In theoretical chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases stability. It is conventionally represented as having alternating single and multiple bonds. Lone pairs, radicals or carbenium ions may be part of the system, which may be cyclic, acyclic, linear or mixed. The term "conjugated" was coined in 1899 by the German chemist Johannes Thiele.[1]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/3/37/Cinnamaldehyde_acsv.svg/175px-Cinnamaldehyde_acsv.svg.png)
![](https://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/1%2C3-pentadien.svg/220px-1%2C3-pentadien.svg.png)
![](https://upload.wikimedia.org/wikipedia/commons/thumb/1/12/Diazomethane-pi-system.png/220px-Diazomethane-pi-system.png)
Conjugation is the overlap of one p-orbital with another across an adjacent
A conjugated system has a region of overlapping p-orbitals, bridging the interjacent locations that simple diagrams illustrate as not having a
Molecules containing conjugated systems of orbitals and electrons are called conjugated molecules, which have overlapping p orbitals on three or more atoms. Some simple organic conjugated molecules are 1,3-butadiene, benzene, and allylic carbocations.[4] The largest conjugated systems are found in graphene, graphite, conductive polymers and carbon nanotubes.
Chemical bonding in conjugated systems
editConjugation is possible by means of alternating single and double bonds in which each atom supplies a p orbital perpendicular to the plane of the molecule. However, that is not the only way for conjugation to take place. As long as each contiguous atom in a chain has an available p orbital, the system can be considered conjugated. For example, furan is a five-membered ring with two alternating double bonds flanking an oxygen. The oxygen has two lone pairs, one of which occupies a p orbital perpendicular to the ring on that position, thereby maintaining the conjugation of that five-membered ring by overlap with the perpendicular p orbital on each of the adjacent carbon atoms. The other lone pair remains in plane and does not participate in conjugation.
In general, any sp2 or sp-hybridized carbon or heteroatom, including ones bearing an empty orbital or lone pair orbital, can participate in conjugated systems, though lone pairs do not always participate in a conjugated system. For example, in pyridine, the nitrogen atom already participates in the conjugated system through a formal double bond with an adjacent carbon, so the lone pair remains in the plane of the ring in an sp2 hybrid orbital and does not participate in the conjugation. A requirement for conjugation is orbital overlap; thus, the conjugated system must be planar (or nearly so). As a consequence, lone pairs which do participate in conjugated systems will occupy orbitals of pure p character instead of spn hybrid orbitals typical for nonconjugated lone pairs.
A common model for the treatment of conjugated molecules is a composite valence bond / Hückel molecular orbital theory (VB/HMOT) treatment, in which the
Sigma (
Pi (
This simple model for chemical bonding is successful for the description of most normal-valence molecules consisting of only s- and p-block elements, although systems that involve electron-deficient bonding, including nonclassical carbocations, lithium and boron clusters, and hypervalent centers require significant modifications in which
Electrons in conjugated
n, to emphasize this behavior. For example, the delocalized
3 and
6 systems, respectively (see the article on three-center four-electron bonding). It is important to recognize that, generally speaking, these multi-center bonds correspond to the occupation of several molecular orbitals (MOs) with varying degrees of bonding or non-bonding character (filling of orbitals with antibonding character is uncommon). Each one is occupied by one or two electrons in accordance with the Aufbau principle and Hund's rule. Cartoons showing overlapping p orbitals, like the one for benzene below, show the basis p atomic orbitals before they are combined to form molecular orbitals. In compliance with the Pauli exclusion principle, overlapping p orbitals do not result in the formation of one large MO containing more than two electrons.
Hückel MO theory is commonly used approach to obtain a zeroth order picture of delocalized
Stabilization energy
editThe quantitative estimation of stabilization from conjugation is notoriously contentious and depends on the implicit assumptions that are made when comparing reference systems or reactions. The energy of stabilization is known as the resonance energy when formally defined as the difference in energy between the real chemical species and the hypothetical species featuring localized
Generalizations and related concepts
editThere are also other types of interactions that generalize the idea of interacting p orbitals in a conjugated system. The concept of hyperconjugation holds that certain
Homoconjugation[12] is an overlap of two
Due to the partial
Two appropriately aligned
Vinylogy is the extension of a functional group through a conjugated organic bonding system, which transmits electronic effects.[17]
Conjugated cyclic compounds
editCyclic compounds can be partly or completely conjugated. Annulenes, completely conjugated monocyclic hydrocarbons, may be aromatic, nonaromatic or antiaromatic.
Aromatic compounds
editCompounds that have a monocyclic, planar conjugated system containing (4n + 2)
Nonaromatic and antiaromatic compounds
editNot all compounds with alternating double and single bonds are aromatic. Cyclooctatetraene, for example, possesses alternating single and double bonds. The molecule typically adopts a "tub" conformation. Because the p orbitals of the molecule do not align themselves well in this non-planar molecule, the
In pigments
editIn a conjugated pi-system, electrons are able to capture certain photons as the electrons resonate along a certain distance of p-orbitals - similar to how a radio antenna detects photons along its length. Typically, the more conjugated (longer) the pi-system is, the longer the wavelength of photon can be captured. Compounds whose molecules contain a sufficient number of conjugated bonds can absorb light in the visible region, and therefore appear colorful to the eye, usually appearing yellow or red.[20]
Many dyes make use of conjugated electron systems to absorb visible light, giving rise to strong colors. For example, the long conjugated hydrocarbon chain in beta-carotene leads to its strong orange color. When an electron in the system absorbs a photon of light of the right wavelength, it can be promoted to a higher energy level. A simple model of the energy levels is provided by the quantum-mechanical problem of a one-dimensional particle in a box of length L, representing the movement of a
Since the box length L increases approximately linearly with the number of C=C bonds n, this means that the energy
Many electronic transitions in conjugated
This absorption of light in the ultraviolet to visible spectrum can be quantified using ultraviolet–visible spectroscopy, and forms the basis for the entire field of photochemistry.
Conjugated systems that are widely used for synthetic pigments and dyes are diazo and azo compounds and phthalocyanine compounds.
Phthalocyanine compounds
editConjugated systems not only have low energy excitations in the visible spectral region but they also accept or donate electrons easily. Phthalocyanines, which, like Phthalocyanine Blue BN and Phthalocyanine Green G, often contain a transition metal ion, exchange an electron with the complexed transition metal ion that easily changes its oxidation state. Pigments and dyes like these are charge-transfer complexes.
Porphyrins and similar compounds
editPorphyrins have conjugated molecular ring systems (macrocycles) that appear in many enzymes of biological systems. As a ligand, porphyrin forms numerous complexes with metallic ions like iron in hemoglobin that colors blood red. Hemoglobin transports oxygen to the cells of our bodies. Porphyrin–metal complexes often have strong colors. A similar molecular structural ring unit called chlorin is similarly complexed with magnesium instead of iron when forming part of the most common forms of chlorophyll molecules, giving them a green color. Another similar macrocycle unit is corrin, which complexes with cobalt when forming part of cobalamin molecules, constituting Vitamin B12, which is intensely red. The corrin unit has six conjugated double bonds but is not conjugated all the way around its macrocycle ring.
Heme group of hemoglobin | The chlorin section of the chlorophyll a molecule. The green box shows a group that varies between chlorophyll types. | Cobalamin structure includes a corrin macrocycle. |
Chromophores
editConjugated systems form the basis of chromophores, which are light-absorbing parts of a molecule that can cause a compound to be colored. Such chromophores are often present in various organic compounds and sometimes present in polymers that are colored or glow in the dark. Chromophores often consist of a series of conjugated bonds and/or ring systems, commonly aromatic, which can include C–C, C=C, C=O, or N=N bonds.
Conjugated chromophores are found in many organic compounds including azo dyes (also artificial food additives), compounds in fruits and vegetables (lycopene and anthocyanidins), photoreceptors of the eye, and some pharmaceutical compounds such as the following:
Conjugated polymer nanoparticles (PDots) are assembled from hydrophobic fluorescent conjugated polymers, along with amphiphilic polymers to provide water solubility. Pdots are important labels for single-molecule fluorescence microscopy, based on high brightness, lack of blinking or dark fraction, and slow photobleaching.[24] [25]
See also
editNotes
edit- ^ For the purposes of this article, we are primarily concerned with delocalized orbitals with
π -symmetry. This is in line with the typical usage of 'conjugated system' to refer toπ (and notσ ) delocalization. Canonical molecular orbitals are fully delocalized, so in a sense, all electrons involved in bonding, including ones making up theσ bonds and lone pairs, are delocalized throughout the molecule. However, while treatingπ electrons as delocalized yields many useful insights into chemical reactivity, treatment ofσ and nonbonding electrons in the same way is generally less profitable, except in cases of multicenterσ -bonding as found in cluster compounds of Li and B. Moreover, the added complexity tends to impede chemical intuition. Hence, for most organic molecules, chemists commonly use a localized orbital model to describe theσ -bonds and lone pairs, while superimposing delocalized molecular orbitals to describe theπ -bonding. This view has the added advantage that there is a clear correspondence between the Lewis structure of a molecule and the orbitals used to describe its bonding.
References
edit- ^ Thiele, Johannes (1899). "Zur Kenntnis der ungesättigten Verbindungen" [[Contribution] to our knowledge of unsaturated compounds]. Justus Liebig's Annalen der Chemie (in German). 306: 87–142. doi:10.1002/jlac.18993060107. On p. 90, Thiele coined the term "conjugated": "Ein solches System benachbarter Doppelbindungen mit ausgeglichenen inneren Partialvalenzen sei als 'conjugirt' bezeichnet." (Such a system of adjacent double bonds with equalized inner partial valences shall be termed "conjugated".)
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "conjugated system (conjugation)". doi:10.1351/goldbook.C01267
- ^ March, Jerry (1985). Advanced Organic Chemistry reactions, mechanisms and structure (3rd ed.). New York: John Wiley & Sons, Inc. ISBN 0-471-85472-7.
- ^ "16 Conjugation, Resonance, and Dienes". Organic Chemistry (PDF) (3rd ed.). Belonia, South Tripura, India: Iswar Chandra Vidyasagar College. Retrieved 19 April 2022.
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "resonance energy". doi:10.1351/goldbook.R05333
- ^ Feller, David; Craig, Norman C. (2009-02-05). "High Level ab Initio Energies and Structures for the Rotamers of 1,3-Butadiene". The Journal of Physical Chemistry A. 113 (8): 1601–1607. Bibcode:2009JPCA..113.1601F. doi:10.1021/jp8095709. PMID 19199679.
- ^ Carey, Francis A.; Guiliano, Robert M. (2013-01-07). Organic chemistry (Ninth ed.). New York, NY. ISBN 9780073402741. OCLC 822971422.
{{cite book}}
: CS1 maint: location missing publisher (link) - ^ Gobbi, Alberto; Frenking, Gernot (1994-10-01). "Resonance Stabilization in Allyl Cation, Radical, and Anion". Journal of the American Chemical Society. 116 (20): 9275–9286. doi:10.1021/ja00099a052. ISSN 0002-7863.
- ^ Barbour, Josiah B.; Karty, Joel M. (2004-01-14). "Resonance Energies of the Allyl Cation and Allyl Anion: Contribution by Resonance and Inductive Effects toward the Acidity and Hydride Abstraction Enthalpy of Propene". The Journal of Organic Chemistry. 69 (3): 648–654. doi:10.1021/jo035189m. PMID 14750787.
- ^ Cotton, Frank Albert (1990). Chemical applications of group theory (3rd ed.). New York: Wiley. ISBN 978-0471510949. OCLC 19975337.
- ^ Braida, Benoit; Prana, Vinca; Hiberty, Philippe C. (2009-07-20). "The Physical Origin of Saytzeff's Rule". Angewandte Chemie International Edition. 48 (31): 5724–5728. doi:10.1002/anie.200901923. ISSN 1433-7851. PMID 19562814.
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "homoconjugation". doi:10.1351/goldbook.H02842
- ^ Some orbital overlap is possible even between bonds separated by one (or more) CH2 because the bonding electrons occupy orbitals which are quantum-mechanical functions and extend indefinitely in space. Macroscopic drawings and models with sharp boundaries are misleading because they do not show this aspect.
- ^ Scott, L. T. (1986-01-01). "Cyclic homoconjugation in neutral organic molecules". Pure and Applied Chemistry. 58 (1): 105–110. CiteSeerX 10.1.1.562.8748. doi:10.1351/pac198658010105. ISSN 1365-3075. S2CID 98131188.
- ^ Stewart, John Mathews; Pagenkopf, Gordon K. (January 1969). "Transmission of conjugation by the cyclopropane ring". The Journal of Organic Chemistry. 34 (1): 7–11. doi:10.1021/jo00838a003. ISSN 0022-3263.
- ^ Maslak, Przemyslaw (May 1994). "Spiroconjugation: An added dimension in the design of organic molecular materials". Advanced Materials. 6 (5): 405–407. Bibcode:1994AdM.....6..405M. doi:10.1002/adma.19940060515. ISSN 0935-9648.
- ^ The Vinylogous Aldol Reaction: A Valuable, Yet Understated Carbon-Carbon Bond-Forming Maneuver Giovanni Casiraghi, Franca Zanardi, Giovanni Appendino, and Gloria Rassu Chem. Rev. 2000; 100(6) pp 1929 - 1972; (Review) doi:10.1021/cr990247i
- ^ Rashid, Zahid; van Lenthe, Joop H. (March 2011). "Generation of Kekulé valence structures and the corresponding valence bond wave function". Journal of Computational Chemistry. 32 (4): 696–708. doi:10.1002/jcc.21655. ISSN 1096-987X. PMID 20941739. S2CID 16526798.
- ^ While the two Kekulé resonance forms contribute to most (>90%) of the
π bond energy, there are also a number of other minor contributors to the wavefunction in the valence bond treatment, including the three Dewar resonance forms, and even smaller contributions from various ionic and singlet diradical forms. See article by Rashid and van Lenthe for a recent computational treatment. - ^ Lipton, Mark (Jan 31, 2017). "Chapter 1. Electronic Structure and Chemical Bonding". Purdue: Chem 26505: Organic Chemistry I (Lipton) (LibreTexts ed.). Purdue University.
- ^ P. Atkins and J. de Paula Physical Chemistry (8th ed., W.H.Freeman 2006), p.281 ISBN 0-7167-8759-8
- ^ Atkins and de Paula p.398
- ^ Autschbach, Jochen (November 2007). "Why the Particle-in-a-Box Model Works Well for Cyanine Dyes but Not for Conjugated Polyenes". Journal of Chemical Education. 84 (11): 1840. Bibcode:2007JChEd..84.1840A. doi:10.1021/ed084p1840. ISSN 0021-9584.
- ^ Wu C, Hansen SJ, Hou Q, Yu J, Zeigler M, Jin Y, Burnham DR, McNeill JD, Olson JM, Chiu DT (2011). "Design of highly emissive polymer dot bioconjugates for in vivo tumor targeting". Angewandte Chemie. 50 (15): 3430–4. doi:10.1002/anie.201007461. PMC 3095208. PMID 21381164.
- ^ Koner AL, Krndija D, Hou Q, Sherratt DJ, Howarth M (2013). "Hydroxy-terminated conjugated polymer nanoparticles have near-unity bright fraction and reveal cholesterol-dependence of IGF1R nanodomains". ACS Nano. 7 (2): 1137–1144. doi:10.1021/nn3042122. PMC 3584654. PMID 23330847.