Baeyer test

Analysis. Butenes are best characterized by their property of decolorizing both a solution of bromine in carbon tetrachloride and a cold, dilute, neutral permanganate solution (the Baeyer test). A solution of bromine in carbon tetrachloride is red the dihaUde, like the butenes, are colorless. Decoloration of the bromine solution is rapid. In the Baeyer test, a purple color is replaced by brown manganese oxide (a precipitate) and a colorless diol. These tests apply to all alkenes. 

The reaction with permanganate constitutes a valuable and much-used test for unsaturation in an organic compound. The substance is dissolved in cold alcohol, a few drops of sodium carbonate solution are added, and then a drop.of dilute permanganate solution. Rapid disappearance of the red colour indicates the presence of a double bond. The Baeyer test can also be carried out in pure glacial acetic acid, which is stable towards permanganate. Another method of detecting double bonds is by the decolorisation of bromine. As a rule, chloroform is used as solvent. 

Reaction of an alkene with hot basic potassium permanganate (KMn04) results in cleavage of the double bond, and formation of highly oxidized carbons. Therefore, unsubstituted carbon atoms become CO2, mono-substituted carbon atoms become carboxylates, and di-substituted carbon atoms become ketones. This can be used as a chemical test (known as the Baeyer test) for alkenes and alkynes, in which the purple colour of the KMn04 disappears, and a brown Mn02 residue is formed. 

The oxidation of alkenes by permanganate is one of the frequently used examples in freshman chemistry. It is also well known as the Baeyer test for unsaturation. There are many reagents that add two hydroxyl groups to a double bond,44 but osmium tetroxide and permanganate are the most prominent ones. The mechanism of the permanganate oxidation is believed to be similar to the oxidation of alkenes by 0s0445 46... 

Hydroxylation of alkenes is the most important method for the synthesis of glycols. Moreover, oxidation by permanganate is the basis of a very useful analytical test known as the Baeyer test (Sec. 6.30). 

Of the compounds we have dealt with so far, alcohols also dissolve in sulfuric acid. Alcohols can be distinguished from alkenes, however, by the fact that alcohols give a negative test with bromine in carbon tetrachloride and a negative Baeyer test—so long as we are not misled by impurities. Primary and secondary alcohols are oxidized by chromic anhydride, CrOa, in aqueous sulfuric acid within two seconds, the clear orange solution turns blue-green and becomes opaque. 

Alkenes can be oxidized by cold, slightly alkaline potassium permanganate (man-ganate(VII)) to make substances called 1,2 diols, containing two OH groups on adjacent carbon atoms. The potassium permanganate solution turns from purple to brown. This is another test (the Baeyer test) that is often used to detect unsaturation in a hydrocarbon ... 

Baeyer test A test for unsaturated compounds in which potassium permanganate is used. Alkenes, for example, are oxidized to glycols, and the permanganate loses its colour ... 

Transfer the pot residues from the distillation of spearmint and/or caraway oil to the container for nonchlorinated organic wastes. After performing the prescribed tests on your distillate, put the isolated natural products into appropriately labeled containers. Place the dichloromethane solution from the bromine test for unsaturation in a container for halogenated organic liquids put the manganese dioxide from the Baeyer test for unsaturation in a container for heavy metals. Neutralize and then filter any excess 2,4-dinitrophenylhydrazine solution. Put the filter cake in the container for nonhazardous solids. Flush the filtrate as well as filtrates obtained from recrystallization down the drain. 

Analysis Weigh the distillate and calculate the yield of products. Then put the container in an ice-water bath until all tests on the distillate have been completed. Test the distillate for unsaturation using the bromine and Baeyer tests (Secs. 25.8A... 

Neutralize the residue remaining in the stillpot with 10% aqueous hydrochloric acid then flush it down the drain. Place the dichloromethane solution from the bromine test for unsaturation in a container for halogenated organic liquids put the manganese dioxide from the Baeyer test for unsaturation in a container for heavy metals. 

Analysis Weigh the distillate and determine the yield of products. Test the distillate for unsaturation using the bromine and Baeyer tests (Secs. 25.8A and B. respectively). Analyze your distillate by GLC or submit a sample of it for analysis. After obtaining the results, calculate the relative percentages of the isomeric alkenes formed assume that the response factors are identical for the isomers. A typical GLC trace of the alkenes from this elimination is shown in Figure 10.10. Obtain IR and NMR spectra of your starting material and compare your spectra with those of an authentic sample (Figs. 10.11 and 10.12). 

Analysis Weigh your product to determine the yield and measure its melting point. Test the product for unsaturation using the bromine in dichloromethane (Sec. 25.8A) and the Baeyer tests (Sec. 25.8B). Obtain IR and El NMR spectra of your starting materials and product and compare them with those of authentic samples (Eigs. 12.1-12.6). 

---