REACTIONS OF THE CARBONYL
GROUP IN ALDEHYDES AND
KETONES
L.O.:
 The mechanism of nucleophilic addition
reactions of carbonyl compounds
 How carbonyl compounds react when oxidised
or reduced.
A test for an aldehyde using Fehling’s solution.
a) In a clean test tube mix together equal volumes of
Fehling's solution A and Fehling's solution B. The
resultant Fehling's test reagent should be a clear dark
blue solution.
b) Add 5 drops of this test reagent to 1 cm3 of sodium
carbonate solution in a test tube containing a few antibumping granules and then add 1 cm3 of ethanal (or
propanal) to this same test tube.
c) Warm the test tube gently for approximately two
minutes in a beaker half filled with hot water and then
gradually bring the beaker of water to boiling and
maintain this temperature for a few minutes.
d) Using the test tube holder, carefully lift the test tube
out of the boiling water and allow its contents to stand
for several minutes.
Tollens’ silver mirror test
Prepare a sample of Tollens’ reagent by adding 5
drops of sodium hydroxide solution to 2 cm3 of silver
nitrate solution in a test tube. To this test tube add just
enough dilute ammonia solution to dissolve the brown
precipitate completely.
Using a beaker of hot water (50 oC to 60 oC), gently
warm approximately 5 cm3 of this test reagent in a test
tube.
Add 10 drops of the aldehyde to the warmed test
reagent in the test tube. Wait a few minutes and note
what happens.
Carbonyl groups consists of a carbonoxygen double bond
The bond is polar due to the difference in
electronegativity
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism
Nucleophilic addition
STEP 1
Step 1
CN¯ acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism
Nucleophilic addition
STEP 1
STEP 2
Step 1
CN¯ acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2
A pair of electrons is used to form a bond with H+
Overall, there has been addition of HCN
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism
Nucleophilic addition
STEP 1
STEP 2
Step 1
CN¯ acts as a nucleophile and attacks the slightly positive C
One of the C=O bonds breaks; a pair of electrons goes onto the O
Step 2
A pair of electrons is used to form a bond with H+
Overall, there has been addition of HCN
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
ANIMATED MECHANISM
CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Watch out for the possibility of optical isomerism in hydroxynitriles
CN¯ attacks from above
CN¯ attacks from below
Many reducing agents will reduce ketones and
aldehydes to alcohols.
NaBH4 (sodium tetrahydroborate(III) ) generates
the nucleophile H-, the hydride ion.
Write the mechanism of the reaction of a
ketone/aldehyde with H-.
Will NaBH4 react with an alkene?
NO! H- is repelled by the electron density of C=C.
CH2 = CHCHO
+
2[H]
———>
CH2 =CHCH2OH
CARBONYL COMPOUNDS - REDUCTION
Example
COMPOUND X
What are the products when Compound X is reduced?
H2
NaBH4
CARBONYL COMPOUNDS - REDUCTION
Example
What are the products when Compound X is reduced?
COMPOUND X
H2
NaBH4
C=O is polar so is attacked by the nucleophilic H¯
C=C is non-polar so is not attacked by the nucleophilic H¯
Oxidation
Fehling test (Cu2+).
Silver mirror test (Ag+)
Is it more difficult to oxidise a ketone than
an aldehyde? Why
THE SILVER MIRROR TEST
Tollen’s Reagent contains [Ag(NH3)2 ]+
When an aldehyde is warmed with Tollen’s
reagent, metallic silver is formed.
Aldehyde is oxidised to carboxylic acid
and Ag+ reduced to metallic silver
RCHO + [o] -> RCOOH
oxidation
[Ag(NH3)2]+ + e- -> Ag + 2NH3 reduction
clip
FEHLING’S SOLUTION
It contains a copper(II) complex ion
giving a blue solution. On warming, it will
oxidise aldehydes. The copper(II) is
reduced to copper(I) and a red precipitate
of copper(I) oxide, Cu2O, is formed.
Ketones do not react with Tollen’s
Reagent or Fehling’s Solution
video 2