Activation (chemistry)

In chemistry, activation is the conversion of a reactant into a state or a chemical compound that allows a certain reaction or type of reaction to take place at a higher rate or yield due to increased reactivity . The possibilities for this are varied and often the subject of intensive research, especially in the development of syntheses that are to be implemented on an industrial scale.

Basics

The activation energy barrier of a chemical reaction with and without a catalyst. The energetically highest position represents the transition state. The activation energy required to reach the transition state is reduced by a catalyst (red line).

In order for a chemical reaction to take place, a certain amount of energy must first be supplied, the so-called activation energy . In the case of spontaneous reactions, the ambient temperature is sufficient as activation energy ; in other reactions, the activation energy barrier can be overcome by increasing the temperature. With an increase in temperature, however, not only does the reaction rate increase; in many chemical processes, side reactions also take place which reduce the yield of the desired product. Therefore, other options are often preferred to a mere temperature increase.

An alternative to supplying thermal energy is irradiation with light ( photochemical activation ).

Catalysts can also be used, the surface of a reaction partner can be changed or the reaction can be carried out via an easily accessible intermediate stage, which in turn can easily be converted to the target molecule.

Use of catalysts

In connection with the use of catalysts , one often speaks of an activation of a substance if, during chemisorption on the catalyst surface, the bonding conditions or the electronic properties of a molecule change in such a way that the activation energy for a reaction is reduced. An example of this is the ammonia synthesis according to the Haber-Bosch process , in which the extremely inert nitrogen can only be converted into ammonia in reasonable yields with hydrogen if the reaction takes place on the surface of an iron oxide mixed catalyst.

Activation through surface modification

By increasing the surface of a substance, its reactivity can be drastically increased. A prominent example of this is Raney nickel , which, due to its porous structure, is used in a large number of laboratory syntheses and industrial processes, mainly in the hydrogenation of unsaturated compounds.

The fine distribution of a substance in small particles can also contribute to its activation. Fine steel wool is easily combustible, but a block of solid steel is not. Fine dusts , for example from flour or wood, can even explode (see dust explosion ).

Chemical activation

Substrates can be chemically activated for certain reactions by converting them into a suitable intermediate compound. In the case of substitution reactions , it depends on the quality of the leaving group whether the reaction takes place or not. If , on the other hand, a hydroxyl group , which is a poor leaving group, is first converted into an ester , a good leaving group, substitutions can easily be made, for example with halides .

A frequently observed step within a reaction mechanism is the protonation of a functional group , which electronically influences the properties of this group or the molecule and thus often makes reactions possible in the first place.

When building peptides , as in the Merrifield synthesis , the free acid groups of the amino acids are first converted into so-called active esters using reagents such as dicyclohexylcarbodiimide (DCC) , which can then be easily coupled with the free amino function of the following amino acid.

In the case of electrophilic aromatic substitution , one speaks of activating substituents when groups already present on the ring system increase the electron density in the aromatic ( + M effect ) and thus facilitate the reaction of an electrophile .

Biochemical activation

In biochemistry , activation is often used to denote the first step in a biochemical reaction sequence that carries out a modification on a comparatively inactive substrate, which sets a reaction cascade in motion. For example, the acetylation by the coenzyme A as acetyl group carrier represents a central reaction step in the fat , carbohydrate and protein metabolism . Acetyl-coenzyme A is therefore also referred to as activated acetic acid . Another example is a phosphoric acid ester bond (and also its variant of the phosphodiester bond ) which is generated by phosphorylation , for example in the case of adenosine triphosphate .

literature

K. Hertwig, L. Martens: Chemical process engineering: calculation, design and operation of chemical reactors , Oldenbourg Wissenschaftsverlag Munich 2007, ISBN 978-3-486-57798-3 , Chapter 4.4: Chemical activation principles in industrial reactors.

Individual evidence

↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 1: A-Cl. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1979, ISBN 3-440-04511-0 , p. 95.

[RÖMPP-1] Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 1: A-Cl. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1979, ISBN 3-440-04511-0 , p. 95.