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Alkynes: Properties

Alkynes: Electronic Structure

The simplest alkyne is ethyne, more frequently called by its trivial name acetylene. It possesses a linear structure and the C-C bond as well as the C-H bond are shorter than those in ethane or ethene.

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3D Model
Bond lengths and bond angle
Tab.1
Comparison between ethane, ethene and ethyne
C-C Bond length [pm]C-H Bond length [pm]C-C-H Bond angle [°]
Ethane 153109.5~110
Ethene 133107.6~120
Ethyne 120106.4180
Fig.1
Construction principle for alkynes: Example ethyne (acetylene)

Each of the two C atoms in ethyne is connected to only two neighbors. The formation of a triple bond between the C atoms assures tetravalency of carbon. The simplest way to explain the electronic structure of the triple bond and the linear geometry is by invoking the hybridization model of carbon. Carbon in ethyne is sp hybridized, e.i. of the four valence orbitals of the C atom (s, p x , p y , p z ) the s and one p orbital are combined into two sp-hybridized orbitals. Two pure p orbitals remain (s + 3p = 2 sp + 2 p). The ethyne molecule can then be constructed from both sp-hybridized C atoms and two H atoms. The two sp-hybridized orbitals each forms a bond to one H atom and the neighboring C atom. The p orbitals combine to form two π bonds perpendicular to each other. The total electron density resembles a cylinder.

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