Heterolytic Bond Formation

HETEROLYTIC BOND FORMATION

The formation of a bond between two atoms can proceed by one of the atoms donating an electron pair and the other atom accepting the electron pair. As before, charge must be conserved and the loss and gain in electrons by the donor and acceptor, respectively, must be accompanied by a corresponding change in formal charge.

The atom donating the electron pair must obviously have an electron pair that is not tightly bound and thus is available for donation. Commonly, lone pairs and π-bonded electron pairs can be most easily donated, but occasionally electron pairs in σ bonds can be donated if the σ bond is weak or electron rich.

The atom accepting the electron pair must have an unfilled orbital available which the donated electron pair can populate. This can be an unfilled valence shell orbital, as is the case if the acceptor atom has a valence sextet, or it can be an accessible antibonding orbital, either σ ^∗ or π^∗.

Thus the reaction of acetone with BF₃ is a Lewis acid – base reaction in which a lone pair of the ketone oxygen atom is donated to an unfilled valence orbital of BF₃. Bond formation is accompanied by the development of formal charges on both oxygen and boron.

The capture of carbocations by alcohols involves a similar donation of a lone pair of electrons on oxygen to the vacant 2p atomic orbital of the sp²-hybridized, sextet carbocation. Note that charge must be conserved so the first formed product is a positively charged oxonium ion.

Reaction of an alkene with a nitronium ion involves donation of the π-electron pair of the alkene into a π^∗ orbital of the nitronium ion. Donation of a bonded electron pair necessarily means that the bond from which it comes is broken. Likewise population of an antibonding level by electron donation generally results in breaking of the bond to which the antibonding orbital corresponds. In this case electron donation of the olefinic π-electron pair results in the rupture of the olefinic π bond and acceptance into the N–O π^∗ orbital results in breakage of the N–O π bond as well. Note that a new bond is formed and net charge is conserved.

Addition of an alkyl lithium reagent, which has a very electron rich σ bond between carbon and lithium, with a ketone involves electron donation of the σ electrons to the π^∗ orbital of the ketone to give a new carbon – carbon bond. The lithium counterion also plays a role in the addition by complexing with the lone pairs of the carbonyl oxygen, thus making the carbonyl group more electron deficient. Effectively this lowers the energy of the π^∗ orbital so its energy better matches the energy of the electron donor, and donation of electrons into that orbital is facilitated.

It is important to remember the requirements for any two-electron bond-making process: first, there must be an available pair of electrons to be donated and, second, there must be an unoccupied orbital of suitable energy available into which the electrons can be donated.