Physical Properties of Ketones and Aldehydes

The carbonyl group is a polar group (see Chapter 9 to review polarity). 

When comparing melting points, boiling points, and solubility, you must use compounds of similar masses. 

Aldehydes and ketones with six or fewer carbon atoms are soluble in water. The presence of the oxygen atom, which may be protonated, means they re dissolvable in concentrated sulfuric acid. 

Creating Aldehydes and Ketones lOith Synthesis Reactions 

Aldehydes and ketones can be synthesized in a number of ways. In most cases but not all, either an oxidation or a reduction is necessary. Oxidation may be a problem in the preparation of an aldehyde, because if care isn t taken, a carboxylic acid may be formed instead. 

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The physical properties of aldehydes and ketones are described as follows. 

Examining the structure and physical properties of aldehydes and ketones Finding out how aldehydes and ketones are formed Mastering the reactions of aldehydes and ketones Reviewing their spectroscopy... 

The carbonyl group is polarised in such a way that the oxygen is slightly negative and the carbon is slightly positive. Both the polarity of the carbonyl group and the presence of the weak n bond explain much of the chemistry and the physical properties of aldehydes and ketones. The polarity of the bond also means that the carbonyl group has a resultant dipole moment. 

TABLE 12.2.1 Summary of physical properties of aldehydes and ketones Molecular Compound CAS no. formula Molecular weight, MW g/mol m.p. b.p. °C Fugacity ratio, F at 25°C Molar volume, V, cmVmol ... 

Aldehydes and ketones exhibit dipole-dipole interactions because of their polar carbonyl group. Because they have no O-H bond, two molecules of RCHO or RCOR are incapable of intermolecular hydrogen bonding, making them less polar than alcohols and carboxylic acids. How these intermolecular forces affect the physical properties of aldehydes and ketones is summarized in Table 21.1. 

The physical properties of aldehydes and ketones are discussed in Section 17.3 (see also Appendix I), and the methods used to prepare aldehydes and ketones are summarized in Appendix IV. 

Compare the physical properties of aldehydes and ketones with those of compounds in other classes. (Section 4.2)... 

The physical properties of aldehydes and ketones can be explained by an examination of their structures. First, the lack of a hydrogen on the oxygen prevents the formation of hydrogen bonds between molecules ... 

The dipole moment of acetone, a typical ketone, is 2.9 D. In contrast, the dipole moment of 2-meth-ylpropane is 0.1 D. The high polarity of the carbonyl group reflects the contribution of the charged structure to the resonance hybrid. The physical properties of aldehydes and ketones reflect the polarity of the carbonyl group (Figure 18.3). 

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