Leaving groups reactivity trend

As Ph2S is a better leaving group that Me2S, the diphenylsulfonium salt is expected to be more reactive. Tlie same trend was observed in the just described copper-catalysed reaction of sulfonium ylides with alkenes (see Section 4.4.1). 

Reactivity trends for D substitution at [Fe(CN)sL] anions will depend on solvent effects on the starting anion and on the separating moieties [Fe(CN)s] and L (L ) in the transition state. The activation parameters determined for a series of anions of this type with L = ammonia, amine, or aniline have been held to indicate the importance of transfer of the leaving group L to the solvent, an importance seemingly not recognized in some other studies. Small differences in reactivity between various complexes with L=NH2(CH2) NH2 or NHa-(CH2) NH3+, 3< <6, have also been attributed to solvation effects at the... 

The reactions shown in the illustration are explained by noting that the NR2 group is the poorest leaving group in this series of acid derivatives, so the reactivity order is Cl > OOCR OR NR2. Note that an amine replaces Cl, 02CMe, and OEt. Experimentally, it is not possible to convert an amide to an ester, an anhydride, or an acid chloride by acyl substitution. This means that amides are the least reactive species and acid chlorides are the most reactive species. This trend has been noted previously. The trend means that amides are at the bottom of the reactivity chain, so they are readily prepared from more reactive acid derivatives such as acid chlorides, acid anhydrides, or esters, as shown. 

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