Reactions of alcohols.

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Oxidation of alcohols, aldehydes and ketones

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Presentation transcript:

Reactions of alcohols

Oxidation of primary alcohols Oxidation by permanganate Methanol is a primary alcohol.

Acidify the methanol with dilute sulfuric acid.

Put it in a beaker of hot water.

Add a few drops of dilute potassium permanganate solution.

The purple colour of the MnO4– ion slowly fades. The final solution is colourless.

Methanol has reduced the MnO4– to Mn2+: MnO4– + 8H+ + 5e– → Mn2+ + 4H2O Methanol is oxidised in this reaction. It is a two-step reaction: You only need to learn that methanol is oxidised to methanoic acid (HCOOH).

Oxidation by dichromate Pour a small amount of ethanol into a test tube and acidify with dilute sulfuric acid.

Place in a beaker of hot water and add a few drops of orange potassium dichromate solution.

Leave in the beaker until no further change takes place. The orange dichromate solution turns blue/green.

The dichromate, Cr2O72–, has been reduced to Cr3+: Cr2O72– + 14H+ + 6e– → 2Cr3+ + 7H2O Ethanol is oxidised by this reaction. The final product of the oxidation reaction is ethanoic acid. CH3CH2OH + H2O → CH3COOH + 4H+ + 4e– 3CH3CH2OH + 3H2O → 3CH3COOH + 12H+ + 12e– 2Cr2O72– + 28H+ + 12e– → 4Cr3+ + 14H2O 3CH3CH2OH + 2Cr2O72– + 16H+ → 3CH3COOH + 4Cr3+ + 11H2O

In 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 dichromate in 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.

Chlorination Alcohols undergo substitution reactions with various chlorinating agents to form chloroalkanes. Important chlorinating agents are: Phosphorus pentachloride, PCl5 Phosphorus trichloride, PCl3 Thionyl chloride, SOCl2 Lucas reagent, HCl/ZnCl2

All these reactions are fastest with tertiary alcohols.

Click on this movie to see what happens when solid PCl5 is added to 2-methyl propan-2-ol (a tertiary alcohol).

Using 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.

Click on this movie to see 1°, then 2° and finally 3° alcohol added to Lucas reagent.

The 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.

Tertiary alcohol turns cloudy immediately Secondary alcohol turns cloudy after gentle heating for a few minutes Primary 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.