Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

CONDENSATION 219
a-acrose (dl-iiuctose). In such condensations several molecules combine
with the formation of new carbon-carbon linkages. Recall the part
played by formaldehyde in the assimilation of carbon dioxide.
The so-called aldol condensation, which all aldehydes of the formula
*R
j£>CH.CHO undergo under the influence of dilute alkalis or acids,
also involves the formation of new carbon-carbon linkages. The hydrogen
atom in the a-position possesses mobility induced by the adjacent
CO-group, and this mobilised hydrogen atom combines with the likewise
very reactive C=O-group of a second molecule :
H 4, OH
O=CH.CH2 + O=CH.CH3 —> O=CH.CH2.CH.CH3.
V___^' Aldol
The aldols are ^-hydroxyaldehydes and, like all ^-hydroxycarbonylcompounds,
they easily lose water and become converted into a-/3unsaturated
aldehydes. From aldol as starting material a technical
route to butadiene is provided and possibly, in the future, to a synthetic
rubber.
Experiment 11.—A few drops of acetaldehyde, dissolved in
about 2 c.c. of water, are heated in a test tube with 0-5 c.c. of dilute
sodium hydroxide solution. A yellow colour develops and the
acetaldehyde is converted by way of aldol into crotonaldehyde, which
can be recognised in the boiling solution by its pungent odour. If
acetaldehyde is heated with concentrated alkali solution yellow
aldehyde resin is precipitated as a result of further condensation.
The brown colour which develops in solutions of ethoxides and of
potassium hydroxide in ethyl alcohol is to be attributed to the formation
of similar substances, following oxidation of the alcohol.
The benzoin reaction and the reaction of Cannizzaro, which are discussed
later, likewise take place because of the tendency of the aldehydes
to undergo condensation. The specific catalyst determines in each case
the particular way along which the condensation will proceed.
Experiment 12. Schardinger's Reaction.—Of two 25 c.c. portions
of fresh milk one is boiled for a short time and cooled; then
1 c.c. of the formaldehyde (prepared in an earlier experiment) and a
few drops of an aqueous solution of methylene blue are added to each
portion. When the two samples are now warmed to about 50° the
dye in the unboiled milk is very rapidly decolorised and the same
happens to further amounts added. In the boiled milk the colour
remains unaltered.