CHEM302 S09 CA5 key

CHEM302 S09 CA5 key - CHEM302 Spring 2009 Dr Ruder 1 Class Activity 5 key Reactions of Carbonyls Aldehydes and Ketones Model 1 Nucleophilic

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CHEM302, Spring 2009 Dr. Ruder 1 Class Activity 5 key Reactions of Carbonyls: Aldehydes and Ketones Model 1 : Nucleophilic Addition to Carbonyls R R + O Nu O Nu R R aldehyde or ketone R R O R R O Questions: 1. For the reaction shown in Model 1: (a). Draw a polarity arrow on the C=O bond to show the direction of electron polarization. See above. (b). In the space above, draw a resonance form of the starting carbonyl compound. See above. (c). Which part of the carbonyl is likely to act as the electrophile? Explain why. The carbon will act as the electrophile because the carbon has a partial positive charge, as can be seen in the resonance form drawn above. (d). Draw curved arrows to show electron movement when the nucleophile reacts with the carbonyl. R R + O Nu O Nu R R 2. (a). Draw the structures for 2-propanone and ethanal. What are the main differences between these two compounds? One is a ketone and the other an aldehyde. O H O (b). If alkyl groups donate electrons, which compound would be the better electrophile? Explain. Ethanal would be the better electrophile because it has only one electron donating alkyl group. Electron donating alkyl groups make the electrophile more electron rich, and less reactive as a nucleophile. (c). Which group, CH 3 or H, is bigger? Based on steric effects, which compound is the better electrophile? Explain. CH3 is bigger than H. The more sterically hindered, the less likely the carbonyl is to act as an electrophile. Thus the aldehyde is the better electrophile because it is less hindered. 3. Draw structures for the following compounds and indicate which would be the most reactive in a nucleophilic addition reaction (rank from most to least reactive). Explain why (two reasons). Pentanal 3-methyl-2-pentanone diisopropyl ketone
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CHEM302, Spring 2009 Dr. Ruder 2 H O O O most reactive next reactive least reactive Carbonyl reactivity is based on steric and electronic effects. The aldehyde is more reactive than the ketone because there are fewer steric effects (H is small). In addition, R groups donate electrons through induction, so the more R groups makes the carbonyl carbon less positive, and therefore less reactive as an electrophile. Model 2: Basic Conditions
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This note was uploaded on 02/17/2011 for the course CHEM 302 taught by Professor Ruder during the Spring '09 term at VCU.

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CHEM302 S09 CA5 key - CHEM302 Spring 2009 Dr Ruder 1 Class Activity 5 key Reactions of Carbonyls Aldehydes and Ketones Model 1 Nucleophilic

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