Melting point hints

The properties of a molecule that are most frequently calculated are geometry, energy (compared to that of other isomers), and spectra. Why is it more of a challenge to calculate simple properties like melting point and density Hint is there a difference between a molecule X and the substance X ... 

Problem 24.17 Phenols are often identified as their aryloxyacetic acids, ArOCH2COOH. Suggest a reagent and a procedure for the preparation of these derivatives. (Hint See Sec. 24.8.) Aside from melting point, what other property of the aryloxyacetic acids would be useful in identifying phenols (Hint See Sec. 18.21.)... 

Analyzing data Based on your observations, which type of flame is hotter the flame formed when the air ports are open or the flame formed when they are closed What is the hottest part of the flame (Hint The melting point of copper is 1083°C.)... 

Problem 19.7. Why do cis double bonds lower the melting points of fatty acids Trans double bonds do not have the same effect as cis double bonds. Why Hint Use appropriate drawings to show the effect of double bonds on molecular shape and indicate how molecular shape affects melting point. 

Explain the difference in the melting points of the following confounds (Hint Only one of them can form intermolecular hydrogen bonds.)... 

The observed melting point of camphor is often low. Look up the molal freezing-point-de-pression constant K for camphor and calculate the expected depression of the melting point of a quantity of camphor that contains 0.5 molal impurity. Hint Look in a general chemistry book under "freezing-point depression" or "colligative properties of solutions."... 

Notice that the details of vacuum distillation are left for you to figure out on your own. However, here are two hints. First, all of the products boil between 100°C and 150°C at 20-mm pressure. Second, if your chosen substrate is anisole, the product will be a solid with a low melting point and will solidify soon after vacuum distillation is completed. The solid can be distilled, but you should not run any cooling water through the condenser. It will also be worthwhile to preweigh the receiving flask, because it will be difficult to transfer the entire solidified product to another container to determine a yield. 

The observed melting point of the 1 1 mixture of fluorene and 9-fluorenone should be relatively sharp, although lower than the melting point of either of the pure compounds. On the other hand, a 3 1 mixture has a broad melting-point range of about 60-90 °C. Explain these observations. (Hint See Sec. 3.3.)... 

It should be stressed here that in general, determination of melting points is a rather difficult task. Our studies with a novel equipment show that published data and our results deviate by more than 10 K [26]. This may be caused by impurities especially water, organic solvents, and chloride. Despite several hints in literature showing the effect of impurities, even now papers are submitted (and even published) that do not specify the purity of the investigated IL. Readers are advised to consult Ref. [27]. 

Yet another possibility is hinted at by the observation (Loewenthal, unpubl.) that the hemithioketal sulphone (26) on heating above its melting point regenerates the original ketone (25) (and, presumably, sulphur dioxide and ethylene) (Fig. 9.3). 

Match up the hydrazones with the aldehydes (without first looking up the melting points of these derivatives), and explain your choices. (Hint See Section 14-11.)... 

PRACTICE EXAMPLE A Francium (Z = 87) is an extremely rare radioactive element formed when actinium (Z = 89) undergoes alpha-particle emission. Francium occurs in natural uranium minerals, but estimates are that little more than 15 g of francium exists in the top 1 km of Earth s crust. Few of francium s properties have been measured, but some can be inferred from its position in the periodic table. Estimate the melting point, density, and atomic (metallic) radius of francium. [Hint Plot each property versus atomic number, Z, and extrapolate to Z = 87.]... 

PRACTICE EXAMPLE B EHscuss the likelihood that element 168, should it ever be synthesized in sufficient quantity, would be a "noble liquid" at 298 K and 1 bar. Could element 168 be a "noble solid" at 298 K and 1 bar Use spdf notation to show the electron configuration you would expect for element 168. [Hint Prepare graphs of boiling point versus atomic number and melting point versus atomic number. Extrapolate to Z = 168.]... 

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