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ALKYNES

CnH2n-2

Dr. Marwa Eid

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AlkyneAlkynes contain a triple bond.General formula is CnH2n-2

Two elements of un-saturation for each triple bond.

Some reactions are like alkenes: addition and oxidation.

Some reactions are specific to alkynes.

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NomenclatureFind the longest chain containing the triple

bond.Change -ane ending to -yne.Number the chain, starting at the end closest

to the triple bond.Give branches or other substituent a number

to locate their position.

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CH3 CH

CH3

CH2 C C CH

CH3

CH3

CH3 C C CH2 CH2 Br

CH3 C CH

propyne

5-bromo-2-pentyne

2,6-dimethyl-3-heptyne

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CH3 C CH

methylacetylene

CH3 CH

CH3

CH2 C C CH

CH3

CH3

isobutylisopropylacetylene

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4-methyl-1-hexen-5-yne

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AcetyleneAcetylene, the simplest alkyne

Acetylene is used in welding torches.

(In pure oxygen, temperature of flame reaches 2800C)

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Synthesis of AcetyleneHeat coke with lime in an electric furnace to

form calcium carbide.Then drip water on the calcium carbide:

coke limeC CaO3 + +CaC2 CO

H C C H Ca(OH)2CaC2 + 2 H2O +

Calcium carbide

Electric furnace

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Synthesis of AcetyleneAcetylene is produced industrially from

methane and steam at high temperature

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Alkyne Addition Reactions:

1. Hydrogenation

2. Halogenation

3. Hydration

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HydrogenationAddition of Hydrogen in the presence of

metal catalyst (Pd, Pt, or Ni) to reduce alkyne to alkane, completely saturated.

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HydrogenationAddition of Hydrogen in the presence of

metal catalyst (Pd, Pt, or Ni) to reduce alkyne to alkane, completely saturated.

You can use a reduced activity catalyst (Lindlar), Pd and Pb, which stops at the alkene. You obtain a cis

A Lindlar catalyst is a catalyst that consists  of palladium  deposited on calcium carbonate treated with various forms of lead.

Hydrogenation

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Addition of HalogensCl2 and Br2 add to alkynes to form vinyl

dihalides.Product is mixture of cis and trans

isomers.Difficult to stop the reaction at dihalide.

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Addition of HX

Addition of HCl, HBr, and HI add to alkynes to form vinyl halides.

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HydrationReaction of alkynes with water

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