3BL-Lecture07 - Welcome to Chemistry 3BL Lecture 7 David...

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Unformatted text preview: Welcome to Chemistry 3BL Lecture 7 David Rabuka Aldol Lab Refections Questions for the Aldol lab are posted on the Chem 3BL Blackboard site. Lets examine the theory in more detail. The Aldol Condensation A carbon-carbon bond forming reaction between an aldehyde electrophile and an enolate nucleophile. The enolate nucleophile is stabilized by resonance. Can produce an ald ehyde-alcoh ol (hence aldol) product or a ketone-alcohol product. Which is formed depends on the source of the enolate. Often the aldol product is dehydrated to produce a more thermodynamically favorable alkene product. The Aldol Mechanism Consider an intramolecular, base catalyzed Aldol. Note that two different cyclic products could be formed, but only one is preferred. O O NaOH (aq) O (Heat) + H 2 O The Aldol Mechanism The base catalyzed Aldol involves an enolate intermediate. Note that if a weak base, such as NaOH, is used, the enolate is only generated in small conc. O O H H H OH Na O O Na + H 2 O The Aldol Mechanism Next, one must consider the enolate attack of an electrophile (typically an aldehyde). Note that ketones are not attacked by enolates, unless its an intramolecular attack of an enolate onto a ketone (as in this example). O Na O O O Na The Aldol Mechanism Next, the addition product can be protonated by water or a protic solvent. Note that protonation of the Aldol product regenerates an equivalent of base. + H 2 O O O Na + NaOH O HO The Aldol Mechanism DEHYDRATION: PART I Finally, an equivalent of base can remove the remaining -hydrogen ( to the carbonyl) in the rst step of a dehydration process. Note that this process is slow. O HO H H OH Na ONa HO + H 2 O The Aldol Mechanism DEHYDRATION: PART II Finally, the hydroxyl group can be expelled in the second step of a dehydration process....
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3BL-Lecture07 - Welcome to Chemistry 3BL Lecture 7 David...

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