Organic reaction mechanisms unimolecular substitution reactions

Early attempts to establish mechanistic models for substitution at metal centers used the labels Sjp and SN2 (substitution, nucleophilic, unimolecular or bimolecular) inherited from Ingold s attempts26 to extend his classic studies in organic reaction mechanisms to substitution at elements other than carbon. Unfortunately, Ingold s attribution of the discontinuity in reaction rates in the replacement of one Cl in m-Co(en)2CI2 1 by various anions A in methanol in the sequence... 

The dissociative, or D, mechanism is also sometimes called an SnI reaction, a form of nomenclature used in organic reaction mechanisms. In the representation SN1, S refers to the process (substitution), subscript N to the character of the leaving and entering group (a nucleophile, equated in this case to a Lewis base with a lone pair of electrons), and 1 to the number of molecules involved in the rate-determining step (unimolecular). For the D mechanism, the reaction rate can be expressed by Equation 5.37 ... 

Unimolecular substitutions are discussed in detail in the following sources (a) C. A. Bunton, Nucleophilic Substitution at a Saturated Carbon Atom, Elsevier, Amsterdam, 1963 (b) C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969 (c) A. Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962 (d) E. R. Thornton, Solvolysis Mechanisms, Ronald Press, New York, 1964. 

The symbols SnI and Sn2 were introduced by Hughes and Ingold for organic reactions. The term SnI is used for dissociative mechanisms and means substitution nucleophilic unimolecular. The term Sn2 is used for mechanisms in which bond making is important and means substitution nucleophilic bimolecular. Let us now look at several systems to see how mechanistic information on substitution reactions of coordination compounds was obtained. 

Nucleophilic anions, i.e. halides, pseudohalides, alkoxides, phenoxides, and thio-phenoxides, are particularly suitable for these reactions. Even anions of lower reactivity in nucleophilic displacements, i.e. carboxylates, nitrates, nitrites and hydroperoxides, find practical application under PTC conditions. Reactions are rigorously Sf,2 in mechanism primary substrates are thus most suitable, since secondary substrates afford elimination products in high yields, especially when reacted at high temperatures, and tertiary substrates only give rise to elimination. This behaviour is consistent with the low polarity of the organic phase, preventing unimolecular mechanisms and favouring elimination over substitution when the reaction center is not a primary carbon atom. 

Christopher Ingold and Edward D. Hughes, in the middle of the last century, used such kinetic properties to devise nomenclature for the mechanisms of organic reactions. For the nucleophilic substitutions. Ingold and Hughes have proposed labels SnI, and Sn2, where S and N refer to substitution and nucleophilic, respectively. Numbers 1 or 2 are used to designate unimolecular or bimolecular mechanisms, respectively. 

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