Double bond rule

The empirically established double bond rule states that the elements of the 3rd period of the periodic table should no longer be able to form stable chemical compounds with (pp) π multiple bonds .

description

The double bond rule is based on the observation that elements of the 2nd period have stable compounds with multiple bonds, e.g. B. Dinitrogen N ₂ and dioxygen O ₂ , form. The homologous elements of the 3rd and higher periods, on the other hand, avoid the formation of multiple bonds. Accordingly, compounds such as diphosphorus P ₂ and disulfur S ₂ only exist in the gas phase at high temperatures. Under normal conditions , however, P ₄ or polymeric phosphorus and S _{8 are} present, which only contain σ-bonds.

validity

The validity of the double bond rule is mostly attributed to poorer overlap of the np orbitals (n≥3) with larger atomic cores. The elements of the higher periods therefore preferably form a larger number of σ-bonds instead of π-bonds.

Recently, however, it has been discussed that this justification is generally untenable and that the causes are much more complex. Much more important is probably the kinetic stability of π bonds in the small elementary atoms of the 2nd period and the kinetic instability in the larger atoms of the higher periods. Also, alkenes and alkynes , ie compounds having carbon-carbon double or triple bond, are often thermodynamically unstable with respect to a polymer formation (goes ethylene in the presence of catalysts exothermic in polyethylene over). Small atoms are more difficult to access and distortions that are necessary to bring the reactants closer together use more energy. This means that the activation energies for smaller atoms are usually significantly greater than for larger ones. The small size of the elements of the 2nd period of the periodic table also makes higher coordination numbers difficult , so that structures that contain π bonds and thus have relatively small coordination numbers - since two bonds go to the same neighboring atom - can also be thermodynamically more stable than alternative structures with σ-bonds.

According to new findings, not only the reason for the double bond rule, but also the double bond rule itself can no longer be regarded as unreservedly correct. In the last 25 years, innumerable chemical compounds of heavy main group elements (n≥3) of groups 13 to 17, which contain formal double and triple bonds , have been represented. These compounds include the heavy homologues of the alkenes and alkynes known from organic carbon chemistry . While there is a bond order of 2 for alkenes and a bond order of 3 for alkynes by definition, the heavy analogues often have significantly lower bond orders than would be expected for true double or triple bonds. In the case of multiple bonds between heavier elements (n≥3), one can only speak of formal double and triple bonds. The first example of a triple bond between heavy main group elements (n≥3) was the formal gallium-gallium triple bond in (Trip = ), the description of which is still very controversial in science and which probably has a bond order less than 2. The largest element-element bond orders between heavier main group elements have so far been demonstrated in the compounds that were only structurally characterized in 2004 and in bond orders of about 2.3 to 2.5 with silicon-silicon or sulfur- sulfur bond orders. ${\ displaystyle \ mathrm {Na_ {2} \ left [\ left (Ga (C_ {6} H_ {3} Trip_ {2} -2.6) \ right) _ {2} \ right]}}$ ${\ displaystyle \ mathrm {C_ {6} H_ {2} iPr_ {3} -2,4,6}}$ ${\ displaystyle ((Me_ {3} Si) _ {2} CH) _ {2} ('' i''Pr) SiSi-SiSi ('' i''Pr) (CH (SiMe_ {3}) _ { 2}) _ {2}}$ ${\ displaystyle S_ {2} I_ {4} (MF_ {6}) _ {2}}$ ${\ displaystyle (M = As, Sb)}$

Schmidt's double bond rule

The double bond rule established by O. Schmidt states that the CC bond adjacent to a double bond or a phenyl radical in a substance is strengthened and the subsequent CC bond is weakened.

This has a direct impact on the chemical reactions of the substances, for example in pyrolysis the carbon chain is split at the weaker bond, with the double bond shifting at the same time. Another example can be found in photochemistry. B. saturated aldehydes in carbon monoxide and hydrocarbon.

literature

Philip P. Power: π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements . In: Chemical Reviews . tape 99 , no. 12 , November 8, 1999, p. 3463-3504 , doi : 10.1021 / cr9408989 .

Individual evidence

^ Walther Awe : Examples and contributions from alkaloid chemistry to the knowledge of the double bond rule by O. Schmidt. (Also VI. Communication on Derivatives of Berbin.) . In: Archives of Pharmacy . tape 276 , no. 5 , 1938, pp. 253-271 , doi : 10.1002 / ardp.19382760502 .

[1] Walther Awe : Examples and contributions from alkaloid chemistry to the knowledge of the double bond rule by O. Schmidt. (Also VI. Communication on Derivatives of Berbin.) . In: Archives of Pharmacy . tape 276 , no. 5 , 1938, pp. 253-271 , doi : 10.1002 / ardp.19382760502 .