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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This note was uploaded on 03/31/2009 for the course CH 318M taught by Professor Bocknack during the Spring '08 term at University of Texas at Austin.

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