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Alkynes (CnH2n-2)

Electronic Structure of Alkyne

Physical Properties of Alkynes

-relatively nonpolar.

-have low density

- low water solubility

- soluble in most organic solvents including acetone, diethyl ether, dichloromethane,chloroform and alcohols.

-acetylene, propyne and butyne are gases at room temperature

Example of Alkyne

Acetylene

-Most important commercial alkyne

-used as fuel for oxyacetylene welding torch-colorless- foul smelling gas that burns in air with a smoky sooty flame. When flame is

supplied with pure O2, the color turns to light blue, flame temperature

increases dramatically

- oxyacetylene torch can reach as high as 2000oC.\ -gives most heat per mole of product.

- source: calcium carbide.

C C

sp sp

C CH H

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Classification of alkynes

Terminal Alkyne

General formula

R C C H

where R is an alkyl chain

Examples:

H3C C C H

CH3CH2 CH2 C C H

CH2CH2 CH2CH3CCH

Internal Alkyne

General formula

R C C R'

where R and R are alkyl chains

Examples:

H3C C C CH3

H3C C C CH2CH3

CH3CH2 C C CH2CH3

-Terminal alkynes are weakly acidic

-reacts only with very strong bases such as sodium amide (NANH2) in liquidammonia solution.

-Alkynide is electron rich and a potential nucleophile.

R C C H + Na+

NH2-

NH3R C C Na+ + NH3

terminal alkyne sodium amidesodium alkynide ammonia

Internal alkynes are not acidic at all.

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Reactions of Alkynes

1. Addition of HX (Hydrohalogenation)

2. Addition of X2 (Halogenation)

3. Addition of H2O catalyzed by HgSO4/H2SO4 (Hydration)

4. Hydroboration/Oxidation

R C C R HX

whereHX = HCl,HBr or HI

R

C C

H

X

R

C CR

H

H

R

X

XHX

Markovnikov's Addition

R C C R X2

whereX2 = Cl2 or Br2

RC C

X

X

RX2

C CR

X

X

R

X

X

(Anti addition)

R C C H

RC C

HO

H

H H+H2O

(HgSO4, H2SO4)C

R CH3

O

Ketoneenol

enolAldehyde

CH2CH

R

O

RC C

H

H

OHR C C H

1. BH3

2.H2O2, OH-

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Reactions of Alkynes (continuation.1)

5. Addition of H2 (Hydrogenation)

Lindlar catalyst- finely divided palladium metal that has been precipitated onto a calciumcarbonate support and then deactivated by treatment with lead acetate, and quinoline, an

aromatic amine.

R C C RH2

LindlarCatalyst

C

R

H

C

R

H

H2

LindlarCatalyst

C CH

H

H

H

H

H

H2/Pd(C)

syn addition

6. Conversion into acetylide anions

7. Acetylide ion alkylationR C C + R' CH2Br R C C CH2R' + NaBr

alkyl halide

must be primary

8. Oxidative cleavage

Lindlar

Catalyst

Syn Addition

H2C CR

H

H

R

H

H

HC C

R

H

RH2

R C C R

R C C R' LiNH3

C C

H

R

R'

HNH3

Li C C RR

H

H

H

H

anti-addition

NH3+Na+

sodium alkynide

R C CNaNH2

NH3R C C H

R C C HKMnO4

H3O+

RCOOH + CO2