An Experiment To Identify A Series Of Liquids

Bromide ion               silver ion                      silver bromide

ALCOHOLS

Butan-1-ol is the primary alcohol. Its molecular formula is C4H10O. Methylpropan-2-ol is the tertiary alcohol.

The test for primary alcohols is to warm with acidified potassium dichromate (VI) solution. If there is a primary or secondary alcohol present, the solution will turn from orange to green fairly quickly. Primary alcohols form aldehydes, secondary alcohols form ketones, but tertiary alcohols do not readily form either. To confirm a tertiary alcohol, a carboxylic acid must be added, such as propanoic acid. If the solution is an alcohol, an ester will form. It will give off a characteristic sweet smell. Primary, secondary and tertiary alcohols will all undergo this reaction.

The reaction with acidified potassium dichromate is oxidation, and forms butanal, or from butan-1-ol. The reaction with a carboxylic acid is esterification

Equation:

Oxidation of Butan-1-ol:

3CH3CH2CH2CH2OH +Cr2O72-   + 8H+       ➔ 2Cr3+ + 3CH3CH2CH2CHO + 7H2O

              Butan-1-ol              dichromate ion  hydrogen ion     chromium ion                butanal                      water

Esterification of methylpropan-2-ol:

CH3C(CH3)OHCH3 + CH3CH2COOH ➔ CH3CH2COOCH2CH(CH3)CH3 + H2O

     Methylpropan-2-ol                propanoic acid                    methylisopropane propanoate              water

CARBOXYLIC ACID

Ethanoic acid, also known as acetic acid, is the carboxylic acid in this experiment. Its structural formula is CH3COOH.

The test for carboxylic acids is to add sodium bicarbonate solution. If a carboxylic acid is present, the solution will effervesce or fizz. The gas given off is carbon dioxide. Water is also formed, and the remaining compound is sodium acetate.

The reaction is decomposition, and is reversible.

Equation:

 CH3COOH +   NaHCO3(alc)  ➔  CH3COONa +     CO2        +    H2O

 Ethanoic acid   sodium bicarbonate   sodium acetate  carbon dioxide      water

WATER

Water is the other chemical in this test. Its chemical formula is H2O.

The test for water is to add anhydrous copper (II) sulphate solution. This solution is white, but after adding it to water, the solution will turn blue. The solution is now pentahydrate copper (II) sulphate, or chalcanthite.

The reaction is hydration.

Equation:

 5H2O         +        CuSO4(l)                          ➔       CuSO4.5H2O

 Water        anhydrous copper (II) sulphate     pentahydrate copper (II) sulphate

METHOD

• Place a few cm3 of each liquid into a separate test tube, labelling each with the appropriate letter label from the bottle.
• Place the test tubes in the test tube holder
• Add approximately 3cm3 of bromine solution, which will be orange-brown, to each test tube.

• Shake each bottle separately to check for a reaction
• If the contents of a test tube decolourise to become colourless, this is cyclohexene, the alkene. The other chemicals will not decolourise the solution.
• Rinse out the other test tubes and add the relevant chemicals to the five remaining tubes again.

• Add approximately 3cm3 of sodium bicarbonate solution

• Shake each bottle separately to check for a reaction.
• If the solution in one of the test tubes begins to effervesce or fizz, there must be ethanoic acid in the test tube. This is the carboxylic acid. The other chemicals will not effervesce or fizz.
• Rinse out the other test tubes and add the relevant chemicals to the four remaining tubes again.

• Heat water in the kettle and pour into water bath
• Add approximately 3cm3 of ethanolic silver nitrate solution, and warm in water bath.

• If a cream precipitate forms in one of the test tubes, there is bromoethane present. This is the halogenoalkane, and silver bromide is forming as a precipitate.
• Rinse out the other test tubes and add the relevant chemicals to the three remaining tubes again.

• Add approximately 3cm3 of anhydrous copper (II) sulphate to each test tube.

• If the solution turns from white to blue, there is water present. The solution now contains pentahydrate copper (II) sulphate.
• Rinse out the other test tubes and add the relevant chemicals to the two remaining tubes again.

• Heat water in the kettle and pour into water bath if necessary
• Add approximately 3cm3 of acidified potassium dichromate to each test tube, and warm in water bath.

• If the solution turns from orange to green, there must be butan-1-ol, as this only occurs with primary or secondary alcohols; butan-1-ol is a primary alcohol. The other alcohol, methylpropan-2-ol is a tertiary alcohol, and so will not undergo a colour change.
• Rinse out the remaining test tube and add the relevant chemical.

• Add propanoic acid to the remaining test tube

• The solution will remain colourless, but give off a sweet smell. This means that an ester is formed. This esterification is the confirmation of an alcohol, and means that methylpropan-2-ol must be present if the previous oxidation reaction did not occur.

FLOWCHART

APPARATUS

• Six test tubes
• Test tube holder
• Water bath
• Kettle

CHEMICALS

Unknowns to test:

• Bromoethane
• Butan-1-ol
• Cyclohexene
• Ethanoic acid
• Methylpropan-2-ol
• Water

For testing:

• Bromine water
• Sodium carbonate solution
• Ethanolic silver nitrate solution
• Acidified potassium dichromate (VI)
• Propanoic acid
• Anhydrous copper (II) sulphate

RISK ASSESSMENT

REFERENCES

Doc Brown's Chemistry Clinic

Wikipedia