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Alkynes: Unsaturated Hydrocarbon

Jully TanSchool of Engineering

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Alkynes

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Naming Alkynes

Step 1: Find the longest chain containing the triple bond.Step 2: Change –ene or -ane ending to -yne.Step 3: Number the chain, starting at the end closest to the triple bond.Step 4: Give branches or other substituents a number to locate their

position.

Example

CH3 C CH

propyne

CH3 C C CH2 CH2 Br

5-bromo-2-pentyne

CH3 CH

CH3

CH2 C C CH

CH3

CH3

2,6-dimethyl-3-heptyne

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Step 5: Substitutive nomenclature: As a substituent, an alkyne is named alkynyl.

Step 6: The double bond has a priority over the triple bond when numbered

CCH CH 2 CH CH2

1-Pentene-4-yne

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There are no cis - trans -isomers, because the triple bond is linear

C C H CH2 C C HEthynyl 2-propynyl

CH2 CH CH2 CH

CH3

C CH

4-methyl-1-hexen-5-yne

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Example of Naming Alkynes

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Carbon-carbon triple bond results from sp orbital on each C forming a sigma bond and unhybridized pX and py orbitals forming π bonds.The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond.The bond is shorter and stronger than single or doubleBreaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)

π1

π2σ

CH3 CH3 CH2 CH2 CH CH

1.54 Å 1.33 Å 1.20 Å

1. Electronic Structure Of Alkynes

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Alkynes show three types of isomerism,Chain isomerism Position isomerism Functional isomerism.

Chain isomerism It is due to the different arrangement of carbon atoms in the chain i.e., straight chain or a branched-chain.

Position isomerism It is due to the difference in the location of the triple bond

2. Isomerism in Alkynes

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Functional isomerism Alkynes are isomeric with alkadienes both being represented by the general formula CnH2n-2. So there can be a difference in the nature of the bond (triple to double) giving different functional properties.

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Boiling points of alkynes are close to the boiling points of alkenes and alkanes.

C C HH

Stretch ~2100 cm-1 Strech ~ 3300 cm -1

3. Boiling Point of Alkynes

Alkyne have lower densities, than water and they are insoluble in water.

4. Density & Solubility

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Synthesis of Alkynes & Reaction of Alkynes

Additional to Triple BondHydrogenHalogenHydrogen Halide (HX)Hydration

Oxidation of triple bond

SynthesisDehydration of alkyl dihalidesReaction with 1° alkyl halides

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Synthesis of AlkynesMethod 1: Dehydration of alkyl dihalides using a strong base

1. Vicinal dihalides (compounds with halogen atoms on adjacent carbon atoms)2. Treatment of vicinal dihalides with strong bases, alcoholic KOH and sodium amide

results in alkynes.3. Vicinal dihalides obtained from addition of halogen to alkenes.

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Method 2: Reaction of sodium acetylide with 1° alkyl halides

1. Acetylene & monosubstituted acetylenes contains a H atom attached to triple bonded C atom: Acetylenic hydrogen

2. Acetylenic H are acidic and replaced by certain metals forming salts: metal acetylides.

R C C Na+ + R'CH 2 X R C C CH 2R' + NaX

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Addition Reaction (i) Hydrogenation

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Conversion of Alkynes to cis-Alkenes

Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkeneThe two hydrogens add syn (from the same side of the triple bond)

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Conversion of Alkynes to trans-Alkenes

Anhydrous ammonia (NH3) is a liquid below -33 ºCAlkali metals dissolve in liquid ammonia and function as reducing agents

Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammoniaThe reaction involves a radical anion intermediate

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Addition Reaction (ii) Halogenation

Alkynes reacts with equivalent of halogens forming tetrahalides.

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Electrophilic addition to triple bonds proceeds slower, than addition to double bonds and often requires a catalyst. The Markovnikov’s rule is as valid as for the addition to double bonds. HBr in the presence of peroxides adds against the rule.

2 Step involved: Step 1: producing of haloalkene stageStep 2: Producing gem-dihalide stage (twin =both halogen at

the same

Addition Reaction (iii) Hydrohalogenation

CH3

C CH3

Br

Br

HBr CH3

C

CH2

Br

CH3

C CH HBrperoxides CH3

C

CH

H

74%CH3

C

CH2

Br

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Addition of HX to Alkynes Involves Vinylic Carbocations

Addition of H-X to alkyne should produce a vinylic carbocation intermediate

Secondary vinyl carbocations form less readily than primary alkyl carbocationsPrimary vinyl carbocations probably do not form at all

Nonethelss, H-Br can add to an alkyne to give a vinyl bromide if the Br is not on a primary carbon

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Addition Reaction (iv) HydrationDifferences from hydration of alkenes:1. A Hg2+ catalyst is required2. The product of addition is unstable and isomerizes.3. Water adds and loses a proton.4.A proton from aqueous acid replaces Hg(II)

RC CH OH2

HgSO4, H2OC C

R

OH

H

HC C

R

O

H

HH+

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Mechanism of Mercury(II)-Catalyzed Hydration of Alkynes

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Oxidative Cleavage of AlkynesStrong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing two carboxylic acidsTerminal alkynes are oxidized to a carboxylic acid and carbon dioxideNeither process is useful in modern synthesis – were used to elucidate structures because the products indicate the structure of the alkyne precursor

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