The Flippin–Lodge angle is one of two angles used by organic and biological chemists studying the relationship between a molecules 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 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'.
Because chemical reactions take place in three dimensions, their quantitative description is, in part, a geometry problem. Two angles, first the Bürgi–Dunitz angle, , and later the Flippin–Lodge angle, , were developed to describe the approach of the reactive atom of a nucleophile (a point off of a plane) to the reactive atom of an electrophile (a point on a plane). The  is an angle that estimates the displacement of the nucleophile, at its elevation, toward or away from the particular R and R' substituents attached to the electrophilic atom (see image). The  is the angle between the approach vector connecting these two atoms and the plane containing the electrophile (see the Bürgi–Dunitz article). Reactions addressed using these angle concepts use nucleophiles ranging from single atoms (e.g., chloride anion, Cl) and polar organic functional groups (e.g., primary amines, R"-NH), to complex chiral catalyst reaction systems and enzyme active sites. These nucleophiles can be paired with an array of planar electrophiles: aldehydes and ketones, carboxylic acid-derivatives, and the carbon-carbon double bonds of alkenes. Studies of  and  can be theoretical, based on calculations, or experimental (either quantitative, based on X-ray crystallography, or inferred and semiquantitative, rationalizing results of particular chemical reactions), or a combination of these.
The most prominent application and impact of the Flippin–Lodge angle has been in the area of chemistry where it was originally defined: in practical synthetic studies of the outcome of carbon-carbon bond-forming reactions in solution.  An important example is the aldol reaction, e.g., addition of ketone-derived nucleophiles (enols, enolates), to electrophilic aldehydes that have attached groups varying in size and polarity. Of particular interest, given the three-dimensional nature of the concept, is understanding how the combined features on the nucleophile and electrophile impact the stereochemistry of reaction outcomes (i.e., the "handedness" of new  chiral centers created by a reaction). Studies invoking Flippin–Lodge angles in synthetic chemistry have improved the ability of chemists to predict outcomes of known reactions, and to design better reactions to produce particular stereoisomers (enantiomers and diastereomers) needed in the construction of complex natural products and drugs.