Plane trigonal hybridisation

Similar, but different, redeployment is envisaged when a carbon atom combines with three other atoms, e.g. in ethene (ethylene) (p. 8) three sp2 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 sp1 hybrid atomic orbitals disposed at 180° to each other (idigonal hybridisation) are employed. In each case the s orbital is always involved as it is the one of lowest energy level. 

Plane trigonal hybridisation, 5 Polarisability, 24, 96 Polarisation, 22, 29 Polarised complexes, 108,141, 144 Polar non-protic solvents, 81 Polyenes... 

Trigonal hybridisation The combination of one s atomic orbital with two p atomic orbitals to give three degenerate, i.e. equal in energy, hybrid orbitals which are labelled sp2. They project from the centre towards the corners of an equilateral triangle, and so are all in the same plane. 

The hybridised orbitals and the 2py orbital occupy spaces as far apart from each other as possible. The lobes of the 2p orbital occupy the space above and below the plane of the x and z axes. The three hybridised sp2 orbitals (major lobes shown only) will then occupy the remaining space such that they are as far apart from the 2py orbital and from each other as possible. Thus, they are all placed in the x-z plane pointing towards the corner of a triangle (trigonal planar shape). The angle between each of these lobes is 120. We are now ready to look at the bonding of an sp2 hybridised carbon. 

The increase in the value of the acidity constant can be predicted somewhat more quantitatively by interpreting the hybridisation of boron s orbitals. In the case of unbound boronic acid, boron has an sp trigonal planar geometry with an empty p orbital perpendicular to the plane of the molecule. It is with this unoccupied p orbital on boron that the nucleophilic oxygen lone pair on the approaching water molecule will mix. As the boron-oxygen interaction strengthens, concomitant proton dissociation occurs. By definition, the ease with which this proton is dissociated determines the value of the acidity constant (Scheme 10). 

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