Phthalaldehyde

o-Phthalaldehyde is the chemical compound with the formula C₆H₄(CHO)₂. Often abbreviated OPA, the molecule is a dialdehyde, consisting of two formyl (CHO) groups attached to adjacent carbon centres on a benzene ring. This pale yellow solid is a building block in the synthesis of heterocyclic compounds and a reagent in the analysis of amino acids.

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Synthesis

The molecule was first described in 1887 when it was prepared from α,α,α',α’-tetrachloro-ortho-xylene.^[1] A more modern synthesis is similar: the hydrolysis of the related tetrabromoxylene using potassium oxalate, followed by purification by steam distillation.^[2] The reactivity of OPA is complicated by the fact that with water it forms both a mono- and dihydrate, C₆H₄(CHO)(CH₂OH) and C₆H₄(CH₂OH)₂, respectively.^[3]

Isomeric phthalaldehydes

Related to o-phthalaldehyde are the meta and para isomers, which are respectively named isophthalaldehyde (m.p. 87-88 °C, CAS# 626-19-7) terephthalaldehyde (m.p. 114-116 °C, CAS# 623-27-8).

References

1. ^ Colson, A.; Gautier, H. "Nouveau Mode de Chloruration des Carbures" Annales de Chimie (Paris), 1887, volume 6, number 11, pp. 28.
2. ^ Bill J. C.; Tarbell, D. S. “o-Phthalaldehyde” Organic Syntheses, Collected Volume 4, p.807 (1963). http://www.orgsyn.org/orgsyn/pdfs/CV4P0807.pdf
3. ^ Zuman, H. "Reactions of Orthophthalaldehyde with Nucleophiles" Chemical Reviews 2004; volume 104, pp. 3217-3238. doi:10.1021/cr0304424

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