Qualitative organic analysis

Reactions of alkanes with bromine.

Alkanes decolourise

bromine in the light.

Alkane layer

Water

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

Silver nitrate is colourless

Chloroalkanes give a white ppt, which darkens to purple on exposure to bright light.

Reactions with silver nitrate

Silver nitrate is colourless

Bromoalkanes give a buff ppt, which darkens on exposure to bright

light.

Reactions with silver nitrate

Silver nitrate is colourless

Iodoalkanes give a light yellow ppt,

which is not photo sensitive.

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 togreen chromium (iii) when heated

with acid.

Oxidation of primary alcohols.

Primary alcohols are oxidised as

orange chromium (vi) is

reduced togreen 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 togreen 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 togreen 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.

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.

Effervescence (CO2)

Phenols and alcohols are weaker acids and will not react.

Acids react with

carbonates to give a salt,

water and carbon dioxide.

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.