Geometric Isomers Examples

Geometric Isomers Examples

---

 

http://en.wikipedia.org/wiki/Fumaric_acid

Fumaric acid

Fumaric acid or trans-butenedioic acid is the chemical compound with the formula HO₂CCH=CHCO₂H. This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid. In fumaric acid the carboxylic acid groups are trans (E) and in maleic acid they are cis (Z). Fumaric acid has a fruit-like taste. The salts and esters are known as fumarates. As a food additive, it is used as an acidity regulator and is denoted by the E number E297.

Biology

Fumaric acid is found in fumitory (Fumaria officinalis), bolete mushrooms (specifically Boletus fomentarius var. pseudo-igniarius), lichen, and Iceland moss.

Fumarate is an intermediate in the citric acid cycle used by cells to produce energy in the form of adenosine triphosphate (ATP) from food. It is formed by the oxidation of succinate by the enzyme succinate dehydrogenase. Fumarate is then converted by the enzyme fumarase to malate. Human skin naturally produces fumaric acid when exposed to sunlight.^{[citation needed]}

Fumarate is also a product of the urea cycle.

 

Medicine

Fumaric acid esters are used to treat psoriasis, as it has been suggested that the condition is caused by an impairment of fumaric acid production in the skin.^{[1] [2]}A starting dose is 60-105 mg daily, which may be gradually increased to as much as 1,290 mg per day. Side effects include kidney or gastrointestinal disorders, as well as skin flushing; these are mainly caused by excess intake. Decreased white blood cell (WBC) counts have been reported with prolonged use.

A fumaric acid ester is currently under investigation for treatment of multiple sclerosis.

Food

Fumaric acid is a food acidulent used since 1946. It is non-toxic. It is generally used in beverages and baking powders for which requirements are placed on purity. It is generally used as a substitute for tartaric acid and occasionally in place of citric acid, at a rate of 1.36 g of citric acid to every 0.91 grams of fumaric acid to add sourness, similar to the way malic acid is used. It is also used as a coagulant in stovetop pudding mixes.

Chemistry

Fumaric acid was first prepared from succinic acid.^[3] A traditional synthesis involves oxidation of furfural (from the processing of maize) using chlorate in the presence of a vanadium-based catalyst.^[4] Currently, industrial synthesis of fumaric acid is mostly based on catalytic isomerisation of maleic acid in aqueous solutions at low pH. Maleic acid is accessible in large volumes as a hydrolysis product of maleic anhydride, produced by catalytic oxidation of benzene or butane.^[5]

The chemical properties of fumaric acid can be anticipated from its component functional groups. This weak acid forms a diester, it undergoes additions across the double bond, and it is an excellent dienophile.

Fumaric acid does not combust in a bomb calorimeter under conditions where maleic acid deflagrates smoothly. For teaching experiments designed to measure the difference in energy between the cis- and trans- isomers, a measured quantity of carbon can be ground with the subject compound and the enthalpy of combustion computed by difference.

---

 

http://en.wikipedia.org/wiki/Maleic_acid

Maleic acid

Maleic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Maleic acid is the cis isomer of butenedioic acid, whereas fumaric acid is the trans isomer. It is mainly used as a precursor to fumaric acid, and relative to its parent maleic anhydride, maleic acid has few applications.

 

Physical properties

Maleic acid is a less stable molecule than fumaric acid. The difference in heat of combustion is 22.7 kJ·mol⁻¹. The heat of combustion is -1355 kJ/mole ^[3] Maleic acid is more soluble in water, than fumaric acid. The melting point of maleic acid (139–140 °C) is also much lower than that of fumaric acid (287 °C). Both properties of maleic acid can be explained on account of the intramolecular hydrogen bonding^[4] that takes place in maleic acid at the expense of intermolecular interactions, and that are not possible in fumaric acid for geometric reasons.

 

Production and industrial applications

In industry, maleic acid is derived by hydrolysis of maleic anhydride, the latter being produced by oxidation of benzene or butane.^[5]

 

Production and industrial applications

In industry, maleic acid is derived by hydrolysis of maleic anhydride, the latter being produced by oxidation of benzene or butane.^[5]

 

Isomerization to fumaric acid

The major industrial use of maleic acid is its conversion to fumaric acid. This conversion, an isomerization, is catalysed by a variety of reagents, such as mineral acids and thiourea. Again, the large difference in water solubility makes fumaric acid purification easy.

The isomerization is a popular topic in schools. Maleic acid and fumaric acid do not spontaneously interconvert because rotation around a carbon carbon double bond is not energetically favourable. However, conversion of the cis isomer into the trans isomer is possible by photolysis in the presence of a small amount of bromine ^[6]. Light converts elemental bromine into a bromine radical, which attacks the alkene in a radical addition reaction to a bromo-alkane radical; and now single bond rotation is possible. The bromine radicals recombine and fumaric acid is formed. In another method (used as a classroom demonstration), maleic acid is transformed into fumaric acid through the process of heating the maleic acid in 12 M hydrochloric acid solution. Reversible addition (of H⁺) leads to free rotation about the central C-C bond and formation of the more stable and less soluble fumaric acid.

Maleic acid is an industrial raw material for the production of glyoxylic acid by ozonolysis.^[7]

 

Maleates

The maleate ion is the ionised form of maleic acid. The maleate ion is useful in biochemistry as an inhibitor of transaminase reactions. Maleic acid esters are also called maleates, for instance dimethyl maleate.

---

 

WIEL

 

 

 

 

Tweet