projection). The preference for the formation of the threo isomer can be explained by the rule stated above by having the active nucleophile in this reaction attacking the carbonyl group from the least hindered side (see Newman projection A) when the carbonyl is positioned in a staggered formation with the methyl group and the hydrogen atom, which are the two smallest substituents creating a minimum of steric hindrance, in a gauche orientation and phenyl as the most bulky group in the anti conformation. The second reaction is the organic reduction of 1,2-diphenyl-1-propanone 2 with lithium aluminium hydride, which results in the same reaction product as above but now with preference for the erythro isomer (2a). Now a hydride anion (H−) is the nucleophile attacking from the least hindered side (imagine hydrogen entering from the paper plane). In the original 1952 publication, additional evidence was obtained for the structural assignment of the reaction products by applying them to a Chugaev elimination, wherein the threo isomer reacts to the cis isomer of -α-methyl-stilbene and the erythro isomer to the trans version. Felkin model The Felkin model (1968) named after Hugh Felkin also predicts the stereochemistry of nucleophilic addition reactions to carbonyl groups. Felkin argued that the Cram model suffered a major drawback: an eclipsed conformation in the transition state between the carbonyl substituent (the hydrogen atom in aldehydes) and the largest α-carbonyl substituent. He demonstrated that by increasing the steric bulk of the carbonyl substituent from methyl to ethyl to isopropyl to isobutyl, the stereoselectivity also increased, which is not predicted by Cram's rule: The Felkin rules are: The transition states are reactant-like. Torsional strain (Pitzer strain) involving partial bonds (in transition states) represents a substantial fraction of the strain between fully formed bonds, even when the degree of bonding is quite low. The conformation in the TS is staggered and not eclipsed with the substituent R skew with respect to two adjacent groups one of them the smallest in TS A. For comparison TS B is the Cram transition state. The main steric interactions involve those around R and the nucleophile but not the carbonyl oxygen atom. Attack of the nucleophile occurs according to the Dunitz angle (107 degrees), eclipsing the hydrogen, rather than perpendicular to the carbonyl. A polar effect or electronic effect stabilizes a transition state with maximum separation between the nucleophile and an electron-withdrawing group. For instance haloketones