- 學位論文
應用核磁共振技術探討影響巴比妥酸分子之酮-烯醇互變異構之變因
Application of NMR techniques to investigate the variables of Barbituric acid molecules under Keto-Enol tautomerization
摘要
巴比妥酸(Barbituric Acid，以下簡稱 BTA)是一種強酸，pKa為4.01，較醋酸強一些。它的酸性來自於 β-dicarbonyl 結構，在某些條件下，去除一個亞甲基質子，能夠得到一個穩定的烯醇離子。去質子化可能使α碳回到酮型式，或者使氧原子形成乙烯醇而得到烯醇型式。這種內部結構的轉換被稱為互變異構化，而烯醇離子在化學反應中則被視為親核試劑使用。有機教科書中提到，即使平衡時烯醇離子濃度很小，還是能夠當作親核試劑產生反應。當烯醇離子發生親電子反應，使烯醇離子濃度下降，酮烯醇互變異構化平衡便會使反應往烯醇離子移動。 然而，即使有些 BTA 化學反應已印證是烯醇離子所產生，目前卻沒有任何 BTA 在液態時為烯醇型式的數據，其於液態時幾乎皆以100%酮型式出現。另外，由於 X-ray 和 Mass 光譜數據只能描繪分子樣式，很難研究 BTA 在化學反應系統中的互變異構化真實狀況。所以使用核磁共振研究 BTA 在液體中的酮-烯醇互變異構化。 酮-烯醇的轉換應從能量上的關係來作探討。酮溶劑複合體和烯醇溶劑複合體的能量差，分別關係到酮和烯醇的溶劑與烯醇離子的穩定度。此外，因為 BTA 可以透過各種相互氫鍵形成板塊，而實驗發現 BTA 的氫鍵強度對溫度變化非常敏感，故溫度變化對實驗結果影響甚鉅。 基於以上考慮進行實驗，透過改變溶劑、溫度、濃度以及溶劑酸鹼度，觀察到 BTA 能夠以酮和烯醇型式共存，配合 van’t Hoff 方程式得到實驗數據，發現 BTA 的酮-烯醇互變異構化主要是藉由焓控制，一旦發生互換，溫度越低，越有利於烯醇型式存在。
並列摘要
Abstract BTA, as its name suggests, is a moderate strong acid with pKa of 4.01 even stronger than acetic acid. The acidity is believed to its -dicarbonyl structure which under suitable conditions a -methylene proton can be removed to give a stabilized enolate ion. The reprotonation may occur either to the -carbon to return to the keto form or to the oxygen atom to give a vinyl alcohol, the enol form. This kind of interstructure conversion is called the tautomerization. It is the enolate ion which is responsible for the chemical reaction as a nucleophile. As the organic textbooks mention, even though the equilibrium concentration of the enolate ion may be small, it serves as a reactive nucleophile. When an enolate ion reacts with an electrophile, the enolate concentration decrease and the equilibrium shifts to the enolate again. However there is no evidence of enol form structure of BTA in liquid state. Even there is some BTA left after a chemical reaction, the BTA is in 100% keto form. Since X-ray and MASS spectral data relate to the represented molecule ony, it is hard to use to the study the BTA tautomerization phenomenon in real chemical reaction systems. We use NMR to investigate the BTA keto-enol tautomerization in liquids. When studying the enol-to-keto convertion, the energy issues are essential. It involves the solvation energy to the keto and enol forms respectively, the stailization of the enolate ion, and the difference in energy between keto-solvent complex and enol-solvent complex. Also the temperature effect is very important since BTA can form a variety of self-assembly with each other through hydrogen bonds and we found BTA’s hydrogen bond strength is very sensitive to temperature change. Based on these considerations, a series of experiments have been performed by changing the solvent, temperature, concentration. A compiling of the experimental data to the fitting of van’t Hoff equation, we found that the BTA keto-enol tautomerization is principally enthalpy control. By changing the acidity/basicity of the solvents, the coexist of keto and enol forms of BTA can be observed. Once the interconversion happens, lower the temperature will favor the exist of enol form.