate (e) (R)-2-bromohexane (f) bromobenzene
2. What would be the product from the reaction of propene with the following reagent sequence:
(i) Br2 (ii) NaNH2 , heat (iii) NaNH2 then MeI
Final product:
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3. For the reaction 2-butyne with HBr:
(i) reaction with 1 equivalent gives 2-bromo-2-butene (ii) reaction with excess gives 2,2-dibromobutane
(a) What conclusions can you reach about the reactivity of 2-bromo-2-butene compared to 2-butyne?
(b) Consider the two possible carbocations that could be formed in the conversion of 2-bromo-2-butene to 2,2-dibromobutane. Suggest a reason for the observed regioselectivity.
5. Show the major products, with stereochemistry where applicable, for the reactions of: With: 1-pentyne 2-pentyne
(i) excess H2 / Pd
(ii) H2 / Lindlar Pd
(iii) Na / NH3
(iv) 1 equiv. HCl
(v) excess HCl
(vi) aq. H2SO4, HgSO4
(vii) excess Br2
(viii) O3 then H2O
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