Strain (chemistry)

Low voltage energy the deviation of the Enthalpiekomponente the Gibbs energy of a molecule located in a state of tension compared to the ground state is understood. The concept is a guiding principle in the chemistry of alicyclic hydrocarbons. It has no quantum-chemical basis, but based on "mechanical" model considerations ( force field calculations), which as Molecular Mechanics were further developed later.

History

Historically sounds to the subject in 1885, with the consideration of cycloalkanes by Adolf von Baeyer: hydrocarbons, the CCC bond angle ( valence ) from the angle of a tetrahedron (109 ° 28 ') differ, should be " charged". Since the three-dimensional structure of most molecules ( stereochemistry) was not recognized at the time, Baeyer was assumed that the carbon atoms monocyclic hydrocarbons would lie in a plane. The energy that must be expended for a change in the "ideal" tetrahedral angle, is now referred to as voltage or angle Baeyer strain. However, it should be considered when looking at the angle strain from the fact that an angle of 109 ° 28 ' can only be expected in tetrahedral molecules configured with four identical substituents (eg, methane, carbon tetrachloride ). In unbranched alkanes and cycloalkanes, however, the ligands are not identical at the C- atom; two of which are carbon atoms, two H atoms. How many structural analyzes showed have " tension-free " hydrocarbons rather a bond angle of about 112 ° (propane: 112.4 °, cyclohexane: 111.4 °). From these values ​​should therefore be assumed if one hires comparisons of compounds.

Later it was recognized that there are other sources of tension in organic molecules:

• Bond stress (English bond strain )

A change in the distance between the nuclei compared to the standard bond length ( for CC 153 pm ), so also the binding energy changes.

• Torsional strain ( Pitzer strain), Eng. torsional strain, Pitzer strain

If the " staggered " conformation in the lowest torsional angle changed from 60 °, then the conformational changes.

• Van der Waals voltage (non-binding interactions)

Are the " electron clouds " of hydrogen atoms in C-H bonds to close this leads to an increase in the internal energy of the molecule. This phenomenon is referred to as non-binding interaction (English non -bonded interaction). Since it was observed mainly at " medium-sized " carbon rings, the term " transannular strain " ( Prelog voltage) has been chosen for this case.

Energy shares

Strain energy (English strain energy) can be interpreted as the sum of these energy components, ie they can be decomposed into individual contributions.

EB = binding voltage, EW = angle strain, torsional strain = ETors, Enb = nonbonding interactions.

Energy quantity as strain energy in kcal / mol or kJ / mol. It manifests itself in the heat of combustion - better enthalpy of formation - the hydrocarbons. This is compared with a (hypothetical) standard without voltage, that is it needs a reference system. Thus, the strain energy (SE ) is defined as the difference between experimentally determined and calculated enthalpy of formation:

In the course of time, several reference systems have been developed ( see below). Therefore, there are different numerical values ​​for strain energy.

Examples

Cycloalkanes

From the heats of combustion of unbranched, ie " stress-free " alkanes could be deduced that the formation enthalpy changes by minus 4.93 kcal / mol ( 20.63 kJ mol -1), if one extends the hydrocarbon to a methylene group ( CH2). Multiplying this value by the number of carbon atoms in a cycloalkane, the result is the " calculated " heat of formation.

The above value can be defined as an increment of one CH2 group. Similarly, let increments for primary ( CH3 group ), define tertiary and quaternary carbon atoms ("modules "). Thus, for systems that are appropriate to calculate by summing Gruppeninkrementen or Bindungsinkrementen of formation, better estimate. Klages, Laidler, Benson ( see Article Benson method) and Paul of Ragué Schleyer have developed Inkrementsysteme frequently used.

Schleyer preferred value for the increment of the CH2 group -5.13 kcal / mol ( -21.46 kJ mol -1), which refers to alkanes in the all-anti conformation. This is for all the C atoms in front of the lowest torsional angle 180 °, so that the chain of carbon atoms can be projected as a zigzag line in the plane. The value of -4.93 kcal / mol has been derived from alkanes, which are present as Konformerengemische, ie in addition to the all-anti conformation still staggered ( gauche ) conformers included. The strain energies are thus higher.

After the chemistry computers were confiscated, of formation and strain energies could be calculated by the molecular mechanics of the models.

Branched alkanes

Some alkanes have significant strain energy, for example, 2,2,3,3 -tetramethyl butane and " Tris- tert- butylmethane " ( systematic name: 3 - (1,1- dimethylethyl ) -2,2,4,4 - tetramethylpentane ), ( SE = 37.1 kcal mol -1). In contrast, 2,3- dimethylbutane is without voltage; the methyl groups can dodge in a staggered position ( torsion angle 60 °).

Polycyclic hydrocarbons

Many bicycloalkanes are curious, especially the fused cyclobutanes and cyclopropanes, ie Bicyclo [ m.2.0 ] - and [ m.1.0 ] alkanes. Among the bicycloalkanes bicyclo holds [ 1.1.0 ] butane to record strain energy ( 63.9 kcal / mol).

In the second half of the 20th century competed organic chemist in the synthesis of polycyclic hydrocarbons that could have been held earlier because of their voltage for "impossible". Spectacular events were the synthesis of cubane, bicyclo [1.1.1 ] pentane, tetrahedranes and Rotanen. Spiropentane was synthesized as early as 1896.

The strain energy initially describes the " energy content " of a molecule ( hydrocarbon ). In the chemical reactivity it is especially effective in thermolysis ( pyrolysis ), which begin with a homolytic cleavage of carbon-carbon bonds. Examples are discussed in some articles about individual hydrocarbons.