Identification of an unknown organic compound

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Identification of an unknown organic compound


The aim of the identification experiment is to use a variety of tests to ultimately go to one functional group to identify an unknown organic compound. It could be one of the following:

  • Alkenes
  • Primary alcohol
  • Tertiary alcohol
  • Aldehyde
  • Ketones
  • Carboxylic acid
  • Ester
  • Phenol

I will take each of the supplied functional groups and talk about them generally and identify a test which will be most suitable to identify whether that functional group is present in the unknown organic compound we are provided with.

If tests need to be taken to see if oxygen, carbon dioxide or hydrogen gas has been given off, we will do the following:

  • Hydrogen – is present if we light a splint and put it in the test tube and it goes out with a squeaky pop.
  • Oxygen – is present if we light a wooden splint and blow it out and put it in the test tube and it rekindles.
  • Carbon dioxide – is present if when bubbled through lime water- it goes cloudy.

During the experiment I will wear safety goggles, gloves and a laboratory coat throughout the whole experiment as some substances may cause irritant to the skin and be dangerous if in contact with skin and eyes. I will use 8 tests in total to identify the identity of the unknown organic compound.


Alkenes are unsaturated hydrocarbons and contain at least one double C=C bond. Unsaturated means that additional reactions can occur to break the C=C bond. They have the general formula of CnH2n with the structural formula of:


They have a planar shape with bond angles of 120. The test I will be using to identify an alkene will be the bromine test. I will add 5 drops of bromine water then add 2 drops of the unknown organic compound and shake. If the bromine water is decolourised this means there is an alkene present.

Apparatus needed:

  • Bromine water
  • Test tube
  • Unknown organic compound

The major product produced isn't 1, 2-dibromoethane. Water also gets involved in the reaction, and most of the product is 2-bromoethanol.

Primary and tertiary alcohols

Aldehydes are reduced to primary alcohols. Primary alcohols are oxidized on heating with acidified aqueous potassium dichromate: however tertiary alcohols remain unchanged with the reagent. (Chemistry 1 endorsed by OCR page 131 first paragraph). The test I will be using to identify whether it is a primary or tertiary alcohol will be by adding acidified potassium dichromate to the unknown organic compound. Then I will heat it in a water bath. If the colour changes from orange to green this shows it is a primary alcohol (as secondary alcohol was not included in the list of could be functional groups). If the colour remains orange this means it is a tertiary alcohol. To conclude this I would use a lucas test. This is where the unknown compound is added to a solution containing zinc chloride in concentrated aqueous hydrochloric acid. If a tertiary alcohol is present, immediately it forms a cloudy solution as the alcohol will be converted to a water soluble alkyl chloride by a SN1 reaction. If a primary alcohol is present, no reaction will occur. A secondary alcohol will take about 6 and a half minutes for a reaction, however as secondary alcohol is not present in the list of compounds given we will not take this into account.

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The equation for this with a tertiary alcohol and zinc chloride solution with concentrated hydrochloric acid will be :

Apparatus needed:

  • Acidified potassium dichromate
  • Test tube
  • Unknown substance
  • Water bath

Presence of a carbonyl group?

I will do this test in order to see whether a carbonyl group is present. If a carbonyl group is present this means that the unknown organic compound will be a ketone or an aldehyde.

The test I will do will be using 2-4-dinitrophenylhydrazine. First of all I will add 5 drops of 2-4-dinitrophenylhydrazine in a test tube and then ...

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