R2CHX

R2CHX is a shorthand notation used in organic chemistry to denote a secondary alkyl halide. In this representation, the carbon bearing the halogen X is attached to two alkyl groups, indicated by R, plus one hydrogen. X stands for a halogen such as chlorine, bromine, iodine, or fluorine. The two R groups may be the same or different, and the structure emphasizes the carbon framework rather than a specific compound.

In reaction mechanism diagrams, R2CHX is used as a generic substrate to discuss common transformations of secondary alkyl halides, including nucleophilic substitution and elimination. The notation helps illustrate how changes in substituents or reaction conditions can influence the outcome without specifying a particular molecule.

Reactivity and preferred mechanism depend on conditions. SN2 tends to occur with strong nucleophiles in polar aprotic solvents and less hindered R groups, while SN1 and E1 can occur under conditions that stabilize carbocation intermediates (often with polar protic solvents and heat). E2 elimination competes with SN2, typically when a strong base is present or at higher temperatures.

A common concrete example is isopropyl chloride, (CH3)2CHCl, which fits the R2CHX form with R = CH3 and X = Cl. Other secondary alkyl halides, such as sec-bromides or sec-chlorides with various R groups, also conform to this notation.

In practice, R2CHX is a teaching shorthand for secondary alkyl halides; actual compound names are provided when specificity is required. See also alkyl halide, SN1, SN2, E1, and E2.