Classification tests aldehydes

Further classification tests for aldehydes and ketones Sodium metabisulphite test. 

When unknown compounds are identified without the aid of spectroscopy classification tests are used. Reacting the carbonyl in a ketone or aldehyde with an amine (2,4 dinitro-phenylhydrazine) to form an inline is the easiest way to detect a ketone or aldehyde (Reaction l). The iinine that forms is a highly colored solid. The color of the solid also helps to indicate structural characteristics. Ketones and aldehydes with no conjugation tend to form itnines with yellow to orange colors, while highly conjugated ketones or aldehydes form imines with red color. 

The presence of a colored solid confirms the presence of a ketone or aldehyde, but the imine formation does not indicate whether the unknown is a ketone or aldehyde. A second classification test is used to distinguish the two functionalities. This test is called the Tpllens test, and die significant reaction is shown in Reaction 2. 

The classification tests (summarized in Table 31.2), when properly done, can distinguish between various types of aldehydes and ketones. However, these tests alone may not allow for the identification of a specific unknown aldehyde or ketone. A way to correctly identify an unknown compound is by using a known chemical reaction to convert it into another compound that is known. The new compound is referred to as a derivative. Then, by comparing the physical properties of the unknown and the derivative to the physical properties of known compounds listed in a table, an identification can be made. 

Tollens s test is designed to distinguish between aldehydes and ketones. A complete discussion of this test together with an experimental procedure is included with the classification tests for aldehydes in Section 25.7C. 

Several classification tests might also be of assistance in classifying this compound. Does the 2,4-dinitrophenylhydrazine test for an aldehyde or ketone give a positive result Isolate the 2,4-dinitrophenylhydrazone derivative and determine its melting point. Does it correspond to the literature value of 180 °C What further test could be run to determine whether the carbonyl is present as an aldehyde or ketone ... 

Tollens reagent is used as a qualitative test for the presence of the aldehyde functional group (see Chapter 9, Classification Tests). However, benzoin, which does not contain an aldehyde, gives a positive test with this reagent. Explain. 

The procedure outlined in the Classification Test Section for aldehydes and ketones (p. 642) is used. Since the derivative to be isolated is a solid, it may be convenient to run the reaction in a 3-mL vial or in a small test tube. Double the amount of the reagents used. If necessary, the derivative can be recrystallized from 95% ethanol. 

Claisen head adapter, 20, 26,134 Claisen-Schmidt reaction, 309-317 classification tests, 640-654. See also qualitative analysis alcohols, 640-642 aldehydes, 642-644 alkanes, 644-645 alkenes, 645 alkyl halides, 645-646 aUcynes, 645 amides, 647-648 amines, 648-649 ammonium salts, 647-648 aromatic hydrocarbons, 649-650 carboxylic acids, 650 cycloalkanes, 644-645 defined, 630 enols, 653-654 esters, 650-651 ethers, 651... 

To choose proper test substances, McReynolds studied the behavior of a large number of substances of the following compound classes alcohols, glycols, aldehydes, ketones, esters, acetals, ethers, oxides, hydrocarbons, chloro compounds, difunctional and polyfunctional compounds, and other miscellaneous substances. " For the stationary phase classification, he eventually proposed benzene, n-butanol, 2-pentanone, 1-nitropropane, and pyridine to represent compounds of different chemical interactions. " McReynolds characteristic phase constants for these five compounds describe the selectivity of the phase. 

Ketones, and aldehydes (e.g. acetaldehyde) containing the group CH3CO- respond to the following tests, which are often useful in effecting a more precise classification of the substance under examination. 

---