Test tube-scale reactions can be used to distinguish between compounds on the basis of differing reactivity of their functional groups.
Alkene and alkane
Alkenes react by addition.
Aldehydes and ketones
Aldehydes (RCHO) have a hydrogen at the carbonyl carbon (C=O) and are thus oxidised to carboxylic acids (RCOOH).under milder conditions than for a ketone (RCOR') where conversion to a carboxylic acid would require the breaking of a C-C bond,
The oxidant Ag(NH3)2+ reacts with aldehydes but not with ketones to form a silver mirror on the walls of the test tube.
The oxidant Cu2+ (in the presence of alkali and tartrate anion, Fehling's solution) reacts with aldehydes but not with ketones to form a red precipitate of Cu2O.
To identify whether a ketone is a methyl ketone (a ketone where one of the groups bonded to the C=O is CH3).
Iodine reacts with methyl ketones in the presence of alkali to give a carboxylate anion with one fewer carbons and a yellow precipitate of CHI3.
Examples of the functional groups that can be distinguished from one another are aldehydes and ketones, alkenes and alkanes. Furthermore on the basis of a test tube experiment, the presence of a methyl bonded to carbonyl in a ketone can be established.
In order to distinguish between functional groups in a test tube reaction, the reaction must result in a distinctive colour change or formation of a precipitate.Alkene and alkane
Alkenes react by addition.
The bromine in bromine water (a light orange solution) reacts by addition with alkenes to give a colourless product.
Br2 + CH2=CH2
BrCH2CH2Br
Provided the bromine is not added in excess, this results in decolourisation of added bromine.
Permanganate ion reacts with alkenes to give a diol, and a brown precipitate of MnO2 is formed.
MnO4– + CH2=CH2 HOCH2CH2OH + MnO2(s) (not balanced).
Alkanes are characteristically unreactive and do not react with either of these reagents. Br2 + CH2=CH2
BrCH2CH2Br Provided the bromine is not added in excess, this results in decolourisation of added bromine.
Permanganate ion reacts with alkenes to give a diol, and a brown precipitate of MnO2 is formed.
MnO4– + CH2=CH2
HOCH2CH2OH + MnO2(s) (not balanced).Aldehydes and ketones
Aldehydes (RCHO) have a hydrogen at the carbonyl carbon (C=O) and are thus oxidised to carboxylic acids (RCOOH).under milder conditions than for a ketone (RCOR') where conversion to a carboxylic acid would require the breaking of a C-C bond,
The oxidant Ag(NH3)2+ reacts with aldehydes but not with ketones to form a silver mirror on the walls of the test tube.
Ag(NH3)2+ + OH– + CH3CHO CH3CO2– + Ag(s) (not balanced)
CH3CO2– + Ag(s) (not balanced)The oxidant Cu2+ (in the presence of alkali and tartrate anion, Fehling's solution) reacts with aldehydes but not with ketones to form a red precipitate of Cu2O.
Cu2+ (in the presence of tartrate anion) + OH– + CH3CHO CH3CO2– + Cu2O(s) (not balanced)
CH3CO2– + Cu2O(s) (not balanced)To identify whether a ketone is a methyl ketone (a ketone where one of the groups bonded to the C=O is CH3).
Iodine reacts with methyl ketones in the presence of alkali to give a carboxylate anion with one fewer carbons and a yellow precipitate of CHI3.
I2 + OH– + CH3COCH3 CH3CO2– + CHI3(s) (not balanced).
CH3CO2– + CHI3(s) (not balanced).