2444-120211

2444-120211 - EXAM 4 Avg. 79.6 / 100 Stnd. Dev. 18.8 2...

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EXAM 4 Avg. 79.6 / 100 Stnd. Dev. 18.8
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Intramolecular Aldol 2 • Aldol condensation reaction can form a ring to give the usual alkoxide product ( 59 ). • Hydrolysis leads to the expected aldol product 60 . • It It is an intramolecular aldol condensation. condensation. Note that 59 is drawn first with an peculiar looking "extended" bond in order to keep the relative shape the same, but it is then re-drawn in its proper five-membered ring form. • Note that the numbers used are arbitrary, since the IUPAC name of 60 is 2- hydroxycyclopentanecarboxaldehyde (or carbaldehyde). H H O O H H O O H H O O H O O H H O HO H 1 2 3 4 5 6 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 6 6 6 6 5 57 58 59 60 LDA THF -78°C aq. H +
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Unsymmetrical Ketones O LDA , THF –78°C O CHO O H 3 O + O OH Add a different C=O compound In a 2nd step A mixed aldol Unsymmetrical ketones pose a different problem O O O O H b H a O H b O H a + more highly substituted C=C so thermodynamically more stable THERMODYNAMIC H a is more acidic than H b Removal of more acidic H a leads to first formed product KINETIC two enolate anions can be formed from an unsymmetrical ketone H on less substituted C is more acidic and H On more substituted C is less acidic since C groups are electron releasing
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Unsymmetrical Ketones 4 • The stability of the conjugate base plays a major role in the magnitude of K a . The structure of the enolate anion must be considered as a major influence on the position of the equilibrium . • An equilibrium reaction is under thermodynamic control, which will favor the more stable (lower energy) product . • Assume that the more highly substituted enolate anion will be more stable and will be the major product in an equilibrium reaction. Et O H b Et O H b 32 Et O H a H b Et O H a Et O H a 34 36 Base Base K a 1 K a 2
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Kinetic versus Thermodynamic 5 S i n c e 34 i s f o r m e d f a s t e s t , i t i s referred to as the kinetic product . • Since 36 is formed under equilibrium conditions and is thermodynamically more stable, it is referred to as the thermodynamic product . Et O H b Et O H b 32 Et O H a H b Et O H a Et O H a 34 36 Base Base K a 1 K a 2
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Kinetic versus Thermodynamic 6 When LDA (the base) reacts with 26 (the acid) the acid-base reaction shown leads to the conjugate base (enolate anion 34 ) and the conjugate acid (diisopropylamine, 29 ).
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This note was uploaded on 02/25/2012 for the course CHEM 244 taught by Professor Jardin,j during the Fall '08 term at UConn.

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2444-120211 - EXAM 4 Avg. 79.6 / 100 Stnd. Dev. 18.8 2...

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