Conjugation (chemistry)

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Comparison of two structurally isomeric ketones with the empirical formula C 6 H 8 O: The double bond system marked blue is cross-conjugated , the double bond system marked green is conjugated but not cross-conjugated.

In chemistry, conjugation is understood as the alternating overlap of a π bond (π = Pi ) with different σ bonds between two sp 2 - hybridized (carbon) atoms or with further π bonds. In the first case (this corresponds to conjugated radicals , carbocations and carbanions ) the atomic chain consists of an odd number of atoms or p orbitals, whereas in conjugated double bonds it consists of an even number of atoms or p orbitals. The molecular orbitals resulting from the overlap result from the concept of MO theory . Conjugation leads to π systems with delocalized electrons. The term mesomerism is closely related . In cyclic, planar, conjugated systems, aromaticity can occur.

Effects on reactivity and structure

Radicals, carbocations and carbanions

All three species are stabilized by conjugation. The reason is that the delocalization of the electrons across several atoms increases the range of their possible location. Since, according to the particle in box model, the energy of a particle is inversely proportional to the square of the box size, the energy of the particles is also lower here. Conjugated radicals, carbocations and carbanions are thermodynamically more stable than non-conjugated ones due to this effect. This stabilization is known as conjugation energy . According to the Bell-Evans-Polanyi principle , conjugate intermediates in reactions arise faster in comparison.

Since the p orbitals must be aligned in parallel for an overlap, all the substituents on the sp 2 -hybridized atoms are in one plane.

Polyenes

Conjugated polyenes , ie polyenes in which the individual double bonds are only separated by a CC single bond, also benefit from the conjugation energy. Since they are accordingly thermodynamically more stable than their non-conjugated analogues, analogous reactions take place, such as hydrogenation , the addition of HHal, Hal 2 , H 2 O, or the like, or the reaction with hydroperoxides to form epoxides - again according to Bell-Evans -Polanyi - slower.

The bond between two carbon atoms separated by a single bond in a conjugated system is 148 pm, shorter than a normal CC single bond (154 pm). This is due to two effects: As sp 2 -hybridized carbon atoms, the binding partners are more electronegative than sp 3 -hybridized carbon atoms. They attract the binding electron pair more strongly, which causes a shortening. The second is conjugation: the π orbitals can overlap. This is what is known as the partial double bond character . It is also expressed in an increased barrier to rotation around this single bond (this effect also occurs, for example, in the CN single bond in amides ).

With conjugated dienes, in connection with the Woodward-Fieser rules for calculating the UV absorption maximum, it is important to distinguish whether the two double bonds are part of a ring (homoannular) or are distributed over two rings (heteroannular).

Allyl and benzyl halides

Allyl or benzyl derivative-substituted leaving groups react more quickly with nucleophiles according to the S N 2 mechanism, since orbital interactions occur in the transition state, which stabilize the charge through delocalization.

Considerations with the VB theory

Fig. 1 : Heterolysis of two substituted propene derivatives to form an identical carbocation

According to the rules of mesomerism , the charge or the individual electron can be distributed ( delocalized ) over several atoms . The structures that can be formulated are limiting formulas of one and the same compound. The heterolysis of the C-Cl bond of compounds A and B in Fig. 1 results in identical cations. Since the distribution of charge or electron deficiency centers is generally energetically favored, this explains the increased stability of the conjugated species.

Within the framework of valence structure theory ( valence bond or VB theory), the wave function is written as a linear combination of chemically interpretable structures and thus forms a quantum mechanical description of the mesomerism concept. Using modern quantum chemical VB programs, the weights of the various resonance structures can also be calculated (ie what proportion a resonance structure has in the overall wave function) or the intrinsic energies of the hypothetical individual mesomeric structures. The concept of resonance also allows the reactivity and structural properties of conjugated double bonds to be explained and understood using the VB theory. For a complete description of the wave function - depending on the electronic structure of the molecules - zwitterionic or biradical structures can also play an important role. So-called valence bond diagrams can also be used to describe the chemical reactivity.

Considerations with the MO theory

Fig. 2 : MO diagram for the propenyl cation (1)
Fig. 3 : MO diagram for the propenyl cation (2)

In MO theory, n atomic orbitals are combined to form n molecular orbitals (LCAO method). An odd number of p-AOs to be combined results in (n-1) / 2 bonding, (n-1) / 2 antibonding and one non-bonding MO. This is demonstrated here using the example of the propenyl cation ( Fig. 2 ). All three p-AOs have the same energy here. These can be combined in three different ways, so that three molecular orbitals result. The electrons populate the MO with the lowest energy according to the occupation principle.

An alternative with the same result is to combine two atomic orbitals into one π and one π * orbital and combine these two with the remaining p atomic orbitals to form the three molecular orbitals ( Fig. 3 ). In this approach, the conjugation energy can be seen, which stabilizes the molecule in comparison to an isolated double bond and an isolated electron sextet on carbon: it is twice (since two electrons) the energetic distance of the π C = C orbital to the π MO of the molecule .

See also

Individual evidence

  1. ↑ Joseph B. Lambert, Scott Gronert, Herbert F. Shurvell, David A. Lightner: Spectroscopy - structure clarification in organic chemistry. 2nd Edition. Pearson Germany, Munich 2012, ISBN 978-3-86894-146-3 , pp. 646-653.
  2. ↑ S. Shaik, PC Hiberty: Valence Bond theory, its History, Fundamentals and Applications. A primer. In: Reviews of Computational Chemistry. Volume 20, 2004, Chapter 1.
  3. ^ A b Sason S. Shaik, Philippe C. Hiberty: A Chemist's Guide to Valence Bond Theory . John Wiley & Sons, 2007, ISBN 978-0-470-19258-0 .
  4. ^ L. Song, Y. Mo, Q. Zhang, W. Wu: XMVB: A program for ab initio nonorthogonal valence bond computations. In: Journal of Computational Chemistry. Volume 26, No. 5, 2005, p. 514.
  5. ^ J. Li, R. McWeeny: VB2000: Pushing Valence Bond Theory to new limits. In: International Journal of Quantum Chemistry. Volume 89, No. 4, 2002, p. 208.
  6. ↑ In older sources there is sometimes the statement that the VB theory cannot explain these and other concepts. However, this statement is based on a misunderstanding of the theory. Chapter 5 (Are the "Failures" of Valence Bond Theory Real?) Of the book "A Chemist's Guide to Valence Bond Theory" by Shaik ud Hiberty deals with this issue.
  7. ↑ Benoit Braida, Christof Walter, Bernd Engels, Philippe C. Hiberty: A Clear Correlation between the Diradical Character of 1,3-Dipoles and Their Reactivity toward Ethylene or Acetylene . In: Journal of the American Chemical Society . tape 132 , no. 22 , June 9, 2010, p. 7631-7637 , doi : 10.1021 / ja100512d .
  8. Jump up ↑ Sason Shaik, David Danovich, Hui Chen, Chunsen Li, Wenzhen Lai: A tutorial for understanding chemical reactivity through the valence bond approach . In: Chemical Society Reviews . tape 43 , no. 14 , 23 June 2014, ISSN  1460-4744 , p. 4968–4988 , doi : 10.1039 / C4CS00043A ( rsc.org [accessed August 17, 2019]).