The fact that the three compounds we have just introduced were chiral might have surprised you, because at fi rst glance they do look quite ‘symmetrical’. In fact, they do all have an element of symmetry, and it is only one which is compatible with chirality: an axis of symmetry. If a molecule can be rotated through 180° about an axis to give exactly the same structure, then it has twofold axial symmetry, or C₂ symmetry. Compounds with an axis of symmetry will still be chiral, provided they lack either a plane or a centre of symmetry. C₂ symmetry is common in many more everyday molecules than the ones in the last section. Below is an example of a compound with two diastereoisomers. One (we call it the syn diastereoisomer here—the two phenyl rings are on the same side) has a plane of symmetry—it must be achiral (and as it nonetheless has chiral centres, we can also call it the meso diastereoisomer). The other has some degree of symmetry, but it has axial symmetry and can therefore be chiral. The C₂ axis of symmetry is shown in orange. Rotating 180° gives back the same structure, but reflecting in a mirror plane (brown) gives a non superimpos able mirror image.

So far we have used a plane of symmetry as the defining characteristic of an achiral molecule: we have said several times that a molecule is chiral if it lacks a plane of symmetry. We are now going to introduce a second type of symmetry that is not compatible with chirality. If a molecule has a centre of symmetry, it is not chiral. We will now explain how to spot a centre of symmetry. The diamide skeleton in the margin has a plane of symmetry in the plane of the page and also a plane of symmetry at right angles to that plane passing through the two saturated car bon atoms (represented by the green dotted line). If we add substituents R to this structure, we can have two diastereoisomers with the two R groups on the same side (syn) of the fl at ring or on opposite (anti) sides. Although the plane of the paper is no longer a plane of symmetry, neither isomer is chiral as the other plane bisects the substituents and is still a plane of symmetry. So far nothing new.

Now consider the related double amide below. The plane of the page is again a plane of symmetry but there is now no plane of symmetry at right angles. This heterocycle is called a ‘diketopiperazine’ and can be made by dimerizing an amino acid: the compound in the mar gin is the dimer of glycine. With substituted amino acids, such as those below where R ≠ H, there are again two diastereoisomers, syn and anti. But their symmetry properties are different. The syn isomer is chiral but the anti-isomer is not.

The syn diastereoisomer has no plane of symmetry but you should be able to spot a C₂ axis of symmetry running straight through the middle of the ring. The axis is compatible with chirality of course. In this compound both chiral centres are S and it has an enantiomer where both are R.

The anti diastereoisomer has no plane of symmetry, nor does it have an axis. Instead, it has a centre of symmetry. This is marked with a black dot in the middle of the molecule and means that if you go in any direction from this centre and meet, say, an R group, you will meet the same thing if you go in the opposite direction (green arrows). The same thing applies to the brown arrows and, of course, to the ring itself. There is no centre of symmetry in the syn isomer as the green or brown arrows would point to R on one side and H on the other. The anti-isomer is superimposable on its mirror image and is achiral.

●Chirality in terms of planes, centres, and axes of symmetry

• Any molecule which has a plane of symmetry or a centre of symmetry is achiral.

• Any molecule which has an axis of symmetry is chiral, provided it does not also have a plane or a centre of symmetry. An axis of symmetry is the only symmetry element compatible with chirality.

مواضيع ذات صلة

Resolutions can be carried out by chromatography on chiral materials

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

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