Flippin–Lodge angle
The nucelophile (atom or group) is represented here as :Nu. The substituents, R and R', attached to the carbon atom of the carbonyl group may be an atom such as hydrogen (H), alkyl groups such as methyl, ethyl, etc. (including far more complex ones), or other functional groups such as the O- and N-containing groups of esters and amides. In both panels, a second plane, plane II (red), is defined that is orthogonal (perpendicular) to the first, con-taining the carbon and oxygen atoms of the carbonyl, C=O, and bisecting the R—•—R' angle (where the "•" is the carbon atom). A third plane, mutually orthogonal to the first two, plane III (green), contains only the carbon atom of the carbonyl; it corresponds to the plane of the page in the Newman projection at right. In that projection, the vector (arrow) pointing from :Nu to the carbon of the carbonyl is mirrored across plane I to make clear that the nucleophile can approach from either above or below this plane (with resulting differences in the stereochemistry of reaction products, see text). The dotted line from :Nu to plane I indicates the computational process of mathematically projecting the geometric point representing :Nu onto plane I, which is sometimes a necessary maths operation (see text).
The Flippin–Lodge angle is one of two angles used by organic and biological chemists studying the relationship between a molecule's chemical structure and ways that it reacts, for reactions involving "attack" of an electron-rich reacting species, the nucleophile, on an electron-poor reacting species, the electrophile. Specifically, the angles—the Bürgi–Dunitz, , and the Flippin–Lodge, —describe the "trajectory" or "angle of attack" of the nucleophile as it approaches the electrophile, in particular when the latter is planar in shape. This is called a nucleophilic addition reaction and it is plays a central role in the biological chemistry taking place in many biosyntheses in nature, and is a central "tool" in the reaction toolkit of modern organic chemistry, e.g., to construct new molecules such as pharmaceuticals. Theory and use of these angles falls into the areas of synthetic and physical organic chemistry, which deals with chemical structure and reaction mechanism, and within a sub-specialty called structure correlation.
...
Wikipedia