Synthesis, Chemical | Research & Encyclopedia Articles

Synthesis, Chemical

Chemical synthesis is the process of preparing inorganic and organic compounds by union of simpler, readily available starting materials through a series of planned reaction and isolation steps. Stoichiometric relationships derived from the balanced chemical reactions of each of the individual steps: a A + b B c C + d D determine the maximum amount of interemediate(s) and/or product(s) (i.e., chemicals C and D) that can be produced from the given starting materials (i.e., chemicals A and B). For example, if one starts with a moles of chemical A plus b moles of chemical B, then one should theoretically make d moles of chemical D, assuming that the above reaction goes to completion. Chemical reactions rarely go to completion, and often some of the starting materials and products are consumed by side reactions.

The efficiency of a chemical reaction is expressed in terms of the percent yield, which is defined as:

Percent yield = 100 x (Product Synthesized/Theoretical Amount)

the ratio of the amount of product actually synthesized divided by the theoretical amount of product that should have been made based upon the balanced chemical reaction and limiting reagent. The ratio is multiplied then by 100 to convert to percent. In the case of a multi-step synthesis the overall percent yield equals the product of the percent yields of the individual steps. One- and two step processes are generally more efficient than multi-step processes. Considerable effort is devoted in the chemical and pharmaceutical industries in designing more efficient synthetic methods. An increase in the percent yield by only a few tenths of a percent can save a company several millions of dollars in manufacturing costs.

Chemical synthesis is generally used whenever: (a) the desired compound cannot be easily obtained by a few-step reaction of readily available compounds; (b) it is not economically feasible to isolate the desired compound from a naturally occurring source; (c) there is an insufficient amount of the desired chemical in naturally occurring sources to meet consumer demand; or (d) the desired compound does not exist naturally. There is no difference between synthetic and natural compounds. Pure synthetic ethanol is identical to ethanol obtained from natural sources. Chemical synthesis can be performed on both the laboratory and industrial scale. Synthesis thus plays an important role in developing new chemicals.

Sulfuric acid, one of the world's most important industrial chemicals, is synthesized by a four-step contact process.Sulfur burns in dry air to form gaseous sulfur dioxide:

S_(solid) + O_2(gas) SO_2(gas)

which is then oxidized to sulfur trioxide in the presence of a vanadium (V) oxide catalyst:

2 SO_2(gas) + O_2(gas) + heat 2 SO_3(gas)

The sulfur trioxide is absorbed in sulfuric acid to form disulfuric acid, H₂S₂O₇

SO_3(gas) + H₂SO₄H₂S ₂O ₇

Dilution with water gives an aqueous sulfuric acid solution:

H₂S₂O₇ + H₂O2 H₂SO_4(aqueous)

Sulfuric acid is used mostly in manufacturing soluble phosphate and ammonium sulfate fertilizers; in petroleum refining; in iron and steel production; in making paints, pigments, dyes and rayon; and for battery acid. Two other very important inorganic compounds that are synthesized commercially are ammonia and nitric acid.

Synthesis of organic reactions requires a thorough knowledge of the reactions that expand carbon atom skeleton systems and that convert one organic functional group into another. Formation of carbon-carbon atom single bonds is the key construction step in many organic reactions. Such reactions require a carbon nucleophile to provide the two electrons for the chemical bond and a carbon electrophile to accept them. Typically, nucleophiles are either carbonanions, e.g., RCH₂:^- as found in an alkyllithium compound or Grignard reagent) or the pi bonds in alkenes or aromatic ring systems. Electrophiles, on the other hand, are electron-deficient carbon atoms like those of carbonyls, e.g., R₂C=O) or carbon atoms which become electron deficient by departure of a negatively-charged leaving group. The carbon nucleophile and carbon electrophile combine to form the desired carbon-carbon bond as illustrated in the following representative unbalanced chemical reactions:

R'₃CLi + RCH₂Cl R'₃CCH₂R

R'₂C=O + R₃Cli R₃CC(OH)R'₂

CN^- + RCH₂Cl NCCH₂R

where R and R' are alkyl chains having molecular formulas of C_nH_2n+1 and C_mH_2m+1, respectively. In each case the carbon atom skeleton is enlarged by at least one carbon atom. Once the desired skeleton system is obtained, is is usually easy to add functional groups or to alter (or remove or replace) existing functional groups using standard oxidation, reduction, dehydrogenation, dehalohydrogenation, and so on methods. The experimental conditions and reagents needed for the different functional group interconversions are wellestablished.

Polymerization is another type of chemical reaction employed in organic synthesis to couple small monomeric units into a much larger chain. The polymerization of ethylene to polyethylene can be represented as:

n H₂C=CH₂ -[-CH₂CH₂-] _n-

The reaction is started by an initiator, usually a free radical R, that bonds to one of the carbon atoms of ethylene. The carbon- carbon double bond is converted into a single bond:

R + CH₂=CH₂ RCH₂CH₂

leaving one unpaired electron on the terminal carbon atom. The free radical species generated during the initiation step then joins with another ethylene molecule to form an even larger radical,

RCH₂CH₂ + H₂C=C₂RC H₂CH₂CH₂CH ₂

which in turn reacts with an adjacent ethylene molecule. The process continues for several hundreds of steps. The propagation finally terminates whenever a molecule is produced that no longer has an unpaired electron. The combination of two radicals:

R(-CH₂) _nCH₂ + CH₂(CH₂-)_nR' R(-CH₂)_n CH₂CH₂(CH₂-) _nR'

would be a possible termination step.Polypropylene and polystyrene are prepared in similar fashion from 1propene and styrene, respectively.

Polymerization can also be achieved through the condensation reactions that couple dissimilar ends of an organic molecule. For example, the condensation of 6aminohexanoic acid,

HOOC(CH₂)₅ NH₂,nHOOC(C H ₂)_nNH₂ + n HOOC(CH₂)_nNH₂ -[-CO(CH₂)₅ -NHC(=O)-(CH₂)₅ NH-]_n- + 2n H₂O

produces a type of nylon. The OH portion of the carboxylic acid end of one molecule reacts with the hydrogen atom of the -NH₂ (amine) group of an adjacent 6-aminohexanoic acid molecule. A water molecule is released during the formation of the amide bond. A second type of nylon (nylon 6,6) can be prepared by reacting adipic acid with 1,6-hexanediamine, or alternatively by reacting adipoyl chloride with 1,6hexanediamine. The latter chemical reaction generates HCl as a by-product, rather than water. There is often more than one synthetic route for preparing a desired chemical. Nylon is used as a clothing fiber in the textile industry. Organic synthesis is responsible for many of the coatings, adhesives, films, resins, polymers, dyes, medicines, pesticides, perfumes, soaps, preservatives, deodorants and the numerous other organic products that a person encounters in his/her everyday life.

Electrolytic cells are used in the commercial production of several important inorganic and organic compounds.Sodium hydroxide is made by the electrolysis of aqueous sodium chloride brine according to the following two half-reactions:

Anode reaction: 2 Cl^- 2 e^- + Cl_2(gas)

Cathode reaction: 2 H₂O + 2 e^- H_2(gas) + 2 OH^-

Overall reaction: 2 Cl^- + 2 H₂O Cl_2(gas) + H_2(gas) + 2 OH^-

Chlorine and hydrogen gases are by- products of the reaction. The anode and cathode must be separated to prevent the chlorine gas from reacting with hydrogen to form hydrochloric acid, and from reacting with the hydroxide ion to form the hypochlorite ion, OCl^-. In commercial processes the anode and cathode compartments are separated by a cation exchange resin that allows only Na⁺ ions pass between the two compartments. A concentrated sodium chloride brine is used to favor oxidation of the chloride ion over oxidation of water. The resulting sodium hydroxide solution is drained from the cathode compartment. Solid sodium hydroxide is obtained by evaporating the water. Sodium hydroxide is used as a base in a large number of industrial processes to neutralize acidic solutions, and is used in the manufacture of soaps, paper and textiles. The chlorine gas that is generated during the electrolysis is recovered, and is used both in waste water treatment and water purification plants, and in the production of specialty chemicals such as pesticides for crop protection, herbicides for weed control, and vinyl chloride (the starting material for polyvinyl chloride production). High-purity copper for making electrical wiring, aluminum for constructing light-weight containers, and adiponitrile for making nylon are also prepared by electrolysis methods.

Synthesis of the heavier transuranic elements, as well as of radioisotopes of the natural elements, is sometimes included under the broader umbrella of chemical synthesis. In 1940 the first of the transuranium elements, Np-239, was synthesized by bombarding a U-238 nuclei with neutrons:

²³⁸_{9 2}U + ¹₀n ²³⁹_{9 2}U

²³⁹_{9 2}U ²³⁹₉₃Np + ⁰_-1e

Uncharged neutrons are effective projectiles for nuclear bombardment because they are not repelled when they approach a positively- charged nuclei.Neutron bombardment generally produces only small changes in atomic number. Larger changes in atomic number can be achieved during nuclear fusion reactions, which involves the formation of a heavier nuclei from two lighter nuclei. Researchers at the Lawrence Berkeley National Laboratory reportedly produced the unnamed element of atomic number 118 (mass number of 293) by bombardment of a Pb-208 target with an intense beam of high-energy Kr-86 ions. The two nuclei fused to form the new element with emission of a neutron. Element-118 subsequently decayed by alpha particle emission to give a second new element, Element-116 (mass number of 289). A joint Russian-American team of scientists reported the discovery of Element- 114 (mass number of 289). The researchers produced the element through fusion of ²⁴⁴Pu and ⁴⁸Ca nuclei.

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