Chemical synthesis

Chemical synthesis (chemical combination) is the artificial execution of chemical reactions to obtain one or several products. This occurs by physical and chemical manipulations usually involving one or more reactions. In modern laboratory uses, the process is reproducible and reliable.

A chemical synthesis involves one or more compounds (known as reagents or reactants) that will experience a transformation under certain conditions. Various reaction types can be applied to formulate a desired product. This requires mixing the compounds in a reaction vessel, such as a chemical reactor or a simple round-bottom flask. Many reactions require some form of processing ("work-up") or purification procedure to isolate the final product.

The amount produced by chemical synthesis is known as the reaction yield. Typically, yields are expressed as a mass in grams (in a laboratory setting) or as a percentage of the total theoretical quantity that could be produced based on the limiting reagent. A side reaction is an unwanted chemical reaction that can reduce the desired yield. The word synthesis was used first in a chemical context by the chemist Hermann Kolbe.

Strategies

Many strategies exist in chemical synthesis that are more complicated than simply converting a reactant A to a reaction product B directly. For multistep synthesis, a chemical compound is synthesized by a series of individual chemical reactions, each with its own work-up. For example, a laboratory synthesis of paracetamol can consist of three sequential parts. For cascade reactions, multiple chemical transformations occur within a single reactant, for multi-component reactions as many as 11 different reactants form a single reaction product and for a "telescopic synthesis" one reactant experiences multiple transformations without isolation of intermediates.

Organic synthesis

Main articles: Organic synthesis and biochemistry

Organic synthesis is a special type of chemical synthesis dealing with the synthesis of organic compounds. For the total synthesis of a complex product, multiple procedures in sequence may be required to synthesize the product of interest, needing a lot of time. A purely synthetic chemical synthesis begins with basic lab compounds. A semisynthetic process starts with natural products from plants or animals and then modifies them into new compounds.

Inorganic synthesis

Inorganic synthesis and organometallic synthesis are used to prepare compounds with significant non-organic content. An illustrative example is the preparation of the anti-cancer drug cisplatin from potassium tetrachloroplatinate.

Inorganic Steps

Inorganic synthesis can be incredibly complex, and involve several intermediates and elementary steps. The steps outlined below are a few of the types of steps that can occur during an inorganic synthesis.

Oxidative Additiion

Oxidative Addition involves a metal center and a molecule with at least two atoms; the atoms can be of the same species, but do not necessarily have to be. The bond between the two atoms will break, and 2 new bonds will form between those atoms and the metal center.

The general reaction cheme for oxidative addition and reductive elimination

This is referred to as "oxidative" because atoms A and B oxidize the metal; that is, the oxidation state of the metal is +2 relative to the oxidation state of the metal before the oxidative addition took place.

Reductive Elimination

Reductive Elimination is the reverse reaction of Oxidative Addition. It invovles two atoms, A and B, bonded to a metal center. Reductive elimination will see atoms A and B form a bond with each other while both losing their bonds with the metal center.

This is referred to as "Reductive" because this reaction reduces the metla center; that is, the metal center's oxidation state will be 2 lower than it was before the reaction took place.

Ligand Substitution

Ligand Substitution occurs when a chemical species attached the a metal center is replaced with a different one.

Associative Substitution

The steps of associative substitution

Associative Substitution sees an incoming ligand Y coordinate to the metal center as the first step. Only after the association is complete, will the leaving ligand X leave the metal center, completing the substution process.

This mechanism tends to occur with complexes that are unsaturated in both ligands and electrons; that is, ligands with <6 ligands and <18 valence electrons.

The first step is generally rate determining, meaning that the reaction rate is second order; the reaction speed dependsd on both the concentration of the metal center and the concentration of the [Y] species.

Dissociative Substitution

Dissociative substution sees the outgoing ligand X leave the metal center before the incoming lingad Y coordinates with the metal center. Only after X completely leaves does the new ligand Y coordinate to the metal center.

This mechanism tends to occur with ligands that are fully staurated; that is, complexes with 6 ligands and 18 valence electrons.

The rate determing step tends to be the dissociation step. Thus, the rate tends to be first order; the reaction rate depends on the concentration of the metal center, and is independant of both the concentration and identity of ligand Y.

See also

• Beilstein database
• Biosynthesis
• Chemical engineering
• Click chemistry
• Electrosynthesis
• Methods in Organic Synthesis
• Organic synthesis
• Peptide synthesis
• Total synthesis
• Automated synthesis

External links

• The Organic Synthesis Archive
• Natural product syntheses