Selectivity (chemistry)

In chemistry, selectivity describes the phenomenon that in a reaction of several possible reaction products , one is preferably formed. The selectivity can be influenced by the reaction conditions - temperature , pressure , concentrations of reactants and auxiliaries (e.g. catalysts , electrolytes ), ionic strength , stoichiometry , solvents , light , electrochemical potential , reaction time.

Formal definition

The selectivity S indicates what proportion of the total converted starting material i was converted into the desired target product k , taking into account the stoichiometry . It can also be described as the quotient of yield Y and conversion X :

{\ displaystyle {\ begin {aligned} S & = {\ frac {{\ text {amount formed}} (k)} {{\ text {converted amount}} (i)}} \\ & = {\ frac {n_ {k} -n_ {k, 0}} {n_ {i} -n_ {i, 0}}} \ cdot {\ frac {\ nu _ {i}} {\ nu _ {k}}} \\ & = {\ frac {n_ {k} -n_ {k, 0}} {n_ {i, 0} -n_ {i}}} \ cdot {\ frac {| \ nu _ {i} |} {\ nu _ {k}}} \\ & = {\ frac {Y_ {k}} {X_ {i}}} \ end {aligned}}}

Here ν are the stoichiometric numbers of the respective substances, where ν _{i is} negative.

Selectivity is a basic parameter in chemical reaction engineering and has a major influence on the economy of a technical reaction.

Regioselectivity

In a regioselective reaction, certain regions of a molecule are preferentially attacked. For example, the electrophilic second substitution on aromatics takes place regioselectively. Electron donating first substituents bring about a preferred second substitution in the ortho and para positions, while electron withdrawing first substituents induce a second substitution in the meta position.

Chemoselectivity

Conversions in which the reagent in the substrate causes exactly one type of several possible transformations are referred to as chemoselective. For example, a reducing agent in a starting material which contains several carbonyl groups can preferably reduce a certain group, while the remaining carbonyl groups are not reduced. For example, sodium borohydride can be used to chemoselectively reduce a keto group without attacking an ester group .

Heterogeneous Catalysis: Hydrogenation of an alkyne proceeds cis -selectively.

cis / trans selectivity ( E / Z selectivity)

If the cis -alkene is preferentially formed in the synthesis of an alkene , one speaks of a cis -selective reaction. An example of such a reaction is the heterogeneously catalyzed hydrogenation of 2-butyne to cis -2- butene . The homogeneous catalyzed hydrogenation of 2-butyne leads selectively to trans -2-butene. The Wittig reaction of aldehydes or asymmetrical ketones can preferably lead to cis - alkenes .

Stereoselectivity

In stereoselective reactions, one of two or more possible stereoisomers is each formed or converted in preference to the other.

Diastereoselectivity

In the case of diastereoselective reactions, one diastereomer of two or more possible diastereomers is preferentially formed. For example, the reduction (with achiral reducing agents) of the carbonyl group of a chiral unsymmetrical ketone (starting material) can lead diastereoselectively to one of the two possible secondary alcohols. If the chiral unsymmetrical ketone used as starting material is enantiomerically pure , an enantiomeric secondary alcohol is generally preferably formed. If the starting material is a racemate , however , a racemic secondary alcohol is formed under the same reaction conditions with the same diastereoselectivity .

The Diels-Alder reaction proceeds diastereoselectively. In most cases, the formation of the endo product is preferred to the exo product. The formation of endo - dicyclopentadiene is also preferred in the [4 + 2] cycloaddition of two molecules of cyclopentadiene .

Enantioselectivity

In enantioselective syntheses, one of the two possible enantiomers is preferably formed from a prochiral starting material . In the reduction of the carbonyl group of a chiral asymmetrical ketone (starting material) with enantiomerically pure reducing agents or in the presence of suitable enantiomerically pure catalysts , one of the two possible secondary alcohols can preferably be formed enantioselectively. The two possible reaction transition states are diastereomeric to one another.

Thermodynamic and kinetic product

Equilibrium reactions in which one of the products is kinetically and the other thermodynamically preferred are a special case. The formation of the kinetic product requires a lower activation energy and is therefore faster, while the thermodynamic product has a lower energy content and is therefore more stable overall. In such cases, the selectivity can often be controlled via the temperature: At lower temperatures, only the activation energy of the kinetic product can be applied, so that this is the preferred product. At higher temperatures, on the other hand, both activation energies are reached, so that the stability of the product itself is decisive - the thermodynamic product is preferably formed.

Individual evidence

↑ Albert Gossauer: Structure and reactivity of biomolecules . Verlag Helvetica Chimica Acta, Zurich, 2006, p. 28, ISBN 978-3-906390-29-1 .
↑ Sabine Wallbaum and Jürgen Martens : Asymmetric Syntheses with Chiral Oxazaborolidines , In: Tetrahedron: Asymmetry 3 (1992) 1475-1504.