5Add a few drops of dilute potassium permanganate solution.
6The purple colour of the MnO4– ion slowly fades. The final solution is colourless.
7Methanol has reduced the MnO4– to Mn2+: MnO4– + 8H e– → Mn H2OMethanol is oxidised in this reaction.It is a two-step reaction:You only need to learn that methanol is oxidised to methanoic acid (HCOOH).
8Oxidation by dichromate Pour a small amount of ethanol into a test tube and acidify with dilute sulfuric acid.
9Place in a beaker of hot water and add a few drops of orange potassium dichromate solution.
10Leave in the beaker until no further change takes place. The orange dichromate solution turns blue/green.
11The dichromate, Cr2O72–, has been reduced to Cr3+: Cr2O72– H e– → 2Cr H2OEthanol is oxidised by this reaction. The final product of the oxidation reaction is ethanoic acid.CH3CH2OH + H2O → CH3COOH + 4H e–3CH3CH2OH + 3H2O → 3CH3COOH H e–2Cr2O72– H e– → 4Cr H2O3CH3CH2OH + 2Cr2O72– H+ →3CH3COOH + 4Cr H2O
12In the days before electronic alcohol ‘sniffers’, New Zealand police used ‘breathalysers’ to measure breath alcohol. The subject blew through this tube to fill a bag. The crystals of potassium dichromatein the tube turned green if ethanol was present. If there was sufficient ethanol in the breath to turn crystals beyond the line green, then the person was over the legal limit for driving.
13ChlorinationAlcohols undergo substitution reactions with various chlorinating agents to form chloroalkanes.Important chlorinating agents are:Phosphorus pentachloride, PCl5Phosphorus trichloride, PCl3Thionyl chloride, SOCl2Lucas reagent, HCl/ZnCl2
14All these reactions are fastest with tertiary alcohols.
15Click on this movie to see what happens when solid PCl5 is added to 2-methyl propan-2-ol (a tertiary alcohol).
16Using Lucas reagent to distinguish alcohols Lucas reagent is used to distinguish between low mass primary, secondary and tertiary alcohols.Lucas reagent contains anhydrous zinc chloride dissolved in concentrated hydrochloric acid. It contains a very high concentration of chloride ions and the Zn2+ ion acts as a catalyst.Take 1–2 mL of Lucas reagent in a dry test tube, add a few drops of the alcohol and shake. If there is no reaction, place the test tube in a beaker of boiling water for a few minutes.
17Click on this movie to see 1°, then 2° and finally 3° alcohol added to Lucas reagent.
18The primary and secondary alcohol test tubes you’ve just seen were placed in beaker of boiling water.Once heated, the secondary alcohol quickly turned cloudy.The primary alcohol tube is unchanged.
19Tertiary alcohol turns cloudy immediately Secondary alcohol turns cloudy after gentle heating for a few minutesPrimary alcohol does not react.Note:Only add a few drops of alcohol.Lucas reagent is very concentrated. Wash it off skin immediately. It’s also expensive, so use very small quantities.