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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211 LSW ALKYNES F04

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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211 LSW ALKYNES F04

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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211 LSW ALKYNES F04

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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