Qualitative organic analysis

Reactions of alkanes with bromine. Alkane layer Water Alkanes decolourise bromine in the light. But not in the dark!

Reactions of alkenes with bromine Alkenes decolourise bromine in both the dark and the light.

Reactions of alkenes with potassium manganate (vii) Alkenes decolourise acidified manganate (vii) Manganese is reduced from purple +7 To colourless +2

If there is insufficient alkene… the intermediate brown Manganese (iv) is formed.

Reactions of alkanes with manganate (vii) Alkanes do not decolourise acidified manganate (vii) as they are saturated.

Reactions of haloalkanes with silver nitrate Chloroalkanes give a white ppt, which darkens to purple on exposure to bright light. Silver nitrate is colourless

Reactions with silver nitrate Silver nitrate is colourless Bromoalkanes give a buff ppt, which darkens on exposure to bright light. Silver nitrate is colourless

Reactions with silver nitrate Iodoalkanes give a light yellow ppt, which is not photo sensitive. Silver nitrate is colourless

Alcohols give effervescence with sodium This is because they are extremely weak acids. Hydrogen gives a “pop” with a lighted splint.

But as alcohols are extremely weak acids they are unable to neutralise bases.

Reactions with sodium dichromate. Sodium dichromate is an oxidising agent, ie it oxidises other chemicals, being reduced in the process. Orange chromium (vi) is reduced to green chromium (iii) when heated with acid.

Oxidation of primary alcohols. Primary alcohols are oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Two organic products are possible; Aldehydes or Carboxylic Acids.

Oxidation of aldehydes. Aldehydes are oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Only one organic product is possible; a Carboxylic Acids.

Oxidation of secondary alcohols. Secondary alcohols are also oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Only one organic product is possible; a Ketone.

Oxidation of tertiary alcohols Tertiary alcohols cannot be oxidised by acidified potassium dichromate. To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.

Testing for carbonyl compounds. Both aldehydes and ketones give an orange/red precipitate with 2,4 dinitro phenyl hydrazine.

Distinguishing between aldehydes and ketones. Aldehydes are oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Only one organic product is possible; a Carboxylic Acids.

Distinguishing between aldehydes and Ketones Ketones cannot be oxidised by acidified potassium dichromate. To oxidise them a much stronger oxidising agent is needed that can break C/C bonds.

Blue copper (ii) ions are reduced to red copper (i). Reaction with Fehling’s Solution Aldehydes change the colour of Fehling’s Reagent upon heating from blue to red. Blue copper (ii) ions are reduced to red copper (i). Cu 2+ + e- → Cu+

Ketones do not react with Fehling’s Solution as they are not oxidised by mild oxidising agents.

Reaction with Tollen’s Reagent Aldehydes react with Tollen’s Reagent (ammonical silver solution), depositing a silver mirror on the side of the tube. Silver ions are reduced to metallic silver. Ag+(aq) + e- → Ag(s)

But Ketones do not react with Tollen’s reagent as they are not oxidised by mild oxidising agents.

Reactions of carboxylic acids Acids react with metals giving hydrogen. Carboxylic acids react with sodium, producing effervescence. Hydrogen gives a “pop” with a lighted splint.

Reaction of carboxylic acids with sodium carbonate. Acids react with carbonates to give a salt, water and carbon dioxide. Effervescence (CO2) Phenols and alcohols are weaker acids and will not react.

Reactions with Universal Indicator Alcohols, carbonyl compounds and esters are neutral so turn Universal Indicator green.

Alcohols might react with sodium like acids, but… they are unable to neutralise bases.

Carboxylic Acids are acidic, so turn Universal Indicator red.

Carboxylic acids neutralise bases.

Amines are basic so turn Universal Indicator blue.

Amines neutralise acids.