Trigonal planar center

Sn2 does not occur on leaving groups attached to a trigonal planar center. 

Sn2 does not occur on leaving groups attached to a trigonal planar center, or on a highly hindered site like a tertiary or neopentyl center. 

The most common example of electrophilic substitution at a trigonal planar center is electrophilic aromatic substitution, which will serve as our archetype. Aromaticity was covered in Section 1.9, and the reactivity trends of aromatics will be covered in detail in Chapter 5. Chapter 8 has additional reaction examples. Aromatic rings are usually poor nucleophiles therefore excellent electrophiles are needed for the reaction to proceed. 

This reaction was introduced in Section 4.6, Electrophilic Substitution at a Trigonal Planar Center. The electrophile adds to the pi bond of the aromatic ring, path Ag, followed by deprotonation of the cation formed, path Dg, restoring aromatic stabilization. See Section 5.6 Aromatic Rings, for a discussion of electrophilic aromatic substitution on heteroaromatics like pyridine and on condensed aromatics like naphthalene. 

This is the almost exclusive route in basic media for replacement of a leaving group hound to a donble bond. This reaction was introduced in Section 4.5, Nucleophilic Substitution at a Trigonal Planar Center. The most common substrates for this reaction are the carboxyl derivatives. A nucleophile adds first via path Adileaving group departs via the beta elimination from an anion path Ep. 

The two faces of a trigonal planar center are designated re and si, according to the direction of Cahn-Ingold-Prelog priorities (Section 5.7) for the groups bonded at the trigonal center when viewed from one face or the other (re is clockwise, si is counterclockwise) ... 

A term is needed in a force field s energy function in order to insure that the four atoms in a trigonal planar center (see Figure Id), such as the sp" hybridized carbon atom and the three atoms bonded to it in A-methylacetamide, remain planar and exhibit the proper resistance to out-of-plane deformation. The quadratic term in equation (9) is used... 

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