These are a pair of isomeric dicarboxylic acids, having
molecular formula CO2HCH=CHCO2H. A >C=C< double bond constructs a rigid
frame in which all the bond angles are 120 degrees. Maleic acid is the
cis isomer, i.e one in which the two -CO2H groups are situated in
the same side of the C=C plane while fumaric acid is the trans
isomer i.e where the two -CO2H groups are on opposite sides or across the C=C
plane.
Molecular model boxes have different types of
spheres. Choose the spheres with two closely spaced bond-points as double bonded carbons
(here all four carbons are so), red spheres as oxygens and smallest spheres or smallest
spheres attached to short handles as hydrogens. Now connect the spheres with bonds or
sticks judiciously keeping in mind the cis and
trans orientations.
Net dipole of the
cis isomer(maleic acid) will be higher than the
trans isomer(fumaric acid). Here the most polar part is the
carboxylic group and when the two carboxylic groups are pointed to a particular
direction (cis), the resultant is a sort of reinforcement of the two individual
vectors. But when the two carboxylic groups are placed across the double bond (trans),
the resultant is a sort of subtraction of the two individual vectors. This difference
is, however, not very high. For diethyl maleate the dipole moment is 2.54D whereas for
diethyl fumarate it is 2.38D.
The different geometrical
disposition of two carboxylic groups have profound effect on the physical and chemical
properties of the molecules. Apart from the difference in dipole moment as discussed
above, the cis isomer undergoes dehydration easily to produce its anhydride. owing to
the 'field effect' of other CO2H group, maleic acid is more acidic among these two. Its
solubility is also better. But the thermal stability, and melting point of the
trans isomer is higher.
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