PROPERTIES OF ALDEHYDES AND KETONES

In the preceding chapter you learned that nucleophilic addition to the carbonyl group IS one of the fundamental reaction types of organic chemistry In addition to its own reactivity a carbonyl group can affect the chemical properties of aldehydes and ketones m other ways Aldehydes and ketones having at least one hydrogen on a carbon next to the carbonyl are m equilibrium with their enol isomers... 

A general property of aldehydes and ketones is that when heated with hydrocyanic acid, additive compounds, termed nitriles or cyanohydrins, are produced, according to the general equations—... 

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 ... 

TABLE 12.2.2 Summary of selected Compound physical-chemical properties of aldehydes and ketones at 25°C Selected properties Henry s law constant H/(Pa-mVmol) ... 

Oxidation of butan-l-ol gives butanal (1.32) which is characterized by a C=0, a carbonyl group, in this case an aldehyde group. Oxidation of the secondary alcohol butan-2-ol (1.28) gives butan-2-one (1.33), a ketone. There are many common properties of aldehydes and ketones, and others that differ because of the aldehydic C-H. 

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 facts are consistent with the orbital picture of the carbonyl group. Electron diffraction and spectroscopic studies of aldehydes and ketones show that carbon, oxygen, and the two other atoms attached to carbonyl carbon lie in a plane the three bond angles of carbon are very close to 120°. The large dipole moments (2.3 2.8 d) of aldehydes and ketones indicate that the electrons ot me cybonvl group arc quite unequally shared. We shall see how the physical and chemical properties of aldehydes and ketones are determined by the structure of the carbonyl group. 

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 constitutional difference between aldehydes and ketones lies in the substituents at the carbonyl carbon atom. While in aldehydes one of the substituents is always hydrogen in ketones both substituents are alkyl groups. Therefore, the chemical properties of aldehydes and ketones, especially in nucleophilic addition reactions are similar. The simplest aldehyde is methanal or formaldehyde and the parent ketone is propan-2-one or acetone. 

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