Sodium n-propoxide

Mention has already been made of the application of alkoxycyclophos-phazenes, [NP(OR)2] , as flame retardants in rayon. Although the methoxy-derivatives, with their high phosphorus content, were expected to be most efficient in this respect, their water solubility proved a major shortcoming. However, the n-propoxy series, [NP(OPr )2] ( mainly 3—6), were found to impart excellent flame resistance and were well retained by rayon. The cyclophosphazene alkoxides were obtained by the addition of sodium-n-propoxide to the chloride homologues, (NPCl2)n, and were added to the viscose dope before the rayon was spun. The flame resistance imparted by various amino- and thioalkoxy-derivatives was also tested, but found to be inferior to the results obtained with alkoxy-deriva-tives. Several patent applications have resulted from work on this topic. ... 

Heat capacity measurements of sodium alkoxides (methoxide, ethoxide, n-propoxide and iso-propoxide) were carried out using DSC in die temperature range 240-550 K by Chandran and coworkers [219]. From the heat capacity values, odier thermodynamic functions, such as endialpy increments, entropies and Gibbs energy fimctions of these compounds were derived. The Cp 9g values of sodium medioxide, sodium ethoxide, sodium n-propoxide and sodium iso-propoxide were measured and reported. 

Sodium alkoxides, namely, sodium methoxide, sodium ethoxide, sodium n-propoxide, and sodium iso-propoxide were synthesized and characterized by various analytical techniques. Thermal decomposition of these sodium alkoxides was studied under isothermal and non-isothermal conditions by thermogravimetric (TG) method. Non-isothermal experiments were carried out at different linear heating rates. Mass spectrometric technique was followed for identifying the evolved gases. The onset temperatures of decomposition of sodium methoxide, sodium ethoxide, sodium n-propoxide, and sodium iso-propoxide were found to be 623, 573, 590, and 545 K, respectively. These sodium alkoxides form gaseous products of saturated and unsaturated hydrocarbons and leave a mixture of sodium carbonate, sodium hydroxide, and free carbon as the decomposition residue. Activation energy and pre-exponential factor for the decomposition reactions were deduced from the TG data by model-dependent and model-independent (iso-conversion) methods. The probable decomposition mechanism of sodiiun alkoxides is described in this chapter. 

Figure 15.4 Photograph of typical crystalline sodium n-propoxide.

Figure 15.6 IR spectrum of (a) n-propanol, (b) sodium n-propoxide with muU, and (c) sodium n-propoxide with KBr.

The C-O feature for sodium H-propoxide appears as doublet at 1070 and 1088 cm" which shows a marked difference from the C-O feature of n-propanol appearing at 1032 cm k The hydrogen atom of-OH group in the n-propanol is replaced with sodium metal in the propoxide making the electrons to flow towards the O-Na bond thereby making the C-O bond strong. Hence a blue shift of the C-O feature is seen in the IR spectrum of sodium n-propoxide. The IR spectrum of other alkoxides showed a similar trend [27]. 

The reactivity of sodium alkoxides towards moisture decreases with increase in carbon chain. This could be seen from the XRD pattern of sodium methoxide that the relative peak intensity due to sodium hydroxide (peaks at 20 = 10.1 and 37.0) in the case of sodium methoxide is nearly 5 %, while it is less than 1 % in the case of sodium ethoxide (9.0 and 35.0) and the same was absent in sodium n-propoxide. 

Literature survey The only study on the decomposition of sodium methoxide is a patent by Pfeifer et al. [36]. These authors have reported that the sodium methoxide decomposed at 393 and 413 K. The only study on the decomposition of sodium ethoxide has been reported by Blanchard et al. [19]. According to them, the decomposition commenced at temperatures above 570 K. There are no reports on the decomposition of other sodium alkoxides (for example, sodium n-propoxide and iso-propoxide). Even for sodium methoxide and sodium ethoxide, data on the kinetic parameters are not available. The kinetic parameters of the decomposition of sodium methoxide, sodium ethoxide, sodium n-propoxide and sodium iso-propoxide were determined and discussed in this chapter. 

Figure 15.23 Plot of sodium n-propoxide decomposition at different linear heating rate (a) TG trace and (b) fraction decomposed deduced from above data.

Figure 15.26 Plot of In (g(a)/T2) vs 1000/T for (a) sodium n-propoxide decomposition and (b) sodium iso-propoxide decomposition derived from TGA data (non-isothermal run).

The activation energies for the decomposition of sodium n-propoxide and sodium iso-propoxide derived from the isothermal data are slightly higher than those of non-isothermal data. There could be two possible reasons (1) the temperature ranges of the isothermal and non-isothermal measurements are not the same and (2) the initial stage of decomposition under isothermal conditions includes a retardation period and also the specimen experiences a non-isothermal condition till the isothermal temperature is reached. Similar observations are reported in the literature [58,67]. 

Figure 15.28 Typical plot of TGA trace for sodium n-propoxide decomposition at a pre determine temperature (isothermal run).

Figure 15.34 Plot of In [P] vs 1/T for decomposition fraction (a = 0.1-0.9) of sodium n propoxide under non-isothermal data.

R. Nithya, K. Chandran A. Gopalan, K. Sankaran Sastry, V. Ganesan, Crystal structure of sodium n-propoxide, JCPDS - International Centre for Diffraction Data, PDF Card No. 00-057-1658. 

---