Fluorinated phosphorus compounds: Part 5. The boiling points of fluoroalkyl phosphoryl compounds

The relationship between structure and boiling point for several classes of phosphoryl compounds having fluorinated or hydrocarbon ester groups is discussed, i.e. phosphoramidates (R2N)2P(O)OCH2RF and (RFCH2O)2P(O)NR2, phosphates (RO)2P(O)ORF, (RFCH2O)2P(O)OR, (RFO)3PO and (RFCH2O)2P(O)OCH2RF′, and phosphonates (RFO)2P(O)R, where R= alkyl and RF= fluoroalkyl. Fluorination generally produces compounds of similar or lower boiling point than the unfluorinated parent compounds. A key factor governing the boiling point of a fluorinated phosphoryl compound relative to its hydrocarbon analogue is not its molecular weight, but the position and number of fluorine atoms in the ester linkage(s). Molecules with an umbrella of fluorine atoms repel each other, leading to low intermolecular forces: the boiling points of (C3F7CH2O)3PO and (C3H7CH2O)3PO are close despite a molecular weight difference of 378. Molecules with protons capable of intermolecular hydrogen-fluorine bonding (i.e. those containing NHR or CF2H groups) have higher boiling points than those without, due to attractive forces in the liquid state. Synthetic procedures for four unfluorinated phosphates — (MeO)2P(O)O-i-Pr, (MeO)2P(O)O-n-Bu, (EtO)2P(O)O-i-Pr and (s-BuO)3PO are outlined.