Editing Cahn–Ingold–Prelog priority rules (section)

== Faces ==
[[Image:Si Re face approach.svg|class=skin-invert-image|700px|thumb|right|[[Acetophenone]] and [[1-Phenylethanol|α-phenylethanol]]. The H atom has the lowest priority number 4.]]
Stereochemistry also plays a role assigning ''faces'' to trigonal molecules such as [[ketone]]s. A [[nucleophile]] in a [[nucleophilic addition]] can approach the [[carbonyl]] group from two opposite sides or faces. When an achiral nucleophile attacks [[acetone]], both faces are identical and there is only one reaction product. When the nucleophile attacks [[butanone]], the faces are not identical (''[[enantiotopic]]'') and a [[Racemic mixture|racemic product]] results. When the nucleophile is a [[chirality (chemistry)|chiral]] molecule [[diastereoisomer]]s are formed. When one face of a molecule is shielded by substituents or geometric constraints compared to the other face the faces are called [[diastereotopic]]. The same rules that determine the stereochemistry of a stereocenter (''R'' or ''S'') also apply when assigning the face of a molecular group. The faces are then called the '''''Re''-face''' and '''''Si''-face'''.<ref>{{Cite journal | author = Moss, G. P. | title = Basic terminology of stereochemistry (IUPAC Recommendations 1996) | journal = [[Pure and Applied Chemistry]]  | volume = 68 | issue = 12 | pages = 2193–2222 | year = 1996 | doi = 10.1351/pac199668122193| s2cid = 98272391 | doi-access = free }}</ref><ref>{{GoldBookRef|title=Re, Si| file = R05308}}</ref> In the example displayed on the right, the compound [[acetophenone]] is viewed from the ''Re''-face. Hydride addition as in a reduction process from this side will form the (''S'')-enantiomer and attack from the opposite ''Si''-face will give the (''R'')-enantiomer.  However, one should note that adding a chemical group to the prochiral center from the ''Re''-face will not always lead to an (''S'')-stereocenter, as the priority of the chemical group has to be taken into account. That is, the absolute stereochemistry of the product is determined on its own and not by considering which face it was attacked from.  In the above-mentioned example, if [[chloride]] (''Z'' = 17) were added to the prochiral center from the ''Re''-face, this would result in an (''R'')-enantiomer.