Stereochemistry
Stereochemistry isomerism can be divided into double bonds. The interconversion of one pair of
two classes–geometrical isomerism and optical such isomers is illustrated by Experiment 12-A.
isomerism. Although these categories are not Optical isomers are distinguished by their ability
mutually exclusive, it is not uncommon to find to rotate the plane of plane-polarized light. Com-
compounds that exhibit only one or the other of pounds that have nonsuperimposable mirror
the two types. Most geometrical isomers result images can be expected to exhibit this optical
from cyclic systems or restricted rotation about property.
EXPERIMENT 12-A cis-trans isomerism
Maleic acid and fumaric acid are dicarboxylic reacts much more readily. This, coupled with the
acid isomers that can be shown to have the same the fact that mild hydrolysis of the anhydride leads
carbon skeleton, because they both yield succinic to maleic acid, is evidence for the cis structure of
acid on hydrogenation. Each can be converted to maleic acid and the trans structure for fumaric
the identical anhydride by heat, but maleic acid acid:
EXP.
12-A CIS-TRANS ISOMERISM page
2
The interconversion of the cis-trans isomers is water in a small round-bottomed flask. Add 20
possible under chemical or photochemical con- mL of con. hydrochloric acid, attach a reflux
ditions. Maleic acid is converted to the more condenser, and reflex the mixture gently for 30
stable fumaric acid (7 kcal
lower heat of com- minutes.
bustion) by hydrohalic acids: out of the hot solution. Cool the flask and con-
tents to room temperature. Isolate the solid by
H COOH H COOH vacuum filtration with a Buchner funnel, [and
\ / \ / recrystallize it from from 1 M hydrochloric acid
C HCl C (1 part of conc HCl plus 11 parts of H2O)].
║ ----> ║ The melting point of fumaric acid is 300-2oC
C C (sealed tube). It is difficult to determine, because
/ \ / \ the solid substance sublimes rapidly at tempera-
H COOH HOOC H tures above 200oC. Under such conditions, it is
necessary to make a laboratory measurement in
When an aqueous solution of either isomer is a sealed capillary tube. From a practical view-
irradiated by ultraviolet light, an equilibrium point for this experiment, the melting point may
mixture is formed that contains about 75% of be too high, depending on the thermometers
maleic acid: available.
hv Determine the melting point of the maleic acid
Maleic acid ↔ Fumaric acid used as a starting material in the synthesis, and a
75% 25% mixed (50% maleic acid
starting material/ 50% fumaric acid product) melting point, and
if possible Procedure a melting point of the fumaric acid
product. Report
your observations in writing and place your product
Dissolve 10 g of maleic acid in 10 mL of warm in a properly labeled vial to be turned in.
STEREOCHEMISTRY REPORT Name
________________________
Mass of maleic acid _____________
Mass of fumaric acid recovered _____________
Theoretical yield of fumaric acid: (Show set-up, units, and answer – all calculations)
Percentage yield of fumaric acid: (Show all calculations)
Melting point of maleic acid _______ Melting
point of 50% maleic acid/50% fumaric
acid _________
Literature melting point of maleic acid ________ Reference
_________________________________
Questions
1. Provide a mechanism for the acid-catalyzed conversion of maleic acid to fumaric acid.
2. The following are pK values of maleic acid and fumaric acid:
Maleic acid pK1 2.0 Fumaric acid pK1 3.0
pK2 6.3 pK2 4.5
(a) Give an explanation for the fact that pK1 is smaller for maleic acid than for fumaric acid.
(b) Give an explanation for the fact that pK2 is smaller for fumaric acid than for maleic acid.