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2,4,6-Trimethylpyridine, commonly called 2,4,6-collidine or sym-collidine, is a methylated derivative of pyridine. It is used as a tissue fixative for electron microscopy and as a reagent and base in organic synthesis.

Preparation
Pyridine derivatives are most commonly isolated from coal tar. Coking coal at temperatures from 900 to 1200 °C produces coal tars from which 2,4,6-collidine and other methylated pyridines can be isolated either by fractional distillation or via the formation of salts and/or other coordination complexes. Pyridine derivatives can also be isolated from shale oil and some petroleums.

Alternatively, 2,4,6-collidine can be prepared from ethyl acetoacetate, acetaldehyde, and ammonia via the Hantzsch pyridine synthesis.

Commercially available 2,4,6-collidine can contain large amounts of 3,5-lutidine and 2,3,5-collidine. Distillation from boron trifluoride etherate can remove the former, and recrystallization of the hydrochloride salt from ethanol can remove the latter. 2,4,6-Collidine can also be purified via complexes with mercury(II) chloride and urea.

Uses
2,4,6-collidine is a weak base, and many of its uses are based on this property.

Electron Microscopy
In electron microscopy experiments, biological samples are often chemically preserved with a fixative prior to analysis. Osmium tetroxide, a common fixative, is most effective between pH 7.2 and 7.5 and is typically used in a buffered solution to maintain a pH value within this range. Although veronal-acetate buffers can be - and traditionally were - used for this task, they are poorly effective between pH 5.2 and 7.5. sym-Collidine buffers regulate pH most efficiently near pH 7.4, and are thus much better suited for use with osmium tetroxide. Additionally, collidine-based sytems are more stable and have longer shelf lives than veronal-acetate buffers.

Base
Used as a sterically hindered base.

Reagent
Used to make a complex with iodine that serves as a source of electrophilic iodine.