L_2_09 - (recall-benzene and H 2 O form an azeotrope). •...

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318N – Krische, Lecture 2: Th - 01/22/09, Aldehydes and Ketones Through a mechanism strictly analogous to the acid catalyzed hydration of an aldehyde or ketone, alcohols will add to aldehydes and ketones to form hemi-acetals and hemi-ketals, respectively. Unless they are cyclic (as in sugars such as glucose), hemi-acetals and hemi-ketals are typically unstable and under acidic conditions will react further to give acetals and ketals. KNOW THE MECHANISM. Attempted base-catalyzed acetal or ketal formation is not possible – be sure you understand why. Ketone + 2 ROH Ketal + H2O. Ketal and acetal formation belongs to a class of processes known as “condensation reactions” – reactions that liberate a small molecule, typically water. To drive this reaction to completion, we carry out the reactions in benzene using a Dean-Stark trap to remove water
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Unformatted text preview: (recall-benzene and H 2 O form an azeotrope). • Ketals and acetals, which are stable to non-acidic conditions, can be used as protecting groups for ketones and aldehydes. • The mechanism for ketal/acetal hydrolysis is the reverse of the mechanism for ketal/acetal formation. Aqueous acid will promote the “hydrolysis” a ketal/acetal. • The mechanism for formation of thio-acetals (i.e. 1,3-dithianes) is analogous to ketal formation.1,3-dithianes can be deprotonated by n-BuLi. The resulting carbanion is a good nucleophile. Once the dithiane is alkylated, it may be hydrolyzed using HgCl 2 , H 2 O-CH 3 CN to produce a ketone. Thus, using dithiane chemistry, we can convert aldehydes to ketones. • Thus far, we’ve examined the addition of O- and S-nucleophiles to aldehydes and ketones. Next, we discuss additions involving C-nucleophiles....
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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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