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Alkynes. Nomenclature Physical Properties Synthesis Reactions. Alkynes. Contain at least one carbon-carbon triple bond (C ≡ C). Also called acetylenes. Properties of C ≡ C Bonds. sigma ( σ ) bond. pi ( π ) bonds. C ≡ C consists of a σ bond and two π bonds.

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Physical Properties




  • Contain at least one carbon-carbon triple bond (C≡C).

  • Also called acetylenes.

Properties of c c bonds
Properties of C≡C Bonds

sigma (σ) bond

pi (π) bonds

  • C≡C consists of a σ bond and two π bonds.

  • C≡C is shorter than a C=C bond.

Properties of c c bonds1
Properties of C≡C Bonds

  • The pi bonds force the four atoms involved to be linear.

  • The e- density of the pi bonds is a cylinder surrounding the two C atoms.


  • The pi bonds are relatively easy to break, which makes C≡C a functional group.

  • The pi bonds block nucleophilic attack.

Nomenclature of alkynes
Nomenclature of Alkynes

  • When the chain contains more than three C atoms, use a number to give the location of the triple bond.

  • Terminal alkynes have one H on the triple-bonded C. Internal alkynes do not.




internal alkyne



terminal alkyne

Ir of alkynes
IR of Alkynes


a terminal alkyne

Ir of alkynes1
IR of Alkynes


an internal alkyne

Nomenclature of alkynes1
Nomenclature of Alkynes

  • Apply the same rules you learned for the alkanes.

  • Use the root name of the longest chain containing the triple bond, but change -ane to -yne.

  • In 8th edition of Wade, alkenes and alkynes are given equal priority, but “-en” is first alphabetically, so

    • The numbering starts at the end closer to the alkene, and

    • the order of naming is “en-yne.”

Nomenclature of alkynes2
Nomenclature of Alkynes

  • Name the following:



Nomenclature of alkynes3
Nomenclature of Alkynes

  • Alkynes as substituents are called alkynyl groups.


Uses and synthesis of acetylene
Uses and Synthesis of Acetylene

  • Main use is as a fuel for oxyacetylene welding.

    • It is one of the cheapest organic elements.

  • Synthesized from

    • coal: 3C + CaO CaC2 +CO

      CaC2+2H2O HC≡CH + Ca(OH)2

    • natural gas: 2CH4 HC≡CH + 3H2

      • driven by ∆S, high T, and 0.01s heating time

Stability of acetylene
Stability of Acetylene

  • Acetylene (HC≡CH) is thermodynamically unstable:

    • HC≡CH(g)  2C(s) + H2(g)

    • This can happen to the compressed gas.

    • produces a very hot flame when burned in pure oxygen

Physical properties of alkynes
Physical Properties of Alkynes

  • Similar to alkanes and alkenes of comparable molecular weight.

    • nonpolar

    • virtually insoluble in water

    • soluble in organic solvents

Physical properties of alkynes1
Physical Properties of Alkynes

  • Terminal alkynes (-C≡C-H) have an acetylenic Hthat is more acidic than the H’s on other hydrocarbons due to the greater s character of the sp bond. The greater s character makes the ≡C-H bond more polar and the acetylenic H more acidic.

    • pKa of terminal acetylenes ≈25

    • pKa of alkanes ≈ 50

    • pKa of NH3 is 35 (so -:NH2 reacts with -C≡C-H)

    • pKa of alcohols ≈ 16 (and alkoxides don’t)

Formation of acetylide ions
Formation of Acetylide Ions

  • From the reaction of a terminal alkyne with sodium amide

    • CH3CH2C≡CH + NaNH2 CH3CH2C≡C:- Na+ + :NH3

    • CH3C≡CCH3 + NaNH2 NR

  • Acetylide ions are strong nucleophiles.

  • OH- and RO- are not strong enough to remove the terminal H.

Synthesis of alkynes
Synthesis of Alkynes

  • from acetylides(lengthens the C skeleton)

    • an excellent way to make a more complex alkyne

      • alkylation of an acetylide

      • addition to carbonyl groups

  • by elimination reactions

Synthesis of alkynes1
Synthesis of Alkynes

  • from acetylides (lengthens the C skeleton) - an SN2 reaction

    • alkylation of an acetylide

      • if the halide is 2°, there will also be the elimination product…which would be what?

Synthesis of alkynes2
Synthesis of Alkynes

  • Predict the product:

Synthesis of alkynes3
Synthesis of Alkynes

  • from acetylides (lengthens the C skeleton)

    • addition to carbonyl groups

      • the acetylide is the nucleophile

      • addition to aldehydes gives 2° alcohols

      • addition to ketones gives 3°alcohols

Synthesis of alkynes4
Synthesis of Alkynes

  • from acetylide addition to a carbonyl group

an alkoxide ion

a 3° alcohol

Synthesis of alkynes5
Synthesis of Alkynes

  • Predict the product:

Synthesis of alkynes6
Synthesis of Alkynes

  • by elimination reactions

    • A vicinal dihalide or a geminaldihalide can undergo a double dehydrohalogenation to form the alkyne

    • This requires STRONGLY BASIC conditions, and many compounds can’t “take it.”

      • KOH in a sealed tube heated to 200°C

      • NaNH2, 150°C

Dehydrohalogenation with koh
Dehydrohalogenation with KOH

  • The heated base is so strong that the triple bond can migrate along the carbon chain to form the more stable internal alkyne.

terminal alkyne, will rearrange

internal alkyne

Dehydrohalogenation with nanh 2
Dehydrohalogenation with NaNH2

  • NaNH2 is even stronger than fused KOH. It is so strong that it traps the terminal alkyne as the sodium salt, and no rearrangement occurs.

major component

terminal alkyne, will not rearrange

Synthesis of alkynes7
Synthesis of Alkynes

  • Predict the product:

Reactions of alkynes
Reactions of Alkynes

  • Additions

    • reduction to alkanes

    • reduction to alkenes

    • addition of halogens

    • addition of HX

    • addition of water

      • Markovnikov

      • anti-Markovnikov

  • Oxidations

    • to α–diketones

    • cleavage

  • Nucleophilicattack on electrophiles

    • covered in synthesis

Reactions of alkynes1
Reactions of Alkynes

  • Reduction to alkanes by hydrogen.

    • second π bond energy = 226 kJ

    • first π bond energy = 264 kJ

    • Alkynes can undergo double additions

      • These typically go all the way to the alkane.

        R-C≡C-R’ + 2H2(g)  RCH2CH2R’

Pt, Pd, or Ni

Reactions of alkynes2
Reactions of Alkynes

  • Reduction to cis-alkenes

  • the syn addition of hydrogen can be stopped at the alkene stage if a poisoned catalyst is used

    • Ni2B

    • Lindlar’s catalyst: Pd/BaSO4 poisoned with quinoline (in CH3OH to dissolve C≡C)

Reactions of alkynes3
Reactions of Alkynes

  • Reduction to trans-alkenes

  • Na/NH3(l) required

    • Na + NH3(l)  e- •NH3 + Na+

    • The H’s come from NH3.

    • The solvated e- leads to a vinyl radical, which is more stable in the trans geometry.

Reactions of alkynes4
Reactions of Alkynes

  • Addition of halogens

    • One mole X2, syn or anti addition leading to cis- or trans-alkenes

    • Two moles X2 leads to the tetrahalides

Reactions of alkynes5
Reactions of Alkynes

  • Addition of HX

    • Reaction proceeds through a vinyl cation intermediate then a carbocation intermediate, so the addition is Markovnikov.

    • If a peroxide is used with HBr, the anti-Markovnikov product will be formed. Why?

Reactions of alkynes6
Reactions of Alkynes

  • Markovnikov addition of water

    • HgSO4/H2SO4 catalyst

    • product is a ketone, not an alcohol

Keto enol conversion
Keto-enol conversion

  • Can occur in acid or base, but the mechanisms are different.

    • The mechanism shown is the conversion in acid.

Reactions of alkynes7
Reactions of Alkynes

  • anti-Markovnikov addition of water

    • hindered dialkylborane needed

    • product is an aldehyde, not an alcohol

Reactions of alkynes8
Reactions of Alkynes

  • Permanganate oxidations

    • nearly neutral conditions needed

    • product is an α-diketone, not a diol

    • terminal alkynes give keto acids

Reactions of alkynes9
Reactions of Alkynes

  • Permanganate oxidations

    • warm, basic conditions cause cleavage

    • products are salts of carboxylic acids

    • (A second step, acidification, is needed to produce the acids themselves.)

Reactions of alkynes10
Reactions of Alkynes

  • Ozonolysis followed by hydrolysis

Reactions of alkynes11
Reactions of Alkynes

  • Predict the products

Reactions of alkynes12
Reactions of Alkynes

  • Predict the products