Interactions even weaker than ionic bonds can be used to separate enantiomers. Chromato graphic separation relies on a difference in affinity between a stationary phase (often silica) and a mobile phase (the solvent travelling through the stationary phase, known as the eluent) mediated by, for example, hydrogen bonds or van der Waals interactions. If the stationary phase is made chiral by bonding it with an enantiomerically pure compound (often a derivative of an amino acid), chromatography can be used to separate enantiomers.

Chromatography on a chiral stationary phase is especially important when the compounds being resolved have no functional groups suitable for making the derivatives (usually esters or salts) needed for the more classical resolutions described above. For example, the two enantiomers of an analogue of the tranquillizer Valium were found to have quite different biological activities.

In order to study these compounds further, it was necessary to obtain them in enantiomerically pure form. This was done by passing a solution of the racemic compound through a column of silica bonded to an amino-acid-derived chiral stationary phase. The (R) - (–) enantiomer showed a lower affinity for the stationary phase and therefore was eluted from the column fi rst, followed by the (S) - (+)-enantiomer.

Two enantiomers of one molecule may be the same compound, but they are clearly different, although only in a limited number of situations. They can interact with biological systems differently, for example, and can form salts or compounds with different properties when reacted with a single enantiomer of another compound. In essence, enantiomers behave identically except when they are placed in a chiral environment. In Chapter 41 we will see how to use this fact to make single enantiomers of chiral compounds, but next we move on to three classes of reactions in which stereochemistry plays a key role: substitutions, eliminations, and additions.

مواضيع ذات صلة

Axes and centres of symmetry

المرافق الإنزيمي حامض الليبويك Lipoic acid

التأثير التحريضي (الحاث) Inductive effect

تحضر يوديد البوتاسيوم KI بتركيز 0.1M

البوليمرات ذات مجموعة الروابط الاقترانية

تأثير إضافة البولي ستايرين المتكسر في مواصفات الاسفلت

الكشف نقاط التكافؤ Detection of Equivalence Points

الفصل باستخدام تكوين الأيون المعقد Separation by Mean of complex-ion formation

ذوبان الأملاح الأيونية Solubility of ionic salts

الخواص الفيزيائية للفينولات

أكتشاف النشاط الإشعاعي

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