Terephthaldicarboxaldehyde: A Key Intermediate in Organic Synthesis

Introduction

Terephthaldicarboxaldehyde, also known as terephthalaldehyde or 1,4-benzenedicarboxaldehyde, is an aromatic dialdehyde with significant importance in the field of organic synthesis. This article will introduce its use as covalent linker and raw material of flame retardant.

Figure 1 Characteristics of Terephthaldicarboxaldehyde

Covalent Linker

Glass plates and pseudo hexagonal prismatic ZSM-5 crystals tethered with 3-aminopropylsilyl (APS) groups, respectively, were prepared by treating them with 3-aminopropyltriethoxysilane. Subsequent treatment of the APS-tethering glass plates with terephthaldicarboxaldehyde led to facile assembly of benzaldehyde (BA) units over the glass plates via imine (±CHvN±) linkages. When the BA-tethering glass plates and the APS-tethering zeolite crystals were allowed to contact in boiling toluene, the ZSM-5 crystals strongly adhered onto the glass plates in the form of closely packed monolayers with perfect one-dimensional orientation.

Furthermore, the assembled monolayers of zeolite crystals can readily be developed into hosts for sensors and membranes for size-selective separations. In this regard, continuous efforts should be made to explore various other novel methodologies for effective assembly of zeolite crystals. In particular, development of various novel covalent linkers is important in the respects that the strength of the crystal-substrate binding, the orientation of the crystals, the degree of intercrystalline packing between the crystals, and the future application of the surface-bound crystals for various purposes will sensitively depend on the nature of the interconnecting `molecular linkers'. Herein we introduce a novel method to assemble zeolite crystals over the glass substrate using terephthaldicarboxaldehyde (TPDA) as the covalent linker between the terminal amino groups independently tethered to zeolite and glass substrate.¹

Raw Material of Flame Retardant

Two novel phosphorus-rich prepolymers based on epoxy novolac and terephthaldicarboxaldehyde and potential flame retardants, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 2,8-dimethyl-phenoxaphosphin-10-oxide (DPPO) were synthesised by S. Seibold and colleagues². The resultant flame-retardant epoxy resins were cured with 4,4'-diamino-diphenylmethane (DDM) and 4,4'-diamino-dicyclohexylmethane (PACM). Their flammability and burning behavior were characterised by UL 94 and LOI and compared with analogue prepolymers based on diethylphosphite (DEPP). The glass transition temperatures were determined by DSC measurements. 

Furthermore, the structures of two exemplary molecules based on p-tolylaldehyde adducts were examined by XRD and NMR analysis to determine the possibilities of linking the two novel DOPO and DPPO derivatives to the backbone of the epoxy resin. Additionally, the char yields were determined by TG analysis and thermal desorption mass spectroscopy of the thermosets used and compared with each other to obtain more information about the possible mode of flame-retardant action of the different phosphorus compounds. Based on the promising results presented, it may be expected that these bifunctional TDA adducts may be an alternative to the well-described benzoquinone adduct of DOPO.

References

1 Orientation-Controlled Monolayer Assembly of Zeolite Crystals on Glass Using Terephthaldicarboxaldehyde as a Covalent Linker. Tetrahedron 56 (2000) 6965-6968.

2 Phosphorus-Containing Terephthaldialdehyde Adducts—Structure Determination and their Application as Flame Retardants in Epoxy Resins. Journal of Applied Polymer Science, Vol. 108, 264–271(2008).

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