Antibonding Orbitals - Organic Chemistry (2023)

The overlap of AOs to give a new MO in which an electron pair is shared by the interacting atoms was illustrated in Figure. The new MO, which contains the shared electron pair, is of lower energy than the AOs from which it was produced by overlap.


Theoverlap of AOs to give a new MO in which an electron pair is shared by theinteracting atoms was illustrated in Figure 1.2. The new MO, which contains theshared electron pair, is of lower energy than the AOs from which it wasproduced by overlap. This energy change (ΔE) is illustrated in Figure 1.3 (Nrepresents the nucleus of someelement in the bond formation process). The ΔE is related closely to the bondenergy of the bond produced. The same model holds irrespective of the type ofAOs which overlap (simple AOs or hybrid AOs) or the type of bond formed (σ or π).

Whilethis model is easy to visualize and understand, it is actually only half of thestory. When AOs interact, the number of new MOs which are producedfrom that interaction must equal the number of AOs which initially interact.

Furthermore, for each MO produced which is lower in energy than the energy ofthe interacting AOs, there will be one produced which will be higher in energy by the same amount(Figure 1.4). So when two half-filled AOs interact, there will be two MOsproduced, one of lower energy which will contain the electron pair and istermed the bonding MO. The secondmolecular orbital is of higher energy, is unfilled, and is termed the antibonding MO.

Foreach bond in a molecule which is described by the overlap of AOs, there will bea bonding MO which is of lower energy and when filled with an electron pairgives rise to a stable bond between elements. There will also be an antibondingMO which is of higher energy and thus unfilled. Antibonding orbitals correspondto the situation where nuclei are moved to within the bonding distance of oneanother but there is no electronsharing; in fact the electrons and nuclei actually repel one another. Thiselectronic and nuclear repulsion is what increases the energy of theantibonding level. Because the bonding MO is filled and the antibonding MO isunfilled, the system is at a lower net energy than the individual AOs and bondformation takes place. This occurs for both σand π bonds as shown in Figure 1.5 (the antibonding orbitals are indicated bythe asterisk). Overlap of an sp3 AO on a carbon with a 1s AO on ahydrogen gives a σ -bonding MO thatis filled with two electrons and an unfilled, higher energy, antibonding MO termeda σ MO. Likewise, overlap of two 2p AOs on carbon gives a π MO which contains a shared pair of e and a π MO which is ofhigher energy and is unfilled.

Antibonding Orbitals - Organic Chemistry (1)

Antibonding Orbitals - Organic Chemistry (2)

Thusfar it would appear that antibonding orbitals are real orbitals, but they seemto be merely mathematical artifacts since they are unfilled and thus do notenter into bonding or energy considerations. For ground-state molecules this isactually true — all of the electrons are found in bonding orbitals. Why, then,should we even concern ourselves with their existence?

Theanswer lies in the realization that antibonding orbitals are still, in fact,orbitals. They are regions of space where one could have electrons. Inground-state molecules, electrons fill the lower energy bonding orbitals.Suppose, how-ever, you wished to take an electron out of a bonding orbital andmove it to a higher level. Where would it go? Or suppose you wished to addelectrons to a molecule which already had its bonding orbitals filled. Wherewould the electrons go? Suppose an electron-rich reagent were to donateelectrons to a molecule. Where would the electrons go?

Inthese examples the electrons could go into a higher energy, unfilled MO whichcould be a nonbonding orbital (when one is present) or an antibonding orbital(which is always present). Thus it is most common to have the elec-trons gointo an antibonding MO. Although they are of high energy, antibonding orbitalsare usually unfilled and can accept electrons from several sources if sufficientenergy is available to promote electrons into the antibonding energy level.Absorption of light energy can cause an electron to be promoted from thehighest occupied molecular orbital (HOMO), which is usually a bonding MO, tothe lowest unoccupied molecular orbital (LUMO), which is most often anantibonding MO. For example, if an olefin which contains a carbon – carbon π bond is exposed to ultraviolet lightof the correct frequency (and hence energy), the molecule can absorb the energyof the light by promoting a πelectron from the bonding MO into the antibonding MO. This new electronic stateis termed an excited state and is higher in energy than the initialelectron-paired state called the ground state. (The electron spins can bepaired in the singlet excited state or unpaired in the triplet excited state.)Excited states of molecules are high-energy states which are much more reactivethan ground states and can be described in terms of the population ofantibonding orbitals. Consequently, almost all photochemical reactions whichoccur by the reactions of excited-state species are intimately dependent on theexistence of and population of antibond-ing orbitals.

Antibonding Orbitals - Organic Chemistry (3)

Thereduction of organic molecules by the addition of electrons can take place bychemical reagents or at the surface of electrodes. In either case electrons areadded to the organic compound, thus reducing it. Now electrons cannot just goanywhere; they must go into an unfilled orbital. Thus, during a reduction, electronsare injected into the LUMO of the molecule, which is often an anti-bondingorbital. Population of the antibonding orbital raises the total energy of themolecule and subsequent reactions follow. The electrochemical reduction ofalkyl bromides illustrates the process well. An electron is added into the σ orbital of thecarbon – bromine bond, which is the LUMO of a saturated alkyl bromide.Population of the antibonding orbital raises the energy of the molecule andweakens the carbon – bromine bond, which then dissociates to give bromide ionand a carbon-centered free radical which has an unpaired electron in a hybridAO (nonbonded energy level).

Antibonding Orbitals - Organic Chemistry (4)

Almostall dissolving metal and electrochemical reductions follow this same generalsequence. An electron is donated into an unfilled orbital which is usu-ally anantibonding MO, the energy of the molecule is raised, and chemical changeensues.

Whena nucleophile attacks an electrophile, it donates a pair of electrons to theelectrophile. Electron donation must take place by an overlap interactionbetween a filled orbital on the nucleophile which contains the electron pair tobe donated and an unfilled orbital (LUMO) on the electrophile, which is usuallyan antibonding orbital. Population of the LUMO by electron donation raises theenergy of the system leading to bonding change and new bond formation. Additionof an alkoxide to a ketone is a typical example of the process. The electronpair to be donated is in a hybrid AO and therefore is at a nonbonding energylevel (n). Overlap with the π orbital of thecarbonyl group starts to populate the π orbital. This weakens the π bond, and the carbon – oxygen π bond of the carbonyl group is brokenand a new lower energy σ bond isformed between the oxygen of the alkoxide and the carbonyl carbon. Theelectrons of the π bond end up in anonbonding AO on oxygen in the product. This process is shown schematically.

Antibonding Orbitals - Organic Chemistry (5)

Nucleophilicadditions and substitutions are the most widespread of all organic reactions,and all have the same general orbital requirements. An orbital containing anelectron pair of the nucleophile overlaps with an antibonding orbital of theelectrophile, which leads to population of the antibonding level (in mostcases). This raises the energy of the system and bond and electronreorganization follows to give products. The electron pair must be able to bedonated (i.e., not tightly bound or of higher energy) and the antibondingorbital be of sufficiently low energy to ensure effective overlap.

Thusit is seen that, although antibonding orbitals are not a major factor indescribing the bonding of ground-state molecules, they can play a pivotal rolein the reactions of molecules. Therefore it is important to keep in mind theexistence of antibonding orbitals and their ability to accept electrons andcontrol the reactivity of molecules.

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