Aldehydes and Ketones

The connection between the structures of alkenes and alkanes was previously established, which noted that we can transform an alkene into an alkane by adding an H₂ molecule across the C=C double bond.

The driving force behind this reaction is the difference between the strengths of the bonds that must be broken and the bonds that form in the reaction. In the course of this hydrogenation reaction, a relatively strong HH bond (435 kJ/mol) and a moderately strong carbon-carbon  bond (270 kJ/mol) are broken, but two strong CH bonds (439 kJ/mol) are formed. The reduction of an alkene to an alkane is therefore an exothermic reaction.

What about the addition of an H₂ molecule across a C=O double bond?

Once again, a significant amount of energy has to be invested in this reaction to break the HH bond (435 kJ/mol) and the carbon-oxygen  bond (375 kJ/mol). The overall reaction is still exothermic, however, because of the strength of the CH bond (439 kJ/mol) and the OH bond (498 kJ/mol) that are formed.

The addition of hydrogen across a C=O double bond raises several important points. First, and perhaps foremost, it shows the connection between the chemistry of primary alcohols and aldehydes. But it also helps us understand the origin of the term aldehyde. If a reduction reaction in which H₂ is added across a double bond is an example of a hydrogenation reaction, then an oxidation reaction in which an H₂ molecule is removed to form a double bond might be called dehydrogenation. Thus, using the symbol [O] to represent an oxidizing agent, we see that the product of the oxidation of a primary alcohol is literally an "al-dehyd" or aldehyde. It is an alcohol that has been dehydrogenated.

This reaction also illustrates the importance of differentiating between primary, secondary, and tertiary alcohols. Consider the oxidation of isopropyl alcohol, or 2-propanol, for example.

The product of this reaction was originally called aketone, although the name was eventually softened to azetone and finally acetone. Thus, it is not surprising that any substance that exhibited chemistry that resembled "aketone" became known as a ketone.

Aldehydes can be formed by oxidizing a primary alcohol; oxidation of a secondary alcohol gives a ketone. What happens when we try to oxidize a tertiary alcohol? The answer is simple: Nothing happens.

There aren't any hydrogen atoms that can be removed from the carbon atom carrying the OH group in a 3º alcohol. And any oxidizing agent strong enough to insert an oxygen atom into a CC bond would oxidize the alcohol all the way to CO₂ and H₂O.

A variety of oxidizing agents can be used to transform a secondary alcohol to a ketone. A common reagent for this reaction is some form of chromium(VI)chromium in the +6 oxidation state  in acidic solution. This reagent can be prepared by adding a salt of the chromate (CrO₄^2-) or dichromate (Cr₂O₇^2-) ions to sulfuric acid. Or it can be made by adding chromium trioxide (CrO₃) to sulfuric acid. Regardless of how it is prepared, the oxidizing agent in these reactions is chromic acid, H₂CrO₄.

The choice of oxidizing agents to convert a primary alcohol to an aldehyde is much more limited. Most reagents that can oxidize the alcohol to an aldehyde carry the reaction one step further  they oxidize the aldehyde to the corresponding carboxylic acid.

A weaker oxidizing agent, which is just strong enough to prepare the aldehyde from the primary alcohol, can be obtained by dissolving the complex that forms between CrO₃ and pyridine, C₆H₅N, in a solvent such as dichloromethane that doesn't contain any water.

The Nomenclature of Aldehydes and Ketones

The common names of aldehydes are derived from the names of the corresponding carboxylic acids.

The systematic names for aldehydes are obtained by adding -al to the name of the parent alkane.

The presence of substituents is indicated by numbering the parent alkane chain from the end of the molecule that carries the CHO functional group. For example,

The common name for a ketone is constructed by adding ketone to the names of the two alkyl groups on the C=O double bond, listed in alphabetical order.

The systematic name is obtained by adding -one to the name of the parent alkane and using numbers to indicate the location of the C=O group.

Common Aldehydes and Ketones

Formaldehyde has a sharp, somewhat unpleasant odor. The aromatic aldehydes in the figure below, on the other hand, have a very pleasant "flavor." Benzaldehyde has the characteristic odor of almonds, vanillin is responsible for the flavor of vanilla, and cinnamaldehyde makes an important contribution to the flavor of cinnamon.

Aldehydes and ketones play an important role in the chemistry of carbohydrates. The term carbohydrate literally means a "hydrate" of carbon, and was introduced to describe a family of compounds with the empirical formula CH₂O. Glucose and fructose, for example, are carbohydrates with the formula C₆H₁₂O₆. These sugars differ in the location of the C=O double bond on the six-carbon chain, as shown in the figure below. Glucose is an aldehyde; fructose is a ketone.