Chem 4563 Organic Qualitative Analysis
Aromatics, Alkyl Halides and Alkanes - Functional Group Tests
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. H2 SO4 (class I),
however the more activated ones are soluble in H2 SO4 but not H3 PO4 (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. sulfuric
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 H2 SO4 , 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.
SO3H
SO3H
SO3
HSO4
H2SO4
OSO3H
b.
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
O
Y
NN
Y
N N
AlCl3
colored arylazobenzene complex
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 mechanism is shown below.
CHCl3 + AlCl3
CHCl2
AlCl4
+ CHCl2
CHCl2 + AlCl3
CHCl2
CHCl
CHCl
AlCl4
2
CHCl + AlCl3
CH
AlCl4
CH
2
2
CH
3
3
R+
hydride
abstraction
C
+ R-H
3
Colored Triaryl cation
The reaction occurs as three successive Friedel-Crafts reactions, these reactions are promoted by the
Lewis acid catalyst AlCl3 . The final product is triaryl methane which is converted to the resonance
stabilized triaryl methyl cation by hydride abstraction during the reaction process.
The colors produced by the triarylmethyl cation are quite characteristic of the aryl groups
present. Typical colors produced are as follows:
Compound
Benzene
Aryl halides
Naphthalene
Biphenyl
Phenanthrene
Anthracene
3.
Color
orange to red
orange to red
blue
purple
purple
Green
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.
Ag+
slow
R+ + AgX
R X
Ag
R X
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 defficient carbonium
ion 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.
G
C C
X
G
C C
G C
G C X
G = Amino, Alkoxy, Hydroxy, Vinyl
Thioalkoxy, etc.
G C
A summary in order of devreasing reactivity of silver nitrate with various organic compounds
containing halogen is as follows:
1.
Water soluble which give immediate precipitates with AgNO3
a.
Salts
b.
Halogen compounds which are hydrolized in water such as acyl or sulfonyl
halides
2.
Reactivity of water insoluble compounds with immediate precipitation
a.
Benzylic and allylic halides
b.
3˚ alkyl halides
c.
Any alkyl iodide
3.
Silver halide precipitates slowly or not at all at room temperature, but readily when
warmed an a steam bath
a.
1˚ and 2˚ alkyl halides
4.
Inert to hot silver nitrate
a.
b.
c.
b.
aryl and vinyl halides
geminal dihalides
α-haloketones
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.
R
R
NaI + 'R C X
'R C I
+ NaX
R''
R''
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. A summary of the results expected in the NaI/acetone test
follows.
1.
Precipitate within 3 min at room temperture
a.
primary bromides
b.
acyl halides
c.
allyl halides
d.
α-haloketones, -esters, -amides, and -nitriles
2.
Precipitate only when heated at 50˚C for up to six minutes.
a.
primary and secondary chlorides
b.
secondary and tertiary bromides
3.
Do not react in 6 min at 50˚C
a.
tertiary chlorides
b.
aryl halides
c.
vinyl halides