10.5 Carbonyl Compounds
(a) describe:
(i) the formation of aldehydes and ketones from primary and secondary
alcohols respectively using Cr2O72-/H+
(ii) the reduction of aldehydes and ketones e.g. using NaBH4
(b) describe the mechanism of nucleophilic addition reactions of hydrogen
cyanide with aldehydes and ketones
(c) describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect the
presence of carbonyl compounds
(d) deduce the nature (aldehyde or ketone) of an unknown carbonyl compound
from the results of simple tests (i.e. Fehling’s and Tollens’ reagents; ease of
oxidation
Revision: formation of aldehydes and ketones from alcohols;
Primary alcohols give aldehydes:
Ethanol
Ethanal
Secondary alcohols give ketones:
Aldehydes are names systematically with al at the end of the name and ketones
with one; remember to include the number e.g. butan-2-one.
Reduction of aldehydes and ketones
Lithium aluminium hydride (tetrahydridoaluminate) and sodium borohydride
(tetrahydridoborate) reduce aldehydes and ketones to primary and
secondary alcohols.
Sodium borohydride can be dissolved in water (some carbonyl compounds
are soluble in water), but lithium aluminium hydride reacts with water, so
apolar solvents like ethers are used.
Nucleophilic addition of hydrogen cyanide to carbonyl
compounds
Hydrogen cyanide adds across the double bond of carbonyl
compounds to form hydroxynitriles (cyanohydrins). It is an example
of nucleophilic addition using a cyanide ion nucleophile:
CH3COCH3 + HCN
CH3COH(CN)CH3
In this reaction sodium cyanide is mixed with dilute acids
in aqueous solution to generate HCN which immediately reacts with
the carbonyl compounds; pH about 5.
Mechanism of Addition
+
nucleophilic
addition
O
H3 C
CH3
O CN
H3C
H3O+
HO CN
H3 C
CH3
CH3
cyanohydrin
CN-
cyanohydrin formation; a new carbon-carbon bond formed
Notice the polar carbonyl compound to be attacked by the nucleophile (CN-)
and the relative ease of breaking the C=O double bond (p-bond) as oxygen
accepts the negative charge.
Remember, aldehydes often produce chiral centres (*),
but normally in equal amounts of optical isomers.
nucleophilic
addition
O
CN-
+
H
CH3
O CN
H
H3O+
CH3
HO CN
H * CH3
cyanohydrin
cyanohydrin formation; chiral centre generated
Use of 2,4-Dinitrophenylhydrazine (2,4-DNPH)
A method of identification of a carbonyl compound is to make a coloured
crystalline solid derivative. They are easily prepared by mixing the carbonyl
compound with an acid solution of 2,4-DNPH in methanol.
R
H
H
N N
R
C O
+
R'
NO2
H+
H
C
H
N N
NO2
+
R'
NO2
R, R' = alkyl
2,4-DNPH
H2O
NO2
an orange-coloured hydrazone formed
Different carbonyl compounds (R, R’ differ in number of carbon/hydrogen
atoms) have different melting points. This is a nucleophilic addition then
elimination (of water) type of reaction to give the hydrazone.
To distinguish between an aldehyde and ketone
1. Fehling’s solutions
Fehling’s solution A (containing Cu2+) and Fehlings solution B (containing an
alkali and a complexing agent) are warmed with the aldehyde to oxidize it to
the carboxylic acid; meanwhile the Cu2+ is reduced to copper (I) oxide Cu2O
seen as an insoluble red precipitate. There’s no reaction with ketones.
NaOH(aq), methanol
RCHO +
Cu2+(aq)
RCOOH + Cu2O(s)
complexing agent
Silver mirror test
Aldehydes are oxidized by Ag+ ions in aqueous ammonia (Tollen’s reagent);
the aldehyde is oxidized to the carboxylic acid and silver reduced to metallic
silver deposited as a silver mirror around the flask. The mixture is warmed.
NH3(aq)
RCHO +
Ag+(aq)
RCOOH + Ag(s)
Silver mirror
(a) Name and draw the full structures of one ketone A and one aldehyde B,
each with formula C5H10O.
(b) Describe a simple chemical teast which would enable you to differentiate
between samples of A and B. State the observations you would expect to make
for each sample and explain the chemistry involved.
(c) For the reaction of your aldehyde B with HCN, give:
(i) the equation
(ii) the mechanism.