Boiling points, molecular hydrides

Toluene is commonly used. It can be dried by molecular sieves or direct distillation from calcium hydride into the reaction flask. Solvent stored over calcium hydride for several days is usually sufficiently dry to decant directly into the reaction flask, but distillation gives more consistent results. Any solvent with a boiling point sufficiently high to melt sodium is satisfactory. The submitters have also used methyl-cyclohexane and xylene in acyloin condensations. After the sodium is dispersed, the high-boiling solvent can be removed and replaced with anhydrous ether (as noted by the submitters) or can be retained and used in combination with ether (checkers). 

FIGURE 5.8 The boiling points of most of the molecular hydrides of the p-block elements show a smooth increase with molar mass in each group. However, three compounds—ammonia, water, and hydrogen fluoride are strikingly out of line. 

When the boiling-points of the hydrides of sulphur, selenium, and tellurium are compared,1 it is evident that the boiling-point rises steadily with the molecular weight. 

How do the boiling points of the group 4A hydrides change as the molecular masses of the hydrides change ... 

Because most of the covalent hydrides consist of molecular units with only weak intermolecular forces between them, they are volatile compounds. Accordingly, the covalent hydrides are sometimes referred to as the volatile hydrides. The nomenclature, melting points, and boiling points of several covalent hydrides are shown in Table 6.6. 

Fig. 1. Normal boiling points of some molecular hydrides, with noble gas elements for comparison.

This solvent is employed widely in analytical and coordination chemistry. At its boiling point it undergoes partial decomposition to yield dimethylamine and carbon monoxide. The decomposition is catalyzed by various substances, particularly those with acidic or basic properties this must be taken into consideration in the selection of the material used for drying. Under no circumstances may this solvent be refluxed with, for example, potassium hydroxide, sodium hydroxide or calcium hydride. Dimethylformamide can be dehydrated most advantageously with a molecular sieve of pore size 0.4 nm however, calcium sulphate, magnesium sulphate or silica gel may also be employed. After dehydration, the solvent may be purified by vacuum distillation. 

Figure 11.9 Boiling points of the covalent hydrides of the elements in groups 4A-7A as a function of molecular weight. 

Figure 11.24A shows a plot of boiling point versus molecular weight for the hydrides (binary compounds with hydrogen) of the Group VIA elements. If London... 

Boiling point versus molecular weight for hydrides... 

With the exception of ammonia, water and hydrogen fluoride, hydrogen bonds are not present in the hydrides of elements in groups 15, 16 and 17. The increase in the boiling points for the hydrides of each periodic group is therefore due to the increase in London (dispersion) forces as the molecular size increases. 

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