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Table of heteroatom parameters

Source: G. Chemie 100 (1969) 1003

the $HX$-Values ​​of the heteroatoms are determined based on the experimental valence orbital ionization potentials (VOIP) for the ionization of one (.A) or two (: A) electrons; the values ​​thus also follow the gradation of the electronegativity of the atoms.

Tab. 1
Heteroatom parameters
X$HX$C-X$kC-X$
1.C0,00C-C1,00
2F.2,84C-F0,68
3Cl1,45C-Cl0,57
4Br1,16C-Br0,38
5J0,78C-J0,19
6:O2,06C-O:1,31
7: O-CH31,96C = O1,93
8.O1,18C-N:1,30
9: N1,47C = N.1,06
10.N0,83N-O1,95
11: CH30,88C-CH30,18

Hints:

• .A - means that the atom contributes a π electron to the molecule (e.g. pyridine nitrogen)
• : A - means that the atom contributes two π electrons to the molecule (e.g. pyrrole nitrogen)
• Halogens contribute two π electrons (electron pair) to conjugation.
• :$CH3$ corresponds to the "heteroatom model" for hyperconjugation