Depiction of Mechanism

DEPICTION OF MECHANISM

Use of curved-arrow notation to depict the mechanisms of organic reactions requires that appropriate mechanistic principles be superimposed on the correct use of curved arrows to denote movement of electrons. The mechanism of a reaction is the stepwise process by which reactants are converted to products, and generally each step involves bond making and/or bond breaking that can readily be depicted by curved-arrow notation.

Simple substitution reactions are shown in curved-arrow notation as

In this example a nucleophile donates electrons to the electrophile, in this case a carbon with a leaving group attached, to produce a new σ bond. As the new σ bond is formed, the bond to the leaving group breaks and the substitution of one group for another is completed. The iodide nucleophile has unshared pairs of electrons which are donated. The σ ^∗ orbital of the C–Br bond is the acceptor orbital. In any donor – acceptor interaction which leads to a chemical change, identification of both the donor and acceptor is very crucial in predicting what change will occur and determining how the change might occur.

Addition of Grignard reagents to carbonyl groups involves donation of the electrons of an electron-rich carbon – metal σ bond to the π^∗ orbital of the car-bonyl group. As shown below, carbon – carbon bond formation is accompanied by carbon – oxygen π bond cleavage, oxygen – metal bond formation, and a cor-responding change in geometry from trigonal to tetrahedral.

Ring opening of epoxides by alkoxides is used to emphasize that charge must be conserved during each mechanistic step. Because the reactants as written (neglecting spectator ions) have a net negative charge, the products must have a net negative charge.

The conservation of charge is a fundamental law for all processes, such as the addition of nucleophiles to π systems or acid – base reactions. The first step of the basic hydrolysis of nitriles has the hydroxide ion adding to the π bond of the nitrile. For the purposes of mechanistic discussion, the hydroxide is shown without its counterion and the net charge on the reactant side of the equation is −1. Consequently, the product of this first step (and each subsequent step) must also have a net negative charge.

In aqueous solution, proton transfer to the first formed intermediate is very rapid. However, again for illustrating the stepwise changes that must occur on the way from reactants to products using curved-arrow notation, these steps are shown independently.

Similarly, the production of enolates from carbonyl compounds involves base removal of a proton from the α position. The enolate is negatively charged and has delocalized electrons.

Although this process can be written to give a single canonical form A, it must be realized that the enolate is a delocalized species and resonance forms A and B can be generated as discussed previously using curved-arrow notation. This is not a mechanistic step since the delocalized product is a resonance hybrid of A and B— that is A is not converted to B, but rather the curved arrows merely indicate the changes in electron distribution that must be used to describe the canonical form B.

These previous examples are reactions where the electron donor (nucleophile) supplies the “electronic push” to accomplish bond breaking. Many nucleophiles, either neutral or anionic, have lone pairs of electrons that are easily donated. They can be donated to even weak electron acceptors (electrophiles).

There are other reactions, also easily describable by electron movement (curved arrows), in which π electrons are donated. In such reactions the π electrons are bonded electrons and hence the π-donor nucleophile is a weak electron donor. Consequently, a much stronger electron acceptor (stronger electrophile) is required for the “electronic pull” for electron donation to occur successfully. However, such descriptions are simply a matter of semantics because curved-arrow notation only shows changes in electrons, it does not indicate driving force. For a donor – acceptor interaction to occur productively, there has to be an energetic driving force for the process, and the energy levels of the donor and acceptor must be matched so that electron movement from the donor to the acceptor can occur.

For example, the protonation of a double bond has a proton as the electrophile and the π bond as the electron donor.

When bonded electrons are donated, it is important to remember that one of the bonded atoms which shared that pair is now left without a valence octet. That is, removal of a bonded pair must result in a sextet atom. Such is the case after protonation of a double bond as seen above. Because of the instability of a sextet electronic configuration, several strategies are available to stabilize it.

Neighboring atoms having unshared pairs of electrons can undergo bridg-ing interaction with the cationic center to give structures in which all atoms have valence octets. An archetypical example is the bromination of olefins. Electrophilic addition of bromine to the double bond is predicted to give an α-bromocarbocation. However, formation of the bridged bromonium ion avoids a sextet configuration of carbon and thus is formed preferentially. Bridging inter-actions occur when a carbocation is generated vicinal to substituents such as –OR, –Cl, –F, –SR, –NR₂, and so on, all of which have lone pairs capable of bridging interactions.

Resonance stabilization can also make π-electron donation much more effec-tive by avoiding the formation of a sextet carbocation. Lone-pair donation from the oxygen of enol derivatives is very important to the good donor ability of these compounds. The resulting oxonium ion has all valence octets (although positively charged) and is thus stabilized over sextet canonical forms.

Resonance stabilization is important in electrophilic aromatic substitution as well. While each of the canonical forms of the Wheland intermediate has a sextet carbon atom, the charge is distributed over the remaining five atoms of the ring by resonance and is thus greatly stabilized.

The reactions of nucleophiles with electrophiles also relates to the overall oxidative change of a reaction. As is expected, nucleophilic atoms which are more electronegative than carbon are not reductants and usually give no change in oxidation state, for example,

Conversely nucleophiles which are carbanion or hydride equivalents are reductants,

Carbocation or proton electrophiles give no change in oxidation level whereas electrophiles which are electronegative elements (Br₂, Cl₂, NBS, peracids, etc.) are oxidants,

Besides intermolecular reactions, curved-arrow notation is also useful in indicating bonding changes in intramolecular reactions and rearrangement. For example, Cope-type rearrangements are seen to involve changes in three pairs of bonded electrons.

The arrows can be written in either directional sense since these reactions are concerted rearrangements with all bond making – bond breaking taking place at the same time. This example emphasizes the fact that curved-arrow notation is merely an electron bookkeeping method.

Cationic rearrangements are also handled easily by keeping track of where electron pairs come from and where they go. For example, the neophyl-type rearrangement below leads to skeletally rearranged products.

The curved-arrow notation clearly shows the electron flow needed to effect the rearrangement. What curved-arrow notation does not show is the timing of these events — that is, whether loss of a leaving group precedes or is concerted with 1,2-phenyl migration or if a bridged ion is an intermediate. Such considerations, if known, can be included in more detailed mechanistic sequences.

With these considerations, then, the steps one goes through to use electron movement to generate a possible reaction mechanism are as follows:

1. Write a balanced equation for the reaction. While spectator ions may be neglected, it is imperative to write correct Lewis structures for reactants and products. This step is very important but often neglected.

2. Note the connectivity changes that occur, changes in oxidation level that occur, and the reagents or reactant types necessary for the conversion.

3. Write a stepwise process for the reaction using curved arrows to account for bonding changes. The use of curved arrows for electron movement should be guided by bond polarities, donor – acceptor properties, electroneg-ativities, and structural factors and should result in a reasonable series of bonding changes from reactant to product.

4. Evaluate intermediates for stability and valence. If they fit normal chemical expectations, then the mechanism is potentially correct. There may be other mechanisms operating or the timing of individual steps (synchronous, con-certed, etc.) may be different, but the above process can be used to generate them as well.

Thus we see that, used properly, curved-arrow notation for electron movement is indispensable to the organic chemist as a way to depict chemical change in com-plex molecules. Furthermore, it can be extended to include a method for showing the mechanism if the ground rules are understood and followed carefully.