Donors and Acceptors

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Donor / Acceptor Defined

Electrons eventually end up in the lowest energy level relative to the reference level.

A donor is a high energy orbital with one or more electrons. An acceptor is a low energy orbital with one or more vacancies:

  • A donor is an atom or group of atoms whose highest filled atomic orbital or molecular orbital is higher in energy than that of a reference orbital
  • An acceptor is an atom or group of atoms whose lowest unfilled atomic or molecular orbital is lower in energy than that of a reference orbital.

Reference orbitals

Why do we use reference orbital?

The driving force for an electron in an orbital to be transferred (donated) to another orbital (an acceptor orbital) is related to the difference in energy between the orbitals not the absolute energy.

In the diagram in each case the electron will end up on the lowest relative energy level. In organic chemistry we often think of CH3O and (CH3)2N as donors even though nitrogen and oxygen are more electronegative than carbon (i.e. their orbital are lower in energy than carbon?

Why is this?

First to be fair we should compare orbitals with equal occupancy and if we compared CH3O and (CH3)2N to (CH3)2C we would think that the carbanion was a better donor than the methoxy group or the amino group.

Effect of adding donors and acceptors

Molecular orbitals of ethylene with added donor or acceptor groups

Consider what happens to the molecular orbitals of ethylene when we attach donors or acceptor groups to them.

The donor case:

  • When the donor orbital mixes with the molecular orbitals of ethylene it is higher in energy then the HOMO of ethylene.
  • It mixes more with the HOMO than the LUMO because they are closer in energy (perturbation theory).
  • As a result of the mixing two new filled orbitals are created and the highest occupied MO is now higher in energy than either the amine orbital or the original ethylene HOMO. This orbital is delocalized and has both carbon and nitrogen character. In fact the orbital will in many ways resemble those of allyl anion.
  • The HOMO-LUMO energy gap is now smaller.

The acceptor case:

The same arguments will hold in reverse for the acceptor case. The LUMO is going to go down, and become easier to reduce. Adding an amine makes it easier to oxidize and decreases the gap.

Donor and Acceptor on Ethylene

Consider this series in which two orbitals like ethylene behave as donor and acceptor.

  • In the first case the donor orbital is very low energy and the acceptor is very high energy. As a result there is little mixing. It looks mostly like a bridge orbital. There is little dipole moment.
  • If we make the donor stronger there is more mixing and the donor begins to have coefficient and the bridge has coefficient. There is more of dipole moment and some redistribution of charge.
  • In third case there is a substantial coefficient on the donor and a net change of charge from the HOMO to the LUMO and a change in dipole moment.
  • Finally in case 4 the donor and acceptor are equal, it now looks like butadiene.

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