Chapter 2
HYDROXY ALDEHYDES
AND
DICARBONYL COMPOUNDS
Glycolaldehyde
2-Hydroxyacetaldehyde
The simplest hydroxy-aldehyde
Methods of Preparation:
1. Oxidation of glycol by Fenton's reagent (hydrogen
peroxide and ferrous sulphate):
2. Ozonolysis of allyl alcohol:
3. Heating dihydroxymaleic acid, which is
obtained from the oxidation of tartaric acid.
Glycolaldehyde exists in the solid form as the dimmer,
m.p. 96C, but in aqueous solution it exists as the
monomer which forms the stable hydrate,
CH(OH)2CH2OH similar to chloral hydrate(CCl3CH(OH)2)
Reactions:
1-Careful oxidation of glycolaldehyde with bromine
water produces glycolic acid:
2. Glycolaldehyde is a powerful reducing agent, it
reduces ammoniacal silver nitrate and Fehling's
solution at room temperature.
3. With phenylhydrazine it forms the osazone. This
osazone is identical with that formed from glyoxal:
4. Glycolaldehyde undergoes the aldol condensation
in the presence of alkali, with sodium hydroxide
solution a tetrose sugar is formed, and with
sodium carbonate solution, a hexose sugar:
Aldol
Acetaldol, b-Hydroxybutyrladehyde,
3-Hydroxybutanal
Preparation:
Aldol can be prepared via aldol condensation of
acetaldehyde in the presence of NaOH or K2CO3 or
HCl.
When heated, aldol is dehydrated to crotonaldehyde:
Aldol mechanism
Dialdehyde (Glyoxal)
CHO - CHO
The simplest dialdehyde is glyoxal
Preparation:
1. Oxidation of ethanol, ethanal or ethane-1,2-diol
with nitric acid gives glyoxal:
Chemical Reactions
1. It reduces ammoniacal silver nitrate.
2. It reacts with two molecules of hydrogen
cyanide to give the corresponding
cyanohydrine.
3. It reacts with sod. Bisulphate to give the
corresponding addative compound.
4. It undergoes Canizzaro’s reactions to give sod. Glycolate. i. e salt.
The mechanism of this reaction may proceed as follows:
(Canizzaro’s mechanism)
5. It reacts with phenylhydrazine to give the
corresponding di-phenylhydrazone.
6. On reduction it gives glycol.
7. It is oxidized to give oxalic acid.
8. It also combines with o-phenylenediamine to
form quinoxaline (heterocyclic compound).
Diketones
These are classified as α, b, , … diketones according
to the two carbonyl groups are in the 1,2 or 1,3 or 1,4positions, respectively.
b-Diketones
CH3- CO-CH2-CO-CH3
(Pentane-2,4-dione, Acetylacetone)
This is the simplest b-diketone.
Methods of Preparation:
1. By the Claisen condensation between ethyl acetate and
acetone
2. By the condensation of acetic anhydride with
acetone in the presence of boron trifluoride as
catalyst:
3. By the ketonic hydrolysis of acetyl-acetoacetic ester:
Chemical Reactions:
Pentane-2,4-dione is a colourless liquid b.p. 139C /
746 mm.
1. It exhibits tautomerism e.g., it gives a red colour
with ferric chloride:
2- It forms pyrazoles when treated with hydrazine or
its derivatives, e.g., with phenylhydrazine it forms
3,5-dimethyl-1- phenylpyrazole:
3. It condeses with hydroxyl amine to give the
monoxime which on warming undergoes
cyclisation to produce 3,5-dimethylisoxazole:
4. Pentane-2,4-dione is oxidized by selenium
dioxide to pentane-2,3,4-trione: