Digonal hybridisation

Similar, but different, redeployment is envisaged when a carbon atom combines with three other atoms, e.g. in ethene (ethylene) (p. 8) three sp hybrid atomic orbitals disposed at 120° to each other in the same plane plane trigonal hybridisation) are then employed. Finally, when carbon combines with two other atoms, e.g. in ethyne (acetylene) (p. 9) two sp hybrid atomic orbitals disposed at 180° to each other digonal hybridisation) are employed. In each case the s orbital is always involved as it is the one of lowest energy level. 

Dienes, 11 addition to, 194-198 cisoid conformation, 197, 350 conjugated, 11 Cope rearrangement, 354 cycUsation, 346 cycloaddition to, 348 Diels-Alder reaction, 197, 349 excited state, 13 heat of hydrogenation, 16,194 isolated, 11 m.o.s of, 12 polymerisation, 323 Dienone intermediates, 356 Dienone/phenol rearrangement, 115 Dienophiles, 198, 350 Digonal hybridisation, 5 Dimedone, 202 Dimroth s Et parameter, 391 solvatochromic shifts, 391 solvent polarity, 391 Y and,392 Dinitrofluorobenzene proteins and, 172... 

In acetylene, HC=CH, the triple bond is formed by digonal hybridisation , a (T-bond between the carbon atoms and two TT-bonds from each in planes through the axis of the molecule and at right-angles to one another. 

With the admission of d orbitals the number of principal types of hybridisation is increased from 4 (pure p, digonal, trigonal, tetrahedral) to over 40 fortunately only a few of these lead to systems of strong bonds. 

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