Tension energy

Under strain energy deviation of the Enthalpiekomponente is Gibbs energy understood a molecule located in a state of tension as compared to the ground state. The concept is a leitmotif in alicyclic hydrocarbon chemistry . It has no quantum chemical basis, but is based on “mechanical” model considerations ( force field calculations ), which were later further developed as molecular mechanics .

history

Historically, the motif hints at the year 1885, when Adolf von Baeyer looked at the cycloalkanes : hydrocarbons whose CCC bond angles (valence angle ) deviate from the angle of a tetrahedron (109 ° 28 ') should be "tense". Since the three-dimensional structure of most molecules ( stereochemistry ) was not recognized at the time, Baeyer assumed that the carbon atoms of monocyclic hydrocarbons would lie in one plane. The energy that has to be expended when changing the "ideal" tetrahedron angle is known today as angular stress or Baeyer stress . However, when considering the angular stress, it should be assumed that an angle of 109 ° 28 'can only be expected for tetrahedral molecules with four identical substituents (e.g. methane , carbon tetrachloride ). In unbranched alkanes and cycloalkanes, however, the ligands on the carbon atom are not identical; two of them are carbon atoms, two are hydrogen atoms. As numerous structural analyzes have shown, “stress-free” hydrocarbons tend to have a valence angle of approx. 112 ° (propane: 112.4 °, cyclohexane: 111.4 °). These values should therefore be used as a starting point when making comparisons of connections.

It was later recognized that there are other causes of stress in organic molecules:

Binding voltage (Engl. Strain bond )

If the distance between the atomic nuclei changes compared to the standard bond length (for CC 153 pm), the bond energy also changes.

Torsional stress ( Pitzer stress ) torsional strain , Pitzer strain

If the torsion angle of 60 °, which is the best for the “staggered” conformation , is changed, the conformational energy changes.

Van der Waals Stress (Nonbonding Interactions)

If the electron clouds of hydrogen atoms in CH bonds come too close, this leads to an increase in the internal energy of the molecule. This phenomenon is known as non-bonded interaction . Since it was mainly observed in medium-sized carbon rings, the term transannular tension ( prelog tension ) was chosen for this case .

Energy shares

Voltage energy (engl. Strain energy ) can be regarded as a sum of these energy components, d. That is, it can be broken down into individual contributions.

{\ displaystyle SE = E_ {B} + E_ {W} + E_ {Tors} + E_ {nb}}

E _B = bond stress , E _W = angular stress, E _Tors = torsional stress, E _nb = non-binding interactions.

The energy quantity given is tension energy in kJ / mol (old: kcal / mol). It manifests itself in the enthalpy of reaction of the hydrocarbons. This is compared to a (hypothetical) standard without tension, i.e. H. you need a reference system . The stress energy (SE) is defined as the difference between the experimentally determined enthalpy of formation and the calculated enthalpy of formation:

{\ displaystyle SE = \ Delta H_ {f} ^ {0} (exp) - \ Delta H_ {f} ^ {0} (ber)}

Several reference systems have been developed over time (see below). Therefore there are different numerical values for voltage energies.

Examples

Cycloalkanes

From the heat of combustion of unbranched, ie “stress-free” alkanes, it can be deduced that the enthalpy of formation changes by minus 20.63 kJ · mol ⁻¹ if the hydrocarbon is lengthened by a methylene group (CH ₂ ). Multiplying this value by the number of carbon atoms in a cycloalkane gives the calculated enthalpy of formation.

**Enthalpies of formation and stress energies (SE) of cycloalkanes** in the gas phase
Cycloalkane	n	ΔH _f⁰ (exp)	ΔH _f⁰ (exp)	ΔH _f⁰ (ber)	SE (Benson)	SE (Benson)	SE (Schleyer)	SE (Schleyer)
(CH ₂ ) _n		kJ mol ⁻¹	kcal mol ⁻¹	kcal mol ⁻¹	kcal mol ⁻¹	kJ mol ⁻¹	kcal mol ⁻¹	kJ mol ⁻¹
Cyclopropane	3	+53.3	12.7	−14.8	27.5	115.2	28.1	117.7
Cyclobutane	4th	+28.4	6.8	−19.7	26.1	110.9	27.3	114.2
Cyclopentane	5	−76.4	−18.3	−24.6	6.3	26.7	7.4	30.9
Cyclohexane	6th	−123.4	−29.5	−29.6	0	0.4	1.3	5.4
Cycloheptane	7th	−118.1	−28.2	−34.5	6.3	26.3	7.7	32.1
Cyclooctane	8th	−124.4	−29.7	−39.4	9.7	40.6	11.3	47.3
Cyclononane	9	−132.8	−31.7	−44.4	12.7	52.8	14.4	60.4
Cyclodecane	10	−154.3	−36.9	−49.3	12.4	52.0	14.4	60.3
Cycloundecane	11	−179.4	−42.9	−54.2	11.3	47.5	13.5	56.7
Cyclododecane	12	−230.2	−55.0	−59.2	4.2	17.3	6.5	27.3
Cyclotridecane	13	−246.4	−58.9	−64.1	5.2	21.8	7.8	32.6
Cyclotetradecane	14th	−239.2	−57.2	−69.0	11.8	49.6	14.6	61.3

The above value can be defined as an increment for a CH ₂ group. Increments for primary (CH ₃ group), tertiary and quaternary carbon atoms (“building blocks”) can also be defined. In this way, systems are obtained that are suitable for calculating enthalpies of formation by summing up group increments or binding increments, or better: estimating them . Klages, Laidler, Benson (see article Benson method ) and Paul von Ragué Schleyer developed frequently used increment systems.

Schleyer preferred the value −21.46 kJ mol ⁻¹ for the increment of the CH ₂ group , which refers to alkanes in the all-anti conformation. Here the most favorable torsion angle for all carbon atoms is 180 °, so that the chain of carbon atoms can be projected into the plane as a zigzag line. The value −20.61 kJ mol ⁻¹ was derived from alkanes which exist as mixtures of conformers, i.e. that is, which contain staggered ( gauche ) conformers in addition to the all-anti conformation. This increases the voltage energies.

After computers had moved into chemistry, enthalpies of formation and stress energies could also be calculated using molecular mechanics models.

Branched alkanes

Some alkanes have significant voltage energies, e.g. B. 2,2,3,3-tetramethylbutane 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 tension; the methyl groups can move into a staggered position (torsion angle 60 °).

Polycyclic hydrocarbons

Many bicycloalkanes are excited, especially the fused cyclobutanes and cyclopropanes; H. Bicyclo [m.2.0] and [m.1.0] alkanes. Among the bicycloalkanes, bicyclo [1.1.0] butane holds the record for strain energy (267.1 kJ · mol ⁻¹ ).

In the second half of the 20th century, organic chemists competed in the synthesis of polycyclic hydrocarbons that would previously have been considered "impossible" because of their tension. The syntheses of cubane , bicyclo [1.1.1] pentane, tetrahedrans, and rotanes were spectacular events . Spiropentane was synthesized as early as 1896.

The tension energy initially describes the energy content of a molecule (hydrocarbon). In terms of chemical reactivity, it is particularly effective in thermolysis ( pyrolysis ), which begin with a homolytic cleavage of CC bonds. Examples are discussed in some articles on single hydrocarbons.

Individual evidence

^ Adolf Bayer: In: Reports of the German Chemical Society. 18 (1885), p. 2269.
↑ Data from: JB Pedley, RD Naylor, SP Kirby, Thermochemical Data of Organic Compounds, 2nd ed., Chapman and Hall, London, New York, 1986. ISBN 0-412-27100-1
↑ Friedrich Klages: About an improvement in the additive calculation of heat of combustion and the calculation of the mesomeric energy from heat of combustion. In: Chem. Ber. 82 (1949), pp. 358-375, doi : 10.1002 / cber.19490820411 .
↑ Keith J. Laidler: A System of Molecular Thermochemistry for Organic Gases and Liquids . In: Canadian Journal of Chemistry . 34, 1956, pp. 626-648, doi : 10.1139 / v56-086 .
↑ Sidney W. Benson: Thermochemical Kinetics. 2nd Edition. Wiley, New York 1976, ISBN 0-471-06781-4 .
↑ P. v. R. Schleyer, JE Williams, KR Blanchard: In: J. Am. Chem. Soc. 92 (1970), p. 2377.
↑ MA Flamm-Ter Meer, H.-D. Beckhaus, C. Rüchardt, Thermolabile Hydrocarbons. XXIX .: Enthalpy of combustion and sublimation enthalpy of tri-tert-butylmethane , Thermochimica Acta, Vol. 10 (1986), pp. 331-338, ISSN 0040-6031 , doi : 10.1016 / 0040-6031 (86) 85059-6 .

literature

Thomas H. Lowry, Kathleen S. Richardson: Mechanisms and Theory in Organic Chemistry . 1st edition. Verlag Chemie, Weinheim 1980, ISBN 3-527-25795-0 , pp. 17-33.
JD Cox, G. Pilcher: Thermochemistry of Organic and Organometallic Compounds. Academic Press, New York 1970, ISBN 0-12-194350-X .
Sidney W. Benson: Thermochemical Kinetics. 2nd Edition. Wiley, New York 1976, ISBN 0-471-06781-4 .
Armin de Meijere: Sport, games, excitement - the chemistry of the small rings. In: Chemistry in Our Time. 16, 13 (1982).

[1] Adolf Bayer: In: Reports of the German Chemical Society. 18 (1885), p. 2269.

[2] Data from: JB Pedley, RD Naylor, SP Kirby, Thermochemical Data of Organic Compounds, 2nd ed., Chapman and Hall, London, New York, 1986. ISBN 0-412-27100-1

[3] Friedrich Klages: About an improvement in the additive calculation of heat of combustion and the calculation of the mesomeric energy from heat of combustion. In: Chem. Ber. 82 (1949), pp. 358-375, doi : 10.1002 / cber.19490820411 .

[4] Keith J. Laidler: A System of Molecular Thermochemistry for Organic Gases and Liquids . In: Canadian Journal of Chemistry . 34, 1956, pp. 626-648, doi : 10.1139 / v56-086 .

[5] Sidney W. Benson: Thermochemical Kinetics. 2nd Edition. Wiley, New York 1976, ISBN 0-471-06781-4 .

[6] P. v. R. Schleyer, JE Williams, KR Blanchard: In: J. Am. Chem. Soc. 92 (1970), p. 2377.

[7] MA Flamm-Ter Meer, H.-D. Beckhaus, C. Rüchardt, Thermolabile Hydrocarbons. XXIX .: Enthalpy of combustion and sublimation enthalpy of tri-tert-butylmethane , Thermochimica Acta, Vol. 10 (1986), pp. 331-338, ISSN 0040-6031 , doi : 10.1016 / 0040-6031 (86) 85059-6 .