Identification of an Organic Unknown.

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Hassan Patel

Chemistry Coursework

Mr Choudhury

Identification of an Organic Unknown.

In this investigation I will be supplied with an unknown compound containing one of the following functional groups:

  • Alkene
  • Primary alcohol
  • Tertiary alcohol
  • Aldehyde
  • Ketone
  • Carboxylic acid
  • Ester
  • Phenol

Wet tests are chemical tests and are useful because they are qualitative and mainly based on observations. Dry tests are more quantitative and go into more detail because determine exact molecular structure. I will be carrying out wet test instead of dry tests because it is more economical as using chemicals is cheaper then using machines and all we need to determine is the functional group.

By using a number of chemical tests I will need to identify the functional group present. Each test used to identify the functional group is dependant on the previous result. I will use a flowchart to demonstrate the paths.

Alkenes are hydrocarbon chains which contain carbon-carbon double bonds, C=C. These consist of a sigma bond and a pi bond. The electron charge cloud of the C=C is the centre of attraction for electrophiles. In most reactions involving alkenes, electrophiles are added to the C=C to give saturated compounds. This means that the characteristic reactions of alkenes are addition reactions. To test for an alkene, bromine water can be used. The orange bromine decolourises if C=C is present.

Equation for bromine water and ethene:

           C2H4 + Br2                  C2H4Br2           


Alcohols are compounds in which a hydroxyl (-OH) group is bonded to a saturated carbon atom. Primary and secondary alcohols undergo oxidation when reacted with a mixture of potassium dichromate (VI). In the reaction the orange dichromate (VI) ions are reduced to green chromium (III) ions. Primary alcohols oxidise to form aldehydes whereas secondary alcohols oxidise to form ketones. Primary alcohols –OH group is attached to a carbon that has three hydrogens whereas a tertiary alcohol has a –OH group which is attached to a carbon that has three R groups and no hydrogens.

2Cr2O72 –(aq)  +  3C2H5OH(l)  +  16H+(aq)              4Cr3+(aq)   +  3CH3CO2H(aq)  +  11H2O(l)

Tertiary alcohols resist oxidation. To confirm presence of an alcohol the addition of sodium will cause effervescence.

2R-OH + 2Na             2RO-Na+ + H2        

Carbonyl compounds react with 2,4 Dinitrophenylhydrazine to form coloured compounds called hydrazones (orange/ yellow precipitate).  A positive test of an unknown with 2,4 dinitrophenylhydrazine confirms presence of an aldehyde or ketone. This reaction is a condensation reaction.

Oxidation of secondary alcohols produces ketones, e.g. propanone and contains a C=O bond, where the carbon atom is also bonded to a CH3 group. Ketones are not readily oxidised at all. They have no effect on mild oxidising agents. This is because they do not have an oxidisable hydrogen atom joined to the carbonyl group.

Primary alcohols produce aldehydes, e.g. ethanal, these contain a C=O group and here the carbon atom is bonded to a hydrogen. Aldehydes carry a hydrogen atom next to their carbonyl group. This hydrogen is activated by the carbonyl group and is readily oxidised to OH. Aldehydes are therefore readily oxidised to carboxylic acids.

Fehlings reagent is used to detect the presence of an aldehyde. When warmed in Fehlings solution, an alkaline solution of Cu2+ ions (a red precipitate of copper (I) oxide) is produced with a carboxylic acid.

Carboxylic acids contain both a carbonyl (C=O) group and a hydroxyl group (O-H). Both bonds are quite strongly polar (C=O and O-H). Adding sodium hydrogen carbonate can test the presence of a carboxylic acid. If effervescence occurs and the gas given off is carbon dioxide (test gas by bubbling into limewater) then a carboxylic acid group is present.

Equation showing the reaction between carboxylic acid and sodium hydrogen carbonate:

NaHCO3 + HCOOH                     HCOO-Na+ + H20 + CO2

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Esters also contain a C=O bond but here the carbon is attached to another oxygen, which is bonded with another carbon. There is no simple test to carry out for identifying an ester, however esters have a distinct fruity smell. Hydrolysis and then testing for a carboxylic acid can test for the presence of an ester because esters hydrolyse to form carboxylic acids.

Phenols are compounds containing a hydroxyl group attached to an aromatic ring, for example 2-methylphenol. Phenol produces a purple complex when mixed with iron (III) chloride in which the phenol is acting as a ligand. This ...

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