Just as there are enantiotopic and diastereotopic atoms and groups, so we
may distinguish enantiotopic and diastereotopic faces in trigonal molecules.
Again, we have three cases: (1) In formaldehyde or acetone (101), attack by
an achiral reagent A from either face of the molecule gives rise to the same
transition state and product; the two faces are thus equivalent. (2) In butanone
or acetaldehyde (102), attack by an achiral A at one face gives a transition
state and product that are the enantiomers of those arising from attack at the
other face. Such faces are enantiotopic. As we have already seen (p. 153),