Some compounds can exist as a pair of mirror-image forms

Some compounds can exist as a pair of mirror-image forms 

One of the veiy first reactions you met, back in Chapter 6, was between an aldehyde and cyanide. They give a cyanohydrin, 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 first 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. 

Are these two products different If we lay them side by side and tiy to arrange them so diat they look identical, we find that we can t—you can verify this by making models of die two structures. 

The structures are nonsuperimposable—so they are not identical. In fact, diey are mirror images of each odier if we reflected one of the would get a structure that is identical with B. 

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SOME COMPOUNDS CAN EXIST AS A PAIR OF MIRROR-IMAGE FORMS... 

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