(Somewhat) polar C-H bonds

Today’s activity (ChemActivity 34R) used potential surfaces to assign charges to hydrogen atoms in ethylene and acetylene. The charge trend goes like this (I’ve added ethane for good measure):

(least +) H in ethane < H in ethylene < H in acetylene << H in water (most +)

This trend can be rationalized by thinking about the energies of the overlapping atomic orbitals.

A detailed analysis of MOs for polar bonds was provided for HF in the study guide for Chapter 3 (see page 4) and pictures of the BMO and ABMO for HF were displayed in lecture #8, Sept 18 (see slide #6 – “Unequal AO Energies”).

As you probably found while working today’s ChemActivity, the energy trend among carbon’s orbitals is: 2s << sp < sp2 < sp3 < 2p. The energy of hydrogen’s 1s orbital is slightly higher than the energies of carbon’s hybrid orbitals, so the MO for a CH bond are always built from orbitals of different energy (C < H).

The greatest inequality appears when we overlap H 1s and C sp. The MO is weighted towards the lower energy orbital (sp) and the resulting bond is polarized towards carbon. Smaller inequalities occur when C sp2 and sp3 orbitals overlap with H 1s, so these bonds are not as polar.

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