Identification of Organic Compounds

Aim: to identify six organic compounds by a series of laboratory
tests.

Apparatus: Test tubes

Six unknown chemicals

Boiling tubes

Water bath

Droppers

Potassium Manganate

Potassium Dichromate

Sulphuric Acid

Sodium Hydroxide

Nitric acid

Silver nitrate

In this experiment I will attempt to label six colourless liquids. The
six are as follows; alkane, alkene, primary alcohol, chloroalkane,
bromoalkane and iodoalkane. There is no test for alkanes therefore the
sample that gives negative results for all tests would be the alkane.

Safety Precaution:

Lab coats, gloves and proper eye protection must be worn throughout
all tests due to the fact that some of the chemicals are corrosive,
potentially flammable and irritants. Chemicals should be treated as
toxic therefore all tests should be carried out in a fume cupboard.

Procedure:

Alkene (C=C)

Potassium Manganate (VII) in alkaline solution is a weak oxidising
agent. When an alkene is reacted with alkaline Potassium Manganate
(VII) solution, the purple colour fades as alkene is oxidised.

1. 5cm3 of each liquid is placed into test tubes.

2. 2 drops of Potassium Manganate (VII) is added to the test tube.

3. A purple to colourless change would be noticed in one of the test
tubes, this is due to the oxidation of the alkene. The sample that
gives the positive result will be removed from further tests and
labelled Alkene.

Primary Alcohol ( C-OH)

1. 5cm3 samples of the remaining five are placed in boiling tubes.

2. 2cm3 of aqueous potassium dichromate (VI) followed by 4cm3 of
dilute sulphuric acid is added to each of the solutions.

3. Samples are gently boiled in water bath and observed for colour
change. A water bath is used because some chemicals are
potentially flammable.

An orange to green colour change would indicate the alcohol. This
colour change is due to dichromate being reduced from Cr2O72- (orange)
to (green) Cr3+ . As this is the only alcohol we can assume this is
the primary alcohol. A silver mirror test can be carried out to verify
it being a primary alcohol; with a positive test being formation of
the silver mirror. The sample that gives the positive result will be
removed from further tests and labelled Primary Alcohol.

Halogenoalkane (C-C-X)

1. 5cm3 samples of the remaining 4 liquids are placed in boiling
tubes.

2. They are then placed in water bath and heated; a water bath is
used because some chemicals are potentially flammable.

3. 2cm3 sodium hydroxide is then added to each sample in order to
hydrolyse the solution. 2cm3 nitric acid is then added to acidify
the solution. Silver nitrate is added until no further change
takes place.

A precipitate is formed in three of the sample solutions.

A white precipitate of Silver Chloride (Cl-) soluble in aqueous
ammonia would indicate a chloroalkane.

A cream precipitate of Silver Bromide (Br-) soluble in concentrated
ammonia would indicate a bromoalkane.

A yellow precipitate of Silver Iodide (I-) insoluble in ammonia would
indicate an iodoalkane.

One of the sample liquids should have tested negative for each of the
three tests carried out. This sample is the alkane.