CHM 501

16. Consider the following two compounds: CF2C(F)CN (having a C-C-C backbone) and CF2C(F)NC (having a C-C-N backbone). Write the lowest energy Lewis structures for each, give the formal charge and oxidation number for each atom, estimate the bond lengths for each bond and bond angles for each atom along the backbone, and indicate the likely hybridization at each atom along the backbone.

Answer

All formal charges are 0 and oxidation numbers are as shown.

The electronegative F atom will withdraw some of the electron density from the double bonds, causing them to lengthen a bit, so the C=C length in each isomer will be ~ 1.2 Å. The C-F bonds should be fairly typical, ~1.38 Å. In the nitrile isomer (left), the C-C bond is shortened a bit by conjugation to ~1.5 Å and the nitrile bond lengthened a bit to ~1.15-1.16 Å. In the isonitrile isomer (right), conjugation should shorten the C-N bond to ~1.45 Å and lengthen the N=C bond to ~ 1.3 Å. The CF2 and CF centers in both isomers are sp2 hybridized with 120° bond angles. The nitrile C in the left isomer is sp hybridized with a 180o bond angle while the isonitrile N in the right isomer is sp2 hybridized with a bond angle reduced from the ideal 120° to ~116°.