Threose
Threose is a four-carbon monosaccharide with molecular formula C4H8O4. It has a terminal aldehyde group, rather than a ketone, in its linear chain and so is considered part of the aldose family of monosaccharides. The threose name can be used to refer to both the Template:Smcaps- and Template:Smcaps-stereoisomers and more generally to the racemic mixture (Template:Smcaps/L-, equal parts D- and L-) as well as to the more generic threose structure (absolute stereochemistry unspecified).
The prefix "threo-" which derives from threose (and "erythro-" from a corresponding diastereomer erythrose) offer a useful way to describe general organic structures with adjacent chiral centers, where "the prefixes... designate the relative configuration of the centers".<ref name="Rausch">Formulas Using Other Configurational Notations, W. Rausch, accessed 1 March 2011</ref> As is depicted in a Fischer projection of Template:Smcaps-threose, the adjacent substituents will have a syn orientation in the isomer referred to as "threo", and are anti in the isomer referred to as "erythro".<ref name="Rausch"/><ref>Prof. Rausch helpfully notes that the prefixes "may be applied to racemic compounds, as well as pure enantiomers and meso compounds", and that when depicted in the common "zig-zag" representation, adjacent "substituents may lie on the same side of the carbon chain... [syn] or on opposite sides... [anti]", which is opposite of their depiction in a Fischer projection.</ref>
Although often inconsequential, threose in aqueous solution mainly exists as the hydrate owing to the following equilibrium:<ref>Template:Cite journal</ref>