Thermodynamic control over ring size

In this section we have discussed the rate at which rings form: in other words the kinetics of ring formation. However, there are many ring-forming reactions that are under thermo dynamic and not kinetic control. For example, you have already seen that glucose exists pre dominantly as a six-membered ring in solution. It could also exist as a five-membered ring: it doesn’t because although five-membered rings form faster than six-membered ones, they are usually less stable (remember, a six-membered ring is essentially strain-free). For similar thermo dynamic reasons, it doesn’t exist as a seven-membered ring, even though you can draw a reasonable structure for it.

Thermodynamic control is important in other ways in carbohydrate chemistry because control over ring size allows selective protection of the hydroxyl groups of sugars. Compare these two reactions. Both of them give acetals from the same starting material, mannitol.

Don’t be put off by the way in which we have had to twist half the molecule round to draw the left-hand structure: the stereochemistry hasn’t changed. The important thing is that ace tone reacts with mannitol to form three five-membered acetals (dioxolanes) while benzaldehyde forms only two six-membered acetals. This is quite a common result: when there is a choice, acetone prefers to react across a 1,2-diol to give a five-membered acetal, while aldehydes prefer to react across a 1,3-diol to form a six-membered acetal. Drawing a conformational diagram of the product on the right helps to explain why. All of the substituents are equatorial, making this a particularly stable structure. Now imagine what would happen if acetone formed this type of six-membered ring acetal. There would always be an axial methyl group, and the six-membered rings would be less stable.

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اخبار العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظم ندوة عن الأبعاد الفكرية والعقدية لثورة الإمام الحسين (عليه السلام)

المجمع العلمي يواصل برامجه ودوراته القرآنية في عدد من المحافظات العراقية

لليوم الرابع.. قسم العلاقات العامة يواصل إقامة فعاليات مخيّم البذرة الواعدة

العتبة العباسية المقدسة تطلق التشغيل التجريبي لمعمل غسيل السجاد والأغطية

المزيد

الآخبار الصحية

نصائح عملية للحد من التعرق المفرط في الصيف

أطباء يحذرون: مكمل غذائي شائع قد يدمر الجهاز العصبي

تحذير طبي من تأثير جانبي "غير متوقع" لحقن التخسيس

حمامات الثلج قد تكون خطيرة وأحيانا مميتة.. ما السبب؟

المزيد

الآخبار التكنلوجية

أكبر جدارية شمسية في العالم تدخل موسوعة غينيس.. مشروع ضخم

تسارع دوران الأرض يثير القلق.. وتحذيرات من "كوارث محتملة" ؟

"سوذبيز" تطرح أكبر قطعة من المريخ على الأرض في مزاد علني

أوبن أيه آي تعتزم إطلاق متصفح إنترنت يعمل بالذكاء الاصطناعي

المزيد

مواضيع ذات صلة

Baldwin’s rules and ring opening

Combatting ΔS‡—the Thorpe–Ingold effect

Making heterocycles: ring-closing reactions

Related effects in other types of compounds

Some substituents of saturated heterocycles prefer to be axial: the anomeric effect

Determining stereochemistry when coupling constants are no help: the nuclear Overhauser effect

Oxygen heterocycles

Aziridine: ring strain promotes ring opening

Reactions of saturated heterocycles

More heteroatoms in fused rings mean more choice in synthesis

Quinolones also come from anilines by cyclization to an ortho position

Quinolines and isoquinolines