Nucleophilic substitution carbon/oxygen additions

Mechanism. Addition of HgS04 generates a cyclic mercurinium ion, which is attacked by a nucleophilic water molecule on the more substituted carbon. Oxygen loses a proton to form a mercuric enol, which under work-up produces enol (vinyl alcohol). The enol is rapidly converted to 2- butanone. 

A significant portion of the epoxide literature deals with reactions -which, although ostensibly of widely divergent character, nevertheless do possess the following important property in common all involve addition to oxygen, as well as to carbon atoms. Products secured from such reactions, in other words, lack free hydroxyl groups, in contrast with those derived from conventional nucleophilic substitutions.18 1 Insofar as the present author is aware, little effort haa been made to treat all these epoxide reactions as a unit, and not much is known of their mechanisms. 

The topic of nucleophilic attack at an sp2 carbon would be too wide a field to review in one chapter, if one were to discuss all facets and outcomes, some of which are shown in Figure 1. If the nucleophile is anionic an anion is formed, but if the nucleophile is neutral a zwitterion is produced. If X is oxygen or nitrogen, then the outcome can be carbonyl addition, substitution or a Darzens-type reaction. If the X is another carbon then addition leads to conjugate additions or polymerization. Alternatively the nucleophile could be expelled after rotation about the C—C bond to give overall isomerization. Both substitution and cyclization are also observed. 

Nucleophilic substitution of compounds containing carbon/oxygen double bonds, i.e. carbonyl compounds, proceed via the tetrahedral mechanism. The tetrahedral intermediate may be isolated when a suitable substrate is used, e g. the addition of HCN to a carbonyl compound yields a cyanohydrin. If one of the groups attached to the carbonyl carbon is eliminated, then an overall substitution occurs, e g. in the hydrolysis of an acid chloride. 

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