Organic and Pharmaceutical
Chemistry Laboratories
Department
College of Pharmacy
2nd Stage (Sophomore)
2nd Semester
“Lab 3”
By
The Drug Chemist
Ph. MSc. Jaafar
Identification
and Aryl Of Alkyl
Halides
Physical properties
• All alkyl halides and chlorobenzene are colourless liquids when pure except
iodoform (CHI3), which is a yellow crystalline solid with a characteristic odour.
• Methyl iodide, ethyl iodide and bromide, chloroform, and carbon tetrachloride
have sweetish odours. Benzyl chloride has a sharp irritating odour and is
lachrymatory. While, Chlorobenzene possesses aromatic odour.
• Alkyl and aryl halides (R-X, Ar-X) have boiling points higher than the parent
hydrocarbon because of the heavier molecular weight. Accordingly, for a given
compound, iodides have the higher boiling point than bromides and chlorides.
• In spite of their polarity alkyl halides are insoluble in water due to their inability
to form hydrogen bonds. They are soluble in most organic solvents.
• Iodo-, bromo-, and polychloro- compounds are denser than water.
Chemical Reaction
1. Reaction with Alcoholic Silver Nitrate
Alcoholic silver nitrate reagent is useful in classifying halogen. Many
halogen containing compounds react with silver nitrate to give an
insoluble silver halide (AgX), and the rate of this reaction indicates
the degree of reactivity of the halogen atom in the compound.
Besides, the identity of the halogen can sometimes be determined
from the colour of the silver halide produced; silver chloride
is white (turns to purple on exposure to light), silver bromide is
pale yellow, and silver iodide is yellow. These should, of
course, be consistent with results from elemental analysis
(sodium fusion for detection of halogens).
Mechanism?
• SN1 Reaction
• The reactivity of alkyl halides
towards this reagent is:
Precipitate R3CX > R2CHX > RCH2X
Procedure
1. Add one drop or a couple of crystals of the unknown to 2 mL of 2% ethanolic
silver nitrate solution. If no immediate reaction is observed, stand for 5 minutes
at room temperature and observe the result.
2. If no reaction takes place, warm the mixture in water bath for 30 seconds and
observe any change.
3. If there is any precipitate (AgX) add several drops of 1 M nitric acid solution to it;
silver halides are insoluble in this acid.
• Tert- chlorides, methyl and ethyl iodides, allylic chlorides, and ethyl
bromides give fast result at room temperature:
• Tri and sec chlorides, Benzylchloride, 1-chloro-2,4-dinitrobenzene give result only
on warming.
• Chlorobenzene, chloroform, iodoform, carbon tetrachloride, and vinylic chlorides
don't give any positive result even on heating:
• Cyclohexyl halides exhibit a decreased reactivity when compared with
corresponding open chain halides. Cyclohexyl chloride is inactive, and
cyclohexyl bromide is less reactive than 2-bromohexane, although it
will give a precipitate with Alcoholic Silver Nitrate
Sodium iodide in acetone test.
• This test, complementing the alcoholic silver nitrate test, is used to
classify aliphatic chlorides and bromides as primary, secondary, or
tertiary. This test depends on the fact that sodium chloride and
sodium bromide are only very slightly soluble in acetone The
mechanism follows direct displacement (SN2) process; therefore, the
order of reactivity of simple halides is:
primary> secondary> tertiary
• With sodium iodide, primary bromides give a precipitate of sodium
bromide within 3 min at 25°C, whereas the chlorides give no
precipitate and must be heated to 50°C in order to effect a reaction.
Secondary and tertiary bromides react at 50°C, but the tertiary
chlorides fail to react within the time specified. Tertiary chlorides will
react if the test solutions are allowed to stand for a day or two.
These results are consistent with the following SN2 process:
Procedure
To 1 mL of the sodium iodide-acetone reagent in a test tube, add two drops of the
compound, if the compound is a solid dissolve about 0.1 g in the smallest possible
volume of acetone. Add the solution to the reagent. Shake the test tube, and allow
the solution to stand at room temperature for 3 min.
Note:
whether a precipitate is formed and/or the solution turns reddish brown due to the
liberation of free iodine, no change could be possible at room temperature, if so:
• place the test tube in water bath at 50°C for 6 min (Excessive heating causes loss
of acetone and precipitation of sodium iodide, which can lead to false-positive
results). Then, cool to room temperature and note whether a reaction has
occurred. Occasionally, a precipitate forms immediately after combination of the
reagents; this represents a positive test precipitate remains after the mixture is
shaken and allowed to stand for 3 minutes.
Differentiation between alkyl and aryl halides
1. Formaldehyde- sulfuric acid test (Marquis reagent)
With this test aryl halides (chlorobenzene) produce pink, red, or bluish
red colour whereas alkyl halides produce yellow, amber, or brown
colour.
Procedure
1. This reagent is prepared at the time of use by adding 1 drop of
formaldehyde to a test tube containing 1 mL concentrated sulfuric
acid.
2. In another test tube add 1 drop of the compound to be tested to 1
mL of hexane.
3. From this solution take 1-2 drops and add them to 1 mL of the
reagent. Shake well and observe the colour.
Addiction
Another type of addiction
Special tests for chloroform
a) Riemer- Tiemann reaction
Resorcinol results in a red colour with slight fluorescence in the aqueous layer
while α- or β- naphthol results in a deep blue aqueous layer fading to green.
Procedure
To about 0.2 g of the phenol add 1mL of 30% sodium hydroxide solution and 1 mL
of chloroform, heat on water bath, and observe the colour of the aqueous layer
b) Reduction of Fehling's reagent
• Preparation of Fehling's reagent
(1ml of Fehling A + 1ml of Fehling B )
• Boil 1 mL of chloroform gently (water bath) with 3 mL of Fehling's reagent
with constant shaking for 3- 4 minutes.
• Reduction occurs and reddish cuprous oxide slightly separates.
