310-14 - Organic Lecture Series CH 310 N LECTURE 14...

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Organic Lecture Series 1 CH 310 N LECTURE 14 Textbook Assignment: Chap 16 CONT’D Homework (for credit): POW 4 posted (Due Feb 21) Today’s Topics: Acetals; Enamines & Imines Notice & Announcements: “Perspectives on Medicine” Melanie Collins, M.D. - Obstetrics & Gynecology Robert Milman, M.D. – Radiology Today 2-3 WEL1.308 Organic Lecture Series 2 Aldehydes Aldehydes And And Ketones Ketones (Chapter 16) (Chapter 16)
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Organic Lecture Series 3 • Hemiacetals react with alcohols to form acetals Acetal: Acetal: a molecule containing two -OR or -OAr groups bonded to the same carbon Addition of Alcohols to Carbonyls Addition of Alcohols to Carbonyls Nota Bene : acetals are STABLE in base OH OR H-OR H + H 2 O acetal + Ahem iace ta l + Organic Lecture Series 4 Step 1: proton transfer from HA gives an oxonium ion Step 2: loss of water gives a resonance- stabilized cation HO R-C- OCH 3 H HA H H O H 3 A: - + An oxonium ion + + R-C OCH 3 H O H H H R- C OCH 3 H 3 H 2 O + A resonance-stabilized cation + + + Begin Mechanism from the hemiacetal stage:
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Organic Lecture Series 5 Step 3 : reaction of the cation (an electrophile) with methanol (a nucleophile) gives the conjugate acid of the acetal Step 4 : proton transfer to A - gives the acetal and generates a new acid catalyst CH 3 -OH H R- C OCH 3 R-C OCH 3 H O 3 H A protonated acetal + + + A: - R- C OCH 3 H O 3 H 3 H R-C-OCH 3 H-A + (4) An acetal + + Organic Lecture Series 6 – with ethylene glycol and other glycols, the product is a five-membered cyclic acetal – this a method of “protecting” ketones + H + HO OH O O O Cyclic acetal + H 2 O Addition of Alcohols to Carbonyls Addition of Alcohols to Carbonyls
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Organic Lecture Series 7 Dean Dean -Stark Trap Stark Trap Organic Lecture Series 8 Acetals as Protecting Groups Acetals as Protecting Groups • How to bring about a Grignard reaction between these compounds: 5-Hydroxy-5-phenylpentanal (racemic) 4-Bromobutanal Benzaldehyde ??
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310-14 - Organic Lecture Series CH 310 N LECTURE 14...

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