One of the very fi rst reactions you met, back in Chapter 6, was between an aldehyde and cyanide. The product was a compound containing a nitrile group and a hydroxyl group.

How many products are formed in this reaction? Well, the straightforward answer is one— there’s only one aldehyde, only one cyanide ion, and only one reasonable way in which they can react. But this analysis is not quite correct. One point that we ignored when we fi rst talked about this reaction, because it was irrelevant at that time, is that the carbonyl group of the aldehyde has two faces. The cyanide ion could attack either from the front face or the back face, giving, in each case, a distinct product.

As we explained in Chapter 6 (pp. 125–7), the cyanide attacks the π* orbital of the aldehyde more or less at right angles to the plane of the molecule as it forms a new bond with the old p orbital on C. This translates into ‘front’ and ‘back’ on a diagram on paper. Compare the diagram on the left with the others to make sure this is clear. Are these two products different? If we lay them side by side and try to arrange them so that they look identical, we find that we can’t—you can verify this by making models of the two structures. The structures are non-superimposable—so they are not identical. In fact, they are mirror images of each other: if we reflected one of the structures, A, in a mirror, we would get a structure that is identical with B.

We call two structures that are not identical but are mirror images of each other (like these two) enantiomers. Structures that are not superimposable on their mirror image, and can therefore exist as two enantiomers, are called chiral. In this reaction, the cyanide ions are just as likely to attack the ‘front’ face of the aldehyde as they are the ‘back’ face, so we get a 50:50 mixture of the two enantiomers.

●Enantiomers and chirality

• Enantiomers are structures that are not identical, but are mirror images of each other.

• Structures are chiral if they cannot be superimposed on their mirror image.

Now consider another similar reaction—the addition of cyanide to acetone.

Again an adduct (a cyanohydrin) is formed. You might imagine that attacking the front or the back face of the acetone molecule could again give two structures, C and D.

However, this time rotating one to match the other shows that they are superimposable and therefore identical.

Make sure that you are clear about this: C and D are identical molecules, while A and B are mirror images of each other. Reflection in a mirror makes no difference to C or D; they are superimposable on their own mirror images and therefore cannot exist as two enantiomers. Structures that are superimposable on their mirror images are called achiral.

● Achiral structures are superimposable on their mirror images.

مواضيع ذات صلة

Coupling in the proton NMR spectrum Nearby hydrogen nuclei interact and give multiple peaks

Exchange of acidic protons is revealed in proton NMR spectra

Protons on heteroatoms have more variable shifts than protons on carbon

Non-aldehyde protons in the aldehyde region: pyridines

The aldehyde region: unsaturated carbon bonded to oxygen

Structural information from the alkene region

Electron-rich and electron-deficient alkenes

How electron donation and withdrawal change chemical shifts

Uneven electron distribution in aromatic rings

The benzene ring current causes large shifts for aromatic protons

Summary chart of proton NMR shifts

Chemical shifts of CH groups