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Chamras Glendale Community College

Organic Chemistry 105

Examination 5 Materials

Chapter 9: Alkynes General Description: Hydrocarbons with at least 1 triple bond. General Formula: Cn H2n-2 Example: C3H4 Structural Formula: Nomenclature: Saturated Hydrocarbon Rootname + “yne” prop ane propyne Other Example: Physical Properties & Structural Features:

H3C C CH OR

C R'R C !-Bond

"-Bond

"-Bond

C

120pm

Bond Type BDE (kcal.mol–1)

C–C 83

C=C 146

C=C 200

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sp-Hybridization : Relative Electronegativity Acidity:

Alkanes Alkenes Alkynes

% s-character:

pKa: Synthetic Advantage Based on Acidity of Alkyne Hydrogens: An Acetylide Ion Example:

R H

N

HH

Na

N

HH

Na

H

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Synthesis of Alkynes: 1. By Reaction between Acetylide Ion and Alkyl Halides: Mechanism: 2. By Reaction between Acetylide Ion and Carbonyl Groups: ***A Brief Look at the Chemistry of Carbonyl Group Example:

H

1. NaNH2

2. C2H5Br

C

O

H

1. NaNH2

2. CH2=O

3. H3O+

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Mechanism: ***Synthetic Example: Provide the starting materials to synthesize the following compound: 3. By Double Dehydrohalogenation of Dihaloalkanes:

OH

C C

X

R'

X

H

R

H

C C

H

R'

X

X

R

H

Strong Base

R C C R' + 2NH3 + 2NaX

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Example:

Mechanism: Disadvantage of this method:

Cl

Cl

KOH

200oC

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Reactions of Alkynes: Name Type

1. Hydrogenation Addition 2. Metal-Ammonia Reduction Addition 3. Hydrohalogenation Addition 4. Hydration Addition 5. Halogenation Addition 6. Ozonolysis Oxidation 7. Permanganate Oxidations Oxidation

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Hydrogenation General Equation: General Comments: • Functional Group Transformation: Alkyne Alkene Alkane (cis) • Rearrangement • The Choice of Catalyst: Pt, Pd, or Ni

• Stereochemistry of Addition:

Example: ***Can the hydrogenation process be stopped at the alkene stage?

R R' + 2H2

Catalyst

H

H

R

H

R

H

'

2H2, Ni

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How? Example:

Metal-Ammonia Reduction General Equation: General Comments:

• Functional Group Transformation:

• Isomerism of the product:

R R'

Na, NH3

H

R

R'

H

H2, Pd/BaSO4,

, CH3OH

N

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Mechanistic Example:

N

HH

H

+ Na

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Hydro-halogenation General Equation: General Comments:

• Functional Group Transformation: • Regiochemistry of Addition:

**In case of addition of 2 moles of hydrogen halide:

Mechanistic Examples:

R H

H

H

R'

X

HX

HBr

HBr

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Hydration

a) By addition of HgSO4, H2SO4, and H2O. General Equation: General Comments:

• Functional Group Transformation: • Regiochemistry of Addition:

***Keto-Enol Tautomerism:

Example:

R H

HgSO4, H2SO4, H2O

HgSO4, H2SO4, H2O

HgSO4, H2SO4, H2O

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b) By Hydroboration-Oxidation. Example: Hydroboration Oxidation General Comments:

• Functional Group Transformation: • Regiochemistry of Addition:

Mechanisms for Keto-Enol Tautomerization:

a) Acid-Catalyzed:

BH

disiamylborane

H

BSia2H

H2O2, NaOH3 3

H

OHH

H

OH

3

3

H

HO

H

O

HH

H

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b) Base-Catalyzed:

Halogenation General Equation: General Comments: • Functional Group Transformation:

• Stereochemistry of Addition:

• With 1 mole of Halogen:

Composition of Product (Isomeric Mixture): Examples:

R R' + 2X2

H

HO

H

O

H

2Cl2

Cl2

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Ozonolysis

General Equation: General Comments: • Functional Group Transformation:

Examples:

Permanganate Oxidations a) Under Neutral & Mild Conditions: General Equation:

R R'1. O3

2. H2O

1. O3

2. H2O

1. O3

2. H2O

RR 'KMnO4, H2O,

Neutral

RR '

OO

Internal Alkyne

HR

KMnO4, H2O,

Neutral

OHR

OO

Terminal Alkyne

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General Comments: • Functional Group Transformation:

Examples: b) Under Basic & Warm Conditions:

KMnO4, H2O,

Neutral

Internal Alkyne

Terminal Alkyne

KMnO4, H2O,

Neutral

H

RR 'KMnO4, H2O,

KOH, Heat

Internal Alkyne

HR

Terminal Alkyne

KMnO4, H2O,

KOH, Heat

R

O

O

R

O

O'

H3O+R

O

OH

R

O

OH'

H3O+

R

O

OH

C OO

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Examples:

H

KMnO4, H2O,

KOH, Heat

H3O+

KMnO4, H2O,

KOH, Heat

H3O+