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Alkynes: Properties

Alkynes: Physical Properties

Low-molecular alkynes have a slightly higher boiling point than the corresponding alkenes. The boiling point of internal alkynes is higher than their terminal isomers. At roomtemperature, ethyne, propyne and but-1-yne are gases, but starting with but-2-yne, alkynes with medium molecular weight are liquids. Since alkynes show a great tendency to polymerize, sometimes violently, they have to be handled carefully. Though ethyne explodes under pressure, it is available as a solution in acetone in gas cylinders filled with porous material, such as kieselguhr or pumice. If not symmetrical, the high s character of the sp hybrid orbitals is the reason asymmetric alkynes exhibit a dipole moment.

Tab.1
Properties
FormulaNameMp [ °C] Bp [ °C]
C 2H 2 Ethyneunder pressure: -81.5 Tsub: -84.0
C 3H 4 Propyne-102.7-23.2
C 4H 6 But-1-yne-122.58.1
C 4H 6 But-2-yne-32.327.0
C 5H 8 Pent-1-yne-90.039.3
C 5H 8 Pent-2-yne-101.055.5
C 6H 10 Hex-1-yne/-132.071.0
C 6H 10 Hex-2-yne-88.084.0
C 6H 10 Hex-3-yne-105.081.0

Physical properties and reactivity of alkynes are determined by the sp hybridization of the C atoms. Though a C≡C triple bond is shorter and stronger than a C=C double or even a C-C single bond, the bonding energy clearly is less than the threefold energy of a single bond. The π bond, too, is weaker than the one in alkenes. These facts explain the hight reactivity of alkynes.

Tab.2
Bonding energy
Bonding energy [ kJmol-1 ] Difference of bonding energies [ kJmol-1 ]
C-C Single bond348
263
C=C Double bond611
216
C≡C Triple bond837

Similar to alkenes, higher substituted alkynes are more stable than their lower substituted (terminal) isomers. The reason for this behavior is the hyperconjugation of the π orbitals with the sp3 hybrid orbitals of the alkyl groups. For example, terminal alkynes can be isomerized to internal ones with strong base.

Compared to its components (graphite and hydrogen gas), ethyne itself is relatively unstable (enthalpy of formation HB = +227.4 kJmol-1 ). Therefore, alkynes in many cases react with release of energy. Ethyne explosively disintegrates into its components under the influence of small catalytic amounts of copper. Because of hyperconjugation, internal alkynes are more stable than terminal ones.

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