Organic Chem Hw plz help
Molecular modeling assignment : Construct models of maleic and fumaric acids. Determine the net dipole for each of the compounds. how does this affect the properties of the compound?
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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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