<P> To designate two anomers the relative stereodescriptors alpha (α) and beta (β) are used . In the α anomer the anomeric carbon atom and the reference atom do have opposite configurations (R, S) or (S, R), whereas in the β anomer they are the same (R, R) or (S, S). </P> <P> Stereochemistry also plays a role assigning faces to trigonal molecules such as ketones . 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 product results . When the nucleophile is a chiral molecule diastereoisomers 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 . 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) was added to the prochiral center from the Re-face, this would result in an (R) - enantiomer . </P>

What is the cahn ingold prelog priority of the oh group in this molecule