Ketones iodoform test

Since it does not reduce Tokens or Fehltng reagent, (A) must be a ketone. (A) responds to iodoform test. Therefore, it should be a methyl ketone. The molecular formula of (A) Indicates high degree of unsaturation, yet it does not decolourise bromine water or Baeyer s reagent. This indicates the presence of unsaturation due to an aromatic ring. 

Aldehydes are easy to oxidize, but ketones are more challenging. The two important oxidation reactions of ketones are the oxidation with a strong oxidant and the iodoform test. 

For years the iodoform test was a laboratory method for the identification of a methyl ketone (a ketone where one of the R groups is a methyl group). A positive test produced the compound iodoform. Iodoform, CHI3, is a yellow precipitate with a characteristic odor. The oxidation utilizes sodium hypoiodite, which is generated in situ by the reaction of iodine with sodium hydroxide. Figure 10-35 shows an example of the iodoform test. 

In Chapter 10, you see the iodoform test as a means of identifying methyl ketones. Here we reexamine this reaction in light of enols and enolates. 

Phenylhydrazone formation indicates a carbonyl compound. Since the negative Tollens test rules out an aldehyde, (A) must be a ketone. A negative iodoform test rules out the CH,C=0 group, and the reduction product, pentane, establishes the C s to be in a continuous chain. The compound is CHjCHjCOCHjCH,. 

Because iodoform is a pale yellow solid, this reaction is often run as a test for methyl ketones and is called the iodoform test. 

Aldehydes Alkanes Benedict s test Dichromate test 2,4-Dinitrophenylhydrazine (2,4-DNP) Fehling test Iodoform test Oxime Permanganate test Semicarbazone Tollen s test No test Positive for all aldehydes Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes Positive only for acetaldehyde Positive for all aldehydes (and ketones) Positive for all aldehydes Positive for all aldehydes (and ketones) Positive for all aldehydes... 

Iodoform test. Methyl ketones give the yellow precipitate iodoform when reacted with iodine in aqueous sodium hydroxide. 

When the halogen is iodine, the haloform product (iodoform) is a solid that separates out as a yellow precipitate. This iodoform test identifies methyl ketones, which halo-genate three times, then lose CI3 to give iodoform. 

Iodine is an oxidizing agent, and an alcohol can give a positive iodoform test if it oxidizes to a methyl ketone. The iodoform reaction can convert such an alcohol to a carboxylic acid with one less carbon atom. 

Although all ketones with a hydrogens react with base and I2, only methyl ketones form CHI3 (iodoform), a pale yellow solid that precipitates from the reaction mixture. This reaction is the basis of the iodoform test, once a common chemical method to detect methyl ketones. Methyl ketones give a positive iodoform test (appearance of a yellow solid), whereas other ketones give a negative iodoform test (no change in the reaction mixture). 

Iodoform test (Section 23.7B) A test for the presence of methyl ketones, indicated by the formation of the yellow precipitate, CHI3, via the haloform reaction. 

Methyl ketones can be distinguished from other ketones by the iodoform test. The methyl ketone is treated with iodine in a basic solution. Introduction of the first iodine atom increases the acidity of the remaining methyl protons, so halogenation stops only when the triiodo compound has been produced. The base then allows the relatively stable triiodomethyl carban-ion to leave and a subsequent proton transfer gives iodoform, a yellow crystalline solid of mp 119-123°C. The test is also positive for fragments easily oxidized to methyl ketones, such as CH3CHOH— and ethanol. Acetaldehyde also gives a positive test because it is both a methyl ketone and an aldehyde. 

Iodoform Test for Methyl Ketones. Follow the procedure in Chapter 30. A... 

Hypoiodites are used for qualitative tests for methyl ketones (Lieben test). For this purpose, a compound to be tested is stirred with an aqueous solution of sodium hydroxide (80 mol/mol of methyl ketone). Iodine (4.5 mol of 12) is added portionwise with stirring, and the mixture is set aside for 20 min at 25 °C before acidification. In the presence of a methyl keto group, a yellow heavy precipitate of iodoform settles at the bottom of the test tube. Iodoform can be identified easily not only by its characteristic smell but also by its melting point (120-123 °C) [1173], This test applies not only to methyl ketones but to any compound that can be converted in the reaction medium into a species containing the COCH3 group, for example, isopropyl or ethyl alcohol. 

Methyl ketones are characterized through the iodoform test (see iSec. 16.11). 

Methyl ketones, and alcohols which are oxidized to methyl ketones, will give a positive iodoform test. All of the compounds in this problem except pentan-3-one (part (d)) will give a positive iodoform test. 

An important variation is that compounds such as methyl carbinols also undergo the Haloform reaction since they are typically oxidized to the corresponding methyl ketone under the reaction conditions. This reaction is particularly well known to undergraduate organic chemistry students as the "Iodoform Test" in qualitative organic... 

An iodoform test can be used to identify methyl ketones. 

This fact together with the subsequent iodoform test tells us that the carbonyl function is a methyl ketone ... 

Cassia excelsa Shrad. contains cassine, C18H35NO2 (mp 57°-58.5° Wd —0.6° hydrochloride, mp 173°-175° hydronitrate, mp 116°-117°) (179), which has been shown to possess structure CXIV (180,181). IR- and NMR-data indicated a secondary hydroxyl, a secondary C-methyl, and a methyl ketone (confirmed by positive iodoform test). Dehydrogenation furnished optically inactive dehydrocassine (CXV), which showed the characteristic UV-spectrum of a 3-hydroxypyridine. 

The answer is D. Iodoform test is used to identify methyl ketones. Methyl ketones give a positive iodoform test which is indicated by a yellow precipitate. 

The reaction of methyl ketones with sodium iodate(l) gives iodoform (tri-iodomethane), which is a yellow soUd with a characteristic smeU. This reaction is used in the Iodoform test to identify methyl ketones. It also gives a positive result with a secondary alcohol of the formula RCH(OH)CH3 (which is first oxidized to a methylketone) or... 

The short list that you developed by inspection of the tables in Appendix 1 and the structures drawn should suggest that some additional tests may be needed to distinguish among the possibilities. For instance, one compound may be a methyl ketone, and the other may not. The iodoform test is called for to distinguish the two possibilities. The tests for the subsidiary functional groups may also be required. These tests are described in Experiments 52B and 52C. These tests should also be studied carefully there is no substitute for firsthand knowledge about these tests. 

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