Identification of Aromatic and Alkyl Halides
1 . Introduction
Benzene and a large number of its alkylated derivatives are liquids. Most compounds
containing more than one aromatic ring, either fused or unfused, are solids. Aromatic hydrocarbons burn with a characteristic sooty flame; compounds that contain little oxygen and have a high carbon to hydrogen ratio (benzene is 1:1) will burn at a low temperature. A large amount of carbon is produced when such compounds are burned. Aromatic hydrocarbons give negative tests with Br2/CCl4 and basic permanganate. The simple aromatic hydrocarbons are insoluble in conc. H2SO4 (class I), however the more activated ones are soluble in H2SO4 but not H3PO4 (class N2).
Halogen substituents may be found alone or in combination with all other functional groups. Alkyl halides are often first indicated by a positive Beilstein Test and a sodium fusion which gives a positive result for a halogen. Alkyl and aryl halides are insoluble in conc. sulfuric acid (class I). However when another functional group is present, the solubility classification is a result of the other functional group. Often an alkyl halide can be classified by the tendency of the halogen to undergo a substitution process.
There are no suitable general classification tests for alkanes. Saturated hydrocarbons are best identified through ruling out other possible functional groups. Alkanes are insoluble in conc. Sulphuric acid and give a negative test for halogens.
2 . Test for Aromatic Hydrocarbons
Several Classification tests for aromatic hydrocarbons (both activated and deactivated) make use of benzene ring's ability to undergo electrophilic aromatic substitution.
a. Fuming Sulfuric Acid
Simple benzene compounds which do not dissolve in concentrated H2SO4, will dissolve in fuming sulfuric acid. Fuming sulfuric acid is a powerful ionizing and sulfonating agent. Even relatively unreactive aromatics will undergo reaction. The products are soluble in the reaction medium.
Azoxybenzene Test
Aromatic hydrocarbons derived from benzene and their halogenated derivatives will react with azoxybenzene in the presence of AlCl3 as a catalyst to give a colored diazo complex. The mechanism for the formation of this compound first involves complexation of the AlCl3 Lewis acid to the azoxy oxygentation with subsequent electrohilic addition the aromatic hyrocarbon.
Aromatic hydrocarbons produce a deep orange to dark red colored solution, or a colored precipitate. Polynuclear hydrocarbons such as naphthalene and anthracene give brownish colors.
c. CHCl3/AlCl3 Test
Treatment of an aromatic hydrocarbon with chloroform (CHCl3) and aluminum chloride will produce the highly colored triarylcarbonium ion in solution.
The colors produced by the triarylmethyl cation are quite characteristic of the aryl groups
present. Typical colors produced are as follows:
Compound | Color |
Benzene | Orange to red |
Aryl halides | Orange to red |
Naphthalene | Blue |
Biphenyl | Purple |
Phenanthrene | Purple |
Anthracene | Green |
3 . Tests for Alkyl Halides
The classification tests for alkyl halides takes advantage of the halo substituent and its ability to be displaced. The two tests for displaceable halogen are complimentary; one works best on 3° alkyl halides, while the other works best on 1° alkyl halides. Thus, by employing both these tests one can possibly differentiate between 1°,2°, and 3° alkyl halides.
a. Alcoholic Silver Nitrate Test
The test depends on the rapid and quantitative reaction of alcoholic silver nitrate with halide ion to produce insoluble silver halide (except fluoride). Silver and other heavy metals catalyze SN1 reactions of alkyl halides by complexation with the lone-pair electrons of the halide, making the leaving group a metal halide rather than a halide ion.
The rate of precipitation of the silver halide depends on the leaving group, I > Br > Cl, and upon the structure of the alkyl group. Any structural factors which stabilize the electron deficient carboniumion will accelerate the reaction. We thus find the expected order of reactivity: Benzyl allyl > 3° > 2°>1° > methyl > vinyl aryl.
Since a halo substituent is often found in the presence of many other functional groups, several functional groups have been shown to greatly facilitate the loss of a halide in the silver nitrate test.
b. Sodium Iodide in Acetone
This test involves treating a suspected halogen bearing unknown with sodium iodide in acetone and observing the rate at which a precipitate of NaX (where X = Cl or Br) is formed. The rate of the reaction of sodium iodide in acetone with a compound containing a covalently bonded bromine or chlorine is a measure of the reactivity of the compound toward SN2 substitution. This test depends on the fact that sodium iodide is readily soluble in acetone, whereas sodium bromide and sodium chloride are not.
SN2 reactions are concerted and involve backside nucleophilic displacement which result in the inversion of configuration of the carbon atom ath the reaction site. As might be expected,such reactions are sensitive to steric crowding. The overall rate of such reactions forrlows the order: methyl > primary > secondary > tertiary.