ASETOASETİK ESTER KONDENSASYONU

ASETOASETİK ESTER KONDENSASYONU İki karbonil grubu arasında doymuş bir karbon atomu olan bileşiklere β-dikarbonil bileşikleri denir.Bu bileşikler organik sentezlerde son derece yararlı ve gerekli reaktiflerdir. β-karbonil bileşiklerinin kimyasında,iki karbonil grubu arasındaki karbona bağlı protonların asitliği önemli rol oynar.Bu protonların enolat oluşturmaları için alkoksit kuvvetinde bir bazla etkileştirilmeleri yeterlidir. Şimdi; Bilindiği üzere bu protonların asidik özelliklerinin CLAİSEN KONDENSASYONU tepkimesi üzerinden β-dikarbonil bileşiklerinin sentezine nasıl imkan sağladığını mekanizmasıyla görelim.

Asetoasetik ester kondensasyon tepkimesi 2

Asetoasetik ester kondensasyon mekanizması

Etil asetat,sodyum etoksit ile tepkimeye girdiğinde kondensasyon tepkimesine uğrar.Asitlendirmeden sonra ürün bir keto ester,etil asetoasetat(asetoasetik ester)tır.Mekanizmanın ilk basmağında sodyumetoksit gibi kuvvetli bir baz kullanılarak etil asetat enolat anyonuna dönüşür.Daha sonra oluşan bu enolat anyonu tekrar etil asetatın karbonil karbonuna saldırarak oksijenin çiftli bağından bir tanesinin kırılmasına neden olur.Bu kararsız yapıyı sevmeyen oksijen üzerindeki ortaklanmamış elektron çiftleri kapanarak karbonla tekrar çiftli bağ oluşturduğu sırada etoksit ile karbon arasındaki bağ kırılır,etoksit yapıdan uzaklaşır ve asetoasetik ester oluşur.

MAKALE CONDENSATION OF ORTHO ESTERS WITH ACETOACETIC ESTER AND MALONIC ESTER (PETER P.T. SAH)

The condensation of ortho-formic ester with compounds containing a reactive methylene group between two carbonyl groups, such as acetoacetic ester, malonic ester, acetylacetone, etc., was a classical piece of work of Claisen,2 who compared the analogy of this type of condensation to the formation of acetoacetic ester

where R’ and R” are alkyl, aryl or alkoxy groups.

The condensation products, esters of hydroxymethylene acids, formed very stable inorganic salts. The unstable acids were also isolated by Claisen and found to be exceedingly strong acids. They may be considered as derivatives of formic acid in which the carbonyl oxygen is replaced by a carbon atom attached to two negative groups.

By using triethyl ortho-acetate and triethyl orthobenzoate in place of triethyl ortho-formate in the above condensation, we have been able to isolate the methyl and the phenyl derivatives of ethoxymethylene esters and identify them through their copper salts. The condensation is, therefore, a general reaction for the class of ortho esters. The mechanism of the reaction may be expressed according to the following general equation,

where R represents an alkyl or an aryl group

General Procedure for the Condensation All the reagents used were freshly distilled and the apparatus kept absolutely anhydrous. One mole of ortho ester, one mole of acetoacetic ester or malonic ester and two moles of acetic anhydride (freshly distilled over sodium, b. p. 137°) were mixed and refluxed vigorously for four hours on an oil-bath with or without the addition of fused zinc chloride to catalyze the condensation. The resulting ethyl acetate and acetic acid were removed by distillation at atmospheric pressure till the thermometer, which was immersed in the solution, registered 120°.

The mixture was then again refluxed for an hour and some more ethyl acetate was distilled out.6 The residue was then fractionated under a reduced pressure of 3 mm. produced by an oil pump, A small amount of acetic acid and acetic anhydride distilled over first, then the unreacted esters and finally the condensation product. The yield of the ethoxymethylene esters thus obtained varied exceedingly, depending upon the following factors: 1) purity of the reagents used, 2) time of reaction] 3) stability of the ortho ester used, 4) stability of the condensation product 5) the presence of catalysts. With this procedure Claisen's experiments could be easily repeated with a yield of 50-60% under the best condition.

BENİ DİNLEDİĞİNİZ İÇİN TEŞEKKÜR EDİYORUM 

HAZIRLAYAN: BAÜ FEF KİMYA BÖLÜMÜ 3.SINIF 2.Ö. 200620105021 HASNİYE ÖZKAN