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Tests for Functional Groups

Please check your syllabus to see which of the following tests you are required to know.

1. Tests for Unsaturation

1. Dissolve 0.1g of the liquid or solid in 2cm³ of trichloroethane. If the alkene is a gas bubble it through the solvent for 15 seconds. Add 2 or 3 drops of bromine dissolved in the same solvent. Instant decolourisation in the cold without the evolution of HBr indicates the presence of .

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2. Dissolve 0.1g of the liquid or solid in 2cm³ of absolute alcohol. Add 2 or 3 drops of 1% potassium manganate (VII) solution. The rapid appearance of a brown colour or precipitate indicates the presence of . This is known as Baeyer's Test.

   `{: [stackrel {text{MnO}_4^{-}} { rarr }],[stackrel {text{H}_2text{O}} {}] :}`

Click here to see a video of the tests for unsaturation.

2. Alcohol Group

1. Add a small piece of freshly cut sodium metal to the liquid (2cm³). A brisk and well sustained effervescence indicates the evolution of hydrogen! from an alcohol.

   Care must be taken if a 'pop' test is carried out as all alcohols have flammable vapours.

   `2text{CH}_3 text{CH}_2 text{OH}\ +\ 2text{Na} rarr stackrel text{sodium ethoxide} {2text{CH}_3 text{CH}_2 text{O}^{-} text{Na}^{+}}\ +\ text{H}_2`

2. The class of an alcohol is determined by oxidation. Mix equal volumes (2cm³) of dilute sulfuric acid and potassium dichromate solutions. Add 1cm³ of the alcohol and place in a water bath at 70°C.

   1. A rapid change of colour from orange to green indicates a primary alcohol.

      `{: [text{CH}_3 text{CH}_2 text{CH}_2 text{OH}, stackrel {text{H}_2text{SO}_text{4(aq)}} rarr, text{CH}_3 text{CH}_2 text{CHO}], [text{}^red{\ \ \ \ 1 deg}, text{}^{text{Cr}_2text{O}_7text{}^text{2-} text{}_text{(aq)}}, text{}^red{\ \ \ \ "aldehyde"}] :}`

   2. A slow or slower change of colour from orange to green indicates a secondary alcohol.

      `{: [(text{CH}_3)_2 text{CH} text{OH}, stackrel {text{H}_2text{SO}_text{4 (aq)}} rarr, text{CH}_3 text{COCH}_3], [text{}^red{\ \ \ \ 2 deg}, text{}^{text{Cr}_2text{O}_7^{2-}text{}_text{(aq)}}, text{}^red{\ \ \ \ "ketone"}] :}`

      The speed of change is not sufficient to distinguish between a primary and secondary alcohol. This is because the number of carbons in the alcohol have a kinetic effect. If the product of oxidation is distilled out then the presence of an aldehyde (from a 1° alcohol) or a ketone (from a 2° alcohol) can be used to distinguish the two classes.

   3. No change of colour indicates a tertiary alcohol.

Click here to see a video of the tests for alcohol groups.

3. Haloalkanes

Dissolve two or three drops of the haloalkane in 2cm³ of ethanol and an equal volume of dilute silver nitrate solution. Allow to stand at room temperature. The slow formation of a white, cream or yellow precipitate indicates a substance of this class.

It is found that R-I forms a precipitate quicker than R-Br which is quicker than R-Cl. It is also found that a tertiary halide, (R)₃CBr forms a precipitate faster than a secondary halide, (R)₂CHBr which is faster than a primary halide, RCH₂Br.

Halobenzenes do not form precipitates in this way but acid chlorides form an immediate precipitate.

`{: [text{CH}_3text{CH}_2text{CH}_2text{CH}_2text{I}\ , stackrel {text{Ag}^text{+}} rarr ,\ text{CH}_3text{CH}_2text{CH}_2text{CH}_2text{OH},\ +\ , text{AgI}_text{(s)}], [text{},\ stackrel {text{H}_2text{O}} {}\ , text{}, text{}, stackrel text{yellow} {}] :}`

`{: [text{CH}_3text{CH}_2text{CH}_2text{CH}_2text{Br}\ , stackrel {text{Ag}^text{+}} rarr ,\ text{CH}_3text{CH}_2text{CH}_2text{CH}_2text{OH},\ +\ , text{AgBr}_text{(s)}], [text{},\ stackrel {text{H}_2text{O}} {}\ , text{}, text{}, stackrel text{cream} {}] :}`

`{: [text{CH}_3text{CH}_2text{CH}_2text{CH}_2text{Cl}\ , stackrel {text{Ag}^text{+}} rarr ,\ text{CH}_3text{CH}_2text{CH}_2text{CH}_2text{OH},\ +\ , text{AgCl}_text{(s)}], [text{},\ stackrel {text{H}_2text{O}} {}\ , text{}, text{}, stackrel text{white} {}] :}`

Click here to see a video of the tests for haloalkanes.

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