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Chapter 5: Aldehydes, Ketones, and Carboxylic Acids
The focus of this chapter is the chemistry of aldehydes, ketones, and carboxylic acids. This is the first time the chemistry of oxygen comes into play in this course. Aldehydes and ketones are discussed together because they have very similar chemistry and reaction types. Carboxylic acids are also oxygen-related because they have a COOH side chain as their defining characteristic. They also have great reactivity and are seen in nature as fatty acids and other biochemically-important molecules. In all cases, you will come to understand their nomenclature, their physical properties, and some of the most important chemical reactions associated with these molecules.
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Aldehydes and Ketones
Like alkenes, aldehydes and ketones have double bonds; however, the double bonds in aldehydes and ketones include the oxygen atom. Because oxygen is highly electronegative, the carbonyl group that defines these compounds has a dipole moment. Because of this, these compounds will have higher boiling points when compared to alkenes of the same length. They are also somewhat more water soluble. Figure 38 illustrates the dipole moment of a ketone molecule:
Figure 38.
An example of the changes in features of alkenes and ketones is the difference between (CH3)2CO and (CH3)2CH2. This is described in figure 39:
Figure 39.
The fact that the carbonyl group is polar makes it much more polar when compared to the double bonding of alkenes. Water can easily add to the carbonyl group whereas it cannot easily add to alkenes. The bond energy of the carbon double bond is 146 kcal per mole. The sigma part of this bond is 83 kcal per mole; however, the pi-bond adds 63 kcal per mole.
The totality of the energy of the C-O double bond in an aldehyde or ketone is between 170 and 180 kcal per mole with increasing energy when there is an R side group on the molecule versus a hydrogen bond. Just the sigma bond is 86 kcal per mole, which is slightly higher than the C-C sigma bond. This means that the addition of the pi-bond on this linkage adds nearly 100 kcal per mole—much higher than the pi-bond energy of the alkene double bond. What this suggests is that any addition reactions to the carbonyl bond is energetically not favorable.
Adding water to an alkene gives rise to an alcohol. This requires a catalyst because, although it is exothermic, it has a high activation energy. The reverse activity (making an alkene from an alcohol) is even slower and is endothermic. The reaction, in contrast, of adding water to a ketone or aldehyde is extremely fast and exothermic—leading to a geminal-diol (with two OH side chains attached to the same carbon atom).
