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L_6_09 - oxonium ion(en route to the enol is destabilized...

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318N – Krische, Lecture 6: Th - 02/05/09, Aldehydes and Ketones · Previously, we saw that carbonyl groups can be “protected” as ketals or acetals. Today, we examine a protecting group for alcohols – the “THP” group. Specifically, we learned the mechanism for formation and removal of the THP group. Whereas alcohols are acidic, THP groups are not. Hence, these base-stable, non-acidic groups are compatible with Grignard reagents. For example, generation of the Grignard reagent derived from 1-bromo-3-propanol is impossible due to self- protonation. By protecting the hydroxyl group as the THP-ether, we circumvent this problem. · Enols and enolates resemble very electron rich alkenes. Just as it is possible to halogenate an alkene, one may halogenate an enol/enolate. However, there are some differences… · Halogenation under acidic conditions yields mono-halogenated products. Under acidic conditions, the intermediate
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Unformatted text preview: oxonium ion (en route to the enol) is destabilized by the electron withdrawing halide, making subsequent halogenation sluggish. Thus, “over-halogenation” is not a problem. · Halogenation under basic conditions yields products in which ALL alpha-hydrogens have been exchanged for halide. Under basic conditions, the electron withdrawing halide stabilizes the negatively charged enolate. Thus, each bromination accelerates the next under basic conditions. Tri-haloketones can react further to give carboxylates and haloforms. This reaction is known as the haloform reaction. Know the mechanism, · The haloform reaction represents the first example of an “acyl substitution reaction”. This is an important reactions type that we will encounter extensively in forthcoming material. · Midterm Exam I is coming after next weeks two lectures (after lecture 8) – be sure to do homework and attend office hours and review sessions....
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