Dolbier HW Solutions 401

Dolbier HW Solutions 401 - a , b-unsaturated ketones. The...

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Now, mentally disconnect the bond between the a -carbon atom and the carbon that bears the hydroxyl group to reveal the intermediate that undergoes intramolecular aldol condensation. The b -hydroxy ketone is the intermediate formed in the intramolecular aldol addition step, and the diketone that leads to it is the intermediate that is formed in the conjugate addition step. The relationship of the starting materials to the intermediates and product is now more evident. 18.38 ( a ) The reduced C ? O stretching frequency of a , b -unsaturated ketones is consistent with an en- hanced degree of single bond character as compared with simple dialkyl ketones. Resonance is more important in a , b -unsaturated ketones. Conjugation of the carbonyl group with the carbon carbon double bond increases opportunities for electron delocalization. ( b ) Even more single-bond character is indicated in the carbonyl group of cyclopropenone than in that of typical
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Unformatted text preview: a , b-unsaturated ketones. The dipolar resonance form contributes substantially to the electron distribution because of the aromatic character of the three-membered ring. Recall that cyclopropenyl cation satis f es the 4 n 1 2 rule for aromaticity (text Section 11.20). ( c ) The dipolar resonance form is a more important contributor to the electron distribution in diphenylcyclopropenone than in benzophenone. C 6 H 5 C 6 H 5 O C 6 H 5 C 6 H 5 O 2 O O 2 ; O 2 1 Equivalent to an oxyanion-substituted cyclopropenyl cation R 2 C CH CR 9 O R 2 C CH CR 9 O 2 1 R 2 C CH CR 9 O 2 1 RCR 9 O RCR 9 O 2 1 CH 3 OH C 6 H 5 C 6 H 5 O C 6 H 5 CH 3 O Intermediate formed in conjugate addition step C 6 H 5 C C H 2 C C 6 H 5 O CH C 6 H 5 CCH 2 CH 3 2 O C 6 H 5 C 6 H 5 O C 6 H 5 CH 3 O 2 C 6 H 5 C 6 H 5 O C 6 H 5 CH 3 O H 5 C 6 O C 6 H 5 CH 3 OH Disconnect this bond. ENOLS AND ENOLATES 495...
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