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Ingold Mechanisms in MACiE

A rigorous treatment of the Ingold mechanisms lead to a precisely defined set of possibilities. Following are the possible Ingold Mechanisms allowed for in MACiE. It should be noted that whilst we allow for unimolecular mechanisms, due to the fact that we classify reactions on a per-step basis these will only be assigned in exceptional circumstances.

C. K. Ingold was the first to attempt a systematic representation of reactions in the 1930's. His mechanistic nomenclature (SN2, E1, etc.) and notational tool, the "curly arrows" or "arrow pushing", are now familiar to the modern day chemist. He defined a chemical reaction as an electrical transaction, which takes place by virtue of some predominating constitutional affinity either for atomic nuclei or electrons on the part of a reagent; or perhaps for both, i.e. the atoms themselves1. He clusters reagents with an even number of electrons (basically nucleophiles and electrophiles) into the first and second classes and reagents with an odd number of electrons (basically radicals) into the third, rarer, category.

Ingold split his reactions firstly into the categories of homolytic and heterolytic. Homolytic reactions are those reactions in which covalent bond formation (colligation) is by the supply of one electron from each of the reacting species. The reverse of this is homolysis, where the bonding electrons are split equally between the two atoms involved in the bond. Heterolytic reactions are those in which one reacting species supplies both electrons or, in the reverse reaction, receives both electrons. Thus heterolytic reactions can be either nucleophilic or electrophilic, whilst homolytic reactions are simply homolytic. Ingold defines nucleophiles as those reagents that act by donating their electrons, or sharing them with, a foreign atomic nucleus, and electrophiles as those reagents which act by acquiring electrons, or a share in electrons, which had previously belonged exclusively to a foreign molecule.

From this, the reaction can be defined as an addition, substitution or elimination. Unfortunately, there are very few rules as to which species is designated the attacking species and so it falls to pure convention as to whether the reaction is nucleophilic or electrophilic. It is suggested by Ingold that in order to assign a substitution reaction the actual mechanism is not necessary as it is obvious as to the identity of the substitution species:

OET- + CH3I --> CH3OEt + I-

the iodide is obviously being substituted for the ethoxide, hence the reaction is considered to be nucleophilic and termed SN.

NO2+ + Ph-H --> PhNO2 + H+

The nitro group is obviously displacing the proton, hence the reaction is considered to be electrophilic and termed SE. A homolytic substitution is thus termed an SH reaction.

Addition reactions are more difficult as the mechanism becomes more important. However, with the application of chemical knowledge it is not as difficult as it might at first seem:

NH3 + CH3CH=O --> CH3CH(OH)NH2

here the mechanism is nucleophilic in nature and thus it is termed an AdN.

H-Cl + CH3CH=CH2 --> CH3CHClCH3

here the mechanism is electrophilic, with the p-electrons in the double bond attacking the H in H-Cl, thus it is termed an AdE. A homolytic addition is termed an AdH reaction.

Ingold states that all elimination reactions are nucleophilic in nature and as such it is not necessary to term them EN, simply E. However, in MACiE we have a case of homolytic elimination and so we have added the term EH to our definitions.

Ingold adds that there are reactions not treated under the general rules discussed above, and included in these reactions are rearrangements. Rearrangements are described according to their reaction type, i.e. nucleophilic, electrophilic and homolytic.

Finally, reactions can be unimolecular, i.e. the rate determining step only involves one species, bimolecular, i.e. the rate determining step involved two species, or intramolecular, i.e. the rate determining step involves two centres reacting with each other which are separate in space, but belong to the same molecule. It is believed that all reactions can thus be assigned an Ingold Mechanism.

The following are Ingold mechanisms allowed for in MACiE:

Additions

A chemical reaction of two or more reacting molecular entities, resulting in a single reaction product containing all atoms of all components, with formation of at least one chemical bond and a net reduction in bond multiplicity in at least one of the reactants. The following addition reactions are defined in the MACiE dictionary:

Nucleophilic Electrophilic Homolytic
Bimolecular Yes Yes Yes
Aromatic Bimolecular Yes Yes X
Intramolecular Yes Yes Yes

If there is no obviously assignable Ingold mechanism, addition may then be used as a reaction type. It is suggested that addition only be used in these circumstances and requires no further input.

The following are examples of addition reactions in MACiE:

Aromatic Electrophilic Addition
Aromatic bimolecular electrophilic addition (AdE2Ar)

aromatic nucleophilic addition
Aromatic bimolecular nucleophilic addition (AdN2Ar)

Bimolecular Electrophilic Addition
Bimolecular electrophilic addition (AdE2) - shown in the blue box.

Bimolecular Homolytic Addition
Bimolecular homolytic addition (AdH2)

Bimolecular Nucleophilic Addition
Bimolecular nucleophilic addition (AdN2)

Internal Electrophilic Addition
Intramolecular electrophilic addition (AdEi)

Intramolecular Homolytic Addition
Intramolecular homolytic addition (AdHi)

Intramolecular Nucleophilic Addition
Intramolecular nucleophilic addition (AdNi)

Elimination

In an elimination one or more groups are lost, most often from two different centres, with concomitant formation of an unsaturation in the molecule (double bond, triple bond) or formation of a new ring. The following types of elimination are allowed in MACiE and correspond to the Ingold elimination reaction mechansims.

Nucleophilic Homolytic
Bimolecular Yes Yes
Aromatic Bimolecular Yes X
Intramolecular Yes Yes
Aromatic Intramolecular Yes X
Unimolecular Yes Yes
Aromatic Unimolecular Yes X

If there is no obviously assignable Ingold mechanism, elimination may then be used as a reaction type. It is suggested that elimination only be used in these circumstances and requires no further input.

The following are examples of elimination reactions in MACiE:

Aromatic unimolecular elimination from the conjugate base
Aromatic unimolecular elimination from the conjugate base.

Heterolytic bimolecular elimination
Heterolytic bimolecular elimination

Homolytic bimolecular elimination
Bimolecular homolytic elimination (EH2)

Intramolecular Elimination
Intramolecular elimination (Ei)

Unimolecular elimination from the conjugate base
Unimolecular elimination from the conjugate base (E1cb) - shown in the blue box

Unimolecular homolytic elimination
Unimolecular homolytic elimination

Substitutions

A reaction, either elementary or stepwise, in which one atom or group in a molecular entity is replaced by another atom or group. The following substitution reactions are defined in the MACiE dictionary:

Nucleophilic Electrophilic Homolytic
Bimolecular Yes Yes Yes
Aromatic Bimolecular Yes X X
Intramolecular Yes Yes Yes

In addition to the above types of substitutions, there also exist the acidic SN2, acidic SNi, allylic SN2 and allylic SNi.

If there is no obviously assignable Ingold mechanism, substitution may then be used as a reaction type. It is suggested that substitution only be used in these circumstances and requires no further input.

The following are examples of substitution reactions in MACiE:

Aromatic bimolecular nucleophilic substitution
Aromatic bimolecular nucleophilic substitution (SN2Ar).
The substitution is shown in two steps for clarity, the reaction may occur in a single or multiple steps

Acidic bimolecular nucleophilic substitution
Acidic bimolecular nucleophilic substitution (A-SN2) - shown in the blue box

Bimolecular Electrophilic Substitution
Bimolecular electrophilic substitution (SE2)

Bimolecular Homolytic Substitution
Bimolecular homolytic substitution (SH2)

Bimolecular Nucleophilic Substitution
Bimolecular nucleophilic substitution (SN2)

Bimolecular nucleophilic substitution with allylic rearrangement
Bimolecular nucleophilic substitution with allylic rearrangement (SN2')

Intramolecular Electrophilic Substitution
Intramolecular electrophilic substitution (SEi)

Intramolecular Homolytic Substitution
Intramolecular homolytic substitution (SHi)

Internal Nucleophilic Substitution
Intramolecular nucleophilic substitution (SNi)

References

  1. C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd Ed, Cornell University Press, Ithaca, N.Y., Chapters 5-15, (1969).
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