The electronic structure of Alkynes

– we studied theThe electronic structureof Alkynes (a triple bond) in This subject: The Structure of Ethyne (Acetylene): sp Hybridization

– Let’s review The electronic structureof Alkynes, using acetylene as the example.

– The Lewis structure of acetylene shows three pairs of electrons in the region between the carbon nuclei:

– Each carbon atom is bonded to two other atoms, and there are no nonbonding valence electrons.

– Each carbon atom needs two hybrid orbitals to form the sigma bond framework.

– Hybridization of the s orbital with one p orbital gives two hybrid orbitals, directed 180° apart, for each carbon atom.

– Overlap of these sp hybrid orbitals with each other and with the hydrogen s orbitals gives the sigma bond framework.

– Experimental results have confirmed this linear (180°) structure.

The electronic structureof Alkynes

The electronic structureof Alkynes

– Two pi bonds result from overlap of the two remaining unhybridized p orbitals on each carbon atom.

– These orbitals overlap at right angles to each other, forming one pi bond with electron density above and below the C-C sigma bond, and the other with electron density in front and in back of the sigma bond.

– The shape of these pi bonds is such that they blend to form a cylinder of electron density encircling the sigma bond between the two carbon atoms.

The electronic structureof Alkynes

– The carbon–carbon bond length in acetylene is 1.20 Å, and each carbon–hydrogen bond is 1.06 Å.

– Both bonds are shorter than the corresponding bonds in ethane and in ethene.

The electronic structureof Alkynes

– The triple bond is relatively short because of the attractive overlap of three bonding pairs of electrons and the high s character of the sp hybrid orbitals.

– The sp hybrid orbitals are about one-half (s) character (as opposed to one-third (s) character of sp2 hybrids and one-fourth of sp3 hybrids), using more of the closer, tightly held s orbital.

– The sp hybrid orbitals also account for the slightly shorter (C-H) bonds in acetylene compared with ethylene.

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