34 



LIGHT AND LIFE 



For convenience, the "lone-pair" electrons may be classified as 

 originating from one of two types of orbitals: n-orbitals (non-bond- 

 ing) , which cannot conjugate with a Tr-electron system; and /-orbitals 

 ("lone-pair") , which can conjugate with a 7r-electron system. 



Fig. 2 illustrates several cases of these classifications. In the car- 

 bonyl group, Fig. 2 (1) (e.g. formaldehyde, acetone), the ?i-orbital is 

 a 2p orbital on the O atom, in the plane of the carbonyl group with 

 attached atoms (and is approx. non-bonding) , (the second, more 

 tightly bound /?-orbital is not shown and is probably an (approx.) 

 5p-hybrid orbital on the O atom). On an aza-nitrogen. Fig. 2 (2) 

 (e.g. pyridine) , the n-orbital is a hybridized (approx.) sp- orbital 

 on the N atom, in the plane of the molecular ring, as required by 



(1) 



'^2p,0 



u 



(2) 



n sp2, N 



i 



2p,N 



2p,N 



(5) 



l 



2p,C 



n-orbital (non-bonding) j^-orbital (can conjugate) 



Fig. 2. lypes of "lone-pair" orbitals in heteroatom molecules. 



the ring skeleton binding. Both of these ??-orbitals are symmetric 

 with respect to the plane of the molecular skeleton at the heteroatom, 

 thus cannot conjugate with the Tr-orbital electrons, which are always 

 antisymmetric to this plane (the 7r-orbital wavefunction changes sign 

 upon reflection in the skeletal plane) . (N.B. Solid contour line 

 designates (-)-) sign of orbital wavefunction, dashed contour line 

 designates ( — ) sign of orbital wavefunction.) 



The /-orbitals shown in Figs. 2 (3) , 2 (4) , 2 (5) are 2p7r orbitals on 

 an N or C atom, and are antisynnnetric to the skeletal plane at the 

 given N or C atom (solid contour line outside designates (-(-) region 

 of orbital wavefunction above skeletal plane; dashed contour inside 

 designates ( — ) region of orbital wavefunction heloxo skeletal plane) . 

 Fig. 2 (3) represents, e.g., a pyrrole "lone-pair," which is conjugated 

 with the 2pTT orl)itals on the four carbon atoms of the ring. Fig. 2 (4) 

 represents, e.g., a "lone-pair" of the aniline molecule. If the amino 



