4,4′-Bipyridine (abbreviated to 4,4′-bipy or 4,4′-bpy) is an organic compound with the formula . It is one of several isomers of bipyridine. It is a colorless solid that is soluble in organic solvents. is mainly used as a precursor to N,N′-dimethyl-4,4′-bipyridinium [(C5H4NCH3)2]2+, known as paraquat.

History 4,4′-Bipyridine was first obtained in 1868 by the Scottish chemist Thomas Anderson via heating pyridine with sodium metal.See: Anderson called 4,4′-bipyridine "Dipyridine". German translation: See also: See γ-Dipyridyl. However, Anderson's empirical formula for 4,4′-bipyridine was incorrect.Anderson gave the empirical formula for 4,4′-bipyridine as C10H10N2. See: (Anderson, 1868), p. 209. (Fehling, 1890), p. 974 (γ-Dipyridyl). The correct empirical formula, and the correct molecular structure, for 4,4′-bipyridine was provided in 1882 by the Austrian chemist Hugo Weidel and his student M. Russo. The empirical formula for 4,4′-bipyridine (γ-Dipyridyl) appears on p. 856; the molecular structure of 4,4′-bipyridine (γ-Dipyridyl) appears on p. 867.

Uses 4,4'-Bipyridine is an intermediate in the production of paraquat, a widely used herbicide. In this process, pyridine is oxidized to 4,4'-bipyridine in a coupling reaction, followed by dimethylation to form paraquat. One possible interpretation of the synthesis is:

However, the Birch reduction is not performed in a distinct step from the oxidation, and the reaction may not involve complete reduction to the aminoalkane (sodium is used in excess, so the distinction cannot be deduced from stoichiometry). An alternative interpretation is an interrupted reduction: another pyridine molecule captures the Birch pyridinyl radical intermediate to give bypyridinyl, which oxygen then quenches.

Reactions The reducing agent N,N'-bis(trimethylsilyl)-4,4'-bipyridinylidene is produced by reduction of 4,4'-bipyridine in the presence of trimethylsilyl chloride (Me = CH3):

The silylated derivative, which is red, is used in salt-free reductions.

4,4′-bipyridine forms a variety of coordination polymers.

References