Editing Glyceraldehyde (section)

== Structure ==
Glyceraldehyde has one [[chirality (chemistry)|chiral]] center and therefore exists as two different [[enantiomer]]s with opposite optical rotation:

* In the [[Absolute configuration#By relative configuration: D- and L-|{{smallcaps all|D/L}} nomenclature]], either {{smallcaps all|D}} from Latin ''Dexter'' meaning "right", or {{smallcaps all|L}} from Latin ''Laevo'' meaning "left"
* In the [[Absolute configuration#By absolute configuration: R- and S-|R/S nomenclature]], either R from Latin ''Rectus'' meaning "right", or S from Latin ''Sinister'' meaning "left"

{| class="wikitable" style="margin: 1em auto 1em auto"
|-
| align = "center"|
| align="center"| '''{{smallcaps all|D}}-glyceraldehyde'''<br />(''R'')-glyceraldehyde<br />(+)-glyceraldehyde
| align="center"| '''{{smallcaps all|L}}-glyceraldehyde'''<br />(''S'')-glyceraldehyde<br />(−)-glyceraldehyde
|-
| align = "center"| '''[[Fischer projection]]'''
| align = "center"| [[Image:D-Glyceraldehyde 2D Fischer.svg|87px|class=skin-invert-image|D-glyceraldehyde]]
| align = "center"| [[Image:L-Glyceraldehyde 2D Fischer.svg|100px|class=skin-invert-image|L-glyceraldehyde]]
|-
| align = "center"| '''[[Skeletal formula]]'''
| align = "center"| [[Image:D-glyceraldehyde-2D-skeletal.svg|150px|class=skin-invert-image|D-glyceraldehyde]]
| align = "center"| [[Image:L-glyceraldehyde-2D-skeletal.svg|150px|class=skin-invert-image|L-glyceraldehyde]]
|-
|}

While the [[optical rotation]] of glyceraldehyde is (+) for ''R'' and (−) for ''S'', this is not true for all monosaccharides. The stereochemical configuration can only be determined from the chemical structure, whereas the optical rotation can only be determined [[empirically]] (by experiment).

It was by a lucky guess that the molecular {{smallcaps all|D-}} geometry was assigned to (+)-glyceraldehyde in the late 19th century, as confirmed by [[X-ray crystallography]] in 1951.<ref>{{cite journal | doi = 10.1038/168271a0 | title = Determination of the Absolute Configuration of Optically Active Compounds by Means of X-Rays | date = 1951 | last1 = Bijvoet | first1 = J. M. | last2 = Peerdeman | first2 = A. F. | last3 = Van Bommel | first3 = A. J. | journal = Nature | volume = 168 | issue = 4268 | pages = 271–272 | bibcode = 1951Natur.168..271B }}</ref>

===Aqueous and concentrated solutions of glyceraldehyde===
The description above focuses on classification of isomers, but the glyceraldehyde is subject to a further complications: the tendency of hydroxy-aldehydes to exist as hydrates.  [[NMR spectroscopy|NMR measurements]] indicate that in aqueous solution, glyceraldehyde exists in a hydrate owing to this reaction:
:{{chem2|HOCH2CH(OH)CHO + H2O  <->  HOCH2CH(OH)CH(OH)2}}
The same study indicates that concentrated ("syrupy") forms of glyceraldehyde exist as dimers, indicating [[hemiacetal]] formation.<ref>{{cite journal |doi=10.1071/CH9801001 |title=The Composition of Reducing Sugars in Aqueous Solution : Glyceraldehyde, Erythrose, Threose |date=1980 |last1=Angyal |first1=SJ |last2=Wheen |first2=RG |journal=Australian Journal of Chemistry |volume=33 |issue=5 |page=1001 }}</ref>