Aldehydes and Ketones

<body> <p align="center"><font size="6"><strong>Aldehydes and Ketones </strong></font></p> <div align="center"><center><table border="1" width="500"> <tr> <td align="center"><a href="#aldehyde">Aldehydes and Ketones</a></td> <td align="center"><a href="#name">The Nomenclature of Aldehydes and Ketones </a></td> <td align="center"><a href="#common">Common Aldehydes and Ketones </a></td> </tr> </table> </center></div><p align="center"><a name="aldehyde"><em><strong>Aldehydes and Ketones</strong></em></a></p> <p>The <a href="http://chemed.chem.purdue.edu/~genchem/topicreview/bp/1organic/organic.html#double">connection between the structures of alkenes and alkanes was previously established</a>, which noted that we can transform an alkene into an alkane by adding an H<sub>2</sub> molecule across the C=C double bond. </p> <p align="center"><img src="graphics/img41.gif" align="middle" width="333" height="79"> </p> <p>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 H<img src="graphics/em.gif" alt="--" width="22" height="9">H bond (435 kJ/mol) and a moderately strong carbon-carbon <img src="graphics/pismall.gif" alt="pi" width="8" height="7"> bond (<img src="graphics/congrnt.gif" alt="approx." width="20" height="11">270 kJ/mol) are broken, but two strong C<img src="graphics/em.gif" alt="--" width="22" height="9">H bonds (439 kJ/mol) are formed. The reduction of an alkene to an alkane is therefore an exothermic reaction. </p> <p>What about the addition of an H<sub>2</sub> molecule across a C=O double bond? </p> <p align="center"><img src="graphics/img42.gif" align="middle" width="324" height="79"> </p> <p>Once again, a significant amount of energy has to be invested in this reaction to break the H<img src="graphics/em.gif" alt="--" width="22" height="9">H bond (435 kJ/mol) and the carbon-oxygen <img src="graphics/pismall.gif" alt="pi" width="8" height="7"> bond (<img src="graphics/congrnt.gif" alt="approx." width="20" height="11">375 kJ/mol). The overall reaction is still exothermic, however, because of the strength of the C<img src="graphics/em.gif" alt="--" width="22" height="9">H bond (439 kJ/mol) and the O<img src="graphics/em.gif" alt="--" width="22" height="9">H bond (498 kJ/mol) that are formed. </p> <p>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 <em>aldehyde</em>. If a reduction reaction in which H<sub>2</sub> is added across a double bond is an example of a <em>hydrogenation</em> reaction, then an oxidation reaction in which an H<sub>2</sub> molecule is removed to form a double bond might be called <em>dehydrogenation</em>. 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 <strong>aldehyde</strong><em><strong>. </strong></em>It is an <em>al</em>cohol that has been <em>dehyd</em>rogenated. </p> <p align="center"><img src="graphics/26.gif" width="189" height="46"> </p> <p>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. </p> <p align="center"><img src="graphics/27.gif" width="203" height="46"> </p> <p>The product of this reaction was originally called <em>aketone</em>, although the name was eventually softened to <em>azetone</em> and finally <em>acetone</em>. Thus, it is not surprising that any substance that exhibited chemistry that resembled "aketone" became known as a <strong>ketone.</strong> </p> <p>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. </p> <p align="center"><img src="graphics/28.gif" width="135" height="66"> </p> <p>There aren't any hydrogen atoms that can be removed from the carbon atom carrying the <img src="graphics/em.gif" alt="--" width="22" height="9">OH group in a 3º alcohol. And any oxidizing agent strong enough to insert an oxygen atom into a C<img src="graphics/em.gif" alt="--" width="22" height="9">C bond would oxidize the alcohol all the way to CO<sub>2</sub> and H<sub>2</sub>O. </p> <p>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)<img src="graphics/em.gif" alt="--" width="22" height="9">chromium in the +6 oxidation state <img src="graphics/em.gif" alt="--" width="22" height="9"> in acidic solution. This reagent can be prepared by adding a salt of the chromate (CrO<sub>4</sub><sup>2-</sup>) or dichromate (Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup>) ions to sulfuric acid. Or it can be made by adding chromium trioxide (CrO<sub>3</sub>) to sulfuric acid. Regardless of how it is prepared, the oxidizing agent in these reactions is chromic acid, H<sub>2</sub>CrO<sub>4</sub>. </p> <p align="center"><img src="graphics/29.gif" width="271" height="44"> </p> <p>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 <img src="graphics/em.gif" alt="--" width="22" height="9"> they oxidize the aldehyde to the corresponding carboxylic acid. </p> <p align="center"><img src="graphics/30.gif" width="321" height="48"></p> <p>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 CrO<sub>3</sub> and pyridine, C<sub>6</sub>H<sub>5</sub>N, in a solvent such as dichloromethane that doesn't contain any water. </p> <p align="center"><img src="graphics/31.gif" width="207" height="49"> </p> <p align="center"><a href="#top"><img src="graphics/button.gif" alt="return to top" border="0" width="226" height="36"></a></p> <p align="center"><a name="name"><em><strong>The Nomenclature of Aldehydes and Ketones</strong></em> </a></p> <p>The common names of aldehydes are derived from the names of the corresponding carboxylic acids. </p> <p align="center"><img src="graphics/32.gif" width="183" height="157"> </p> <p>The systematic names for aldehydes are obtained by adding -<em>al</em> to the name of the parent alkane. </p> <p align="center"><img src="graphics/33.gif" width="140" height="65"> </p> <p>The presence of substituents is indicated by numbering the parent alkane chain from the end of the molecule that carries the <img src="graphics/em.gif" alt="--" width="22" height="9">CHO functional group. For example, </p> <p align="center"><img src="graphics/34.gif" width="213" height="47"> </p> <p>The common name for a ketone is constructed by adding <em>ketone</em> to the names of the two alkyl groups on the C=O double bond, listed in alphabetical order. </p> <p align="center"><img src="graphics/35.gif" width="221" height="47"> </p> <p>The systematic name is obtained by adding -<em>one</em> to the name of the parent alkane and using numbers to indicate the location of the C=O group. </p> <p align="center"><img src="graphics/36.gif" width="185" height="47"> </p> <p align="center"><a href="#top"><img src="graphics/button.gif" alt="return to top" border="0" width="226" height="36"></a></p> <p align="center"><a name="common"><em><strong>Common Aldehydes and Ketones</strong></em> </a></p> <p>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. </p> <div align="center"><center><table border="0"> <tr> <td><img src="graphics/37.gif" width="112" height="90"></td> <td> </td> <td><img src="graphics/38.gif" width="177" height="114"></td> <td> </td> <td><img src="graphics/39.gif" width="154" height="86"></td> </tr> </table> </center></div><p>Aldehydes and ketones play an important role in the chemistry of carbohydrates. The term <em>carbohydrate</em> literally means a "hydrate" of carbon, and was introduced to describe a family of compounds with the empirical formula CH<sub>2</sub>O. Glucose and fructose, for example, are carbohydrates with the formula C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>. 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. </p> <p align="center"><img src="graphics/img55.gif" align="middle" width="499" height="112"> </p> <p align="center"><a href="#top"><img src="graphics/button.gif" alt="return to top" border="0" width="226" height="36"></a></p> <hr> <p><img src="http://chemed.chem.purdue.edu/Count.cgi?df=toprev_org2_aldehyde.dat"></p> </body>