Editing Conjugated system (section)

{{Short description|System of connected p-orbitals with delocalized electrons in a molecule}}
[[File:Cinnamaldehyde acsv.svg|class=skin-invert-image|thumb|175px|[[Cinnamaldehyde]] is a naturally-occurring compound that has a conjugated system]]
[[File:1,3-pentadien.svg|class=skin-invert-image|thumb|[[Piperylene|penta-1,3-diene]] is a molecule with a conjugated system]]
[[File:Diazomethane-pi-system.png|class=skin-invert-image|thumb|[[Diazomethane]] has a conjugated pi-system]]

In [[physical organic chemistry]], a '''conjugated system''' is a system of connected [[p-orbital]]s with [[delocalized electron]]s in a [[molecule]], which in general lowers the overall energy of the molecule and increases [[Chemical stability|stability]]. It is [[Resonance (chemistry)|conventionally represented]] as having alternating single and multiple [[covalent bond|bonds]]. [[Lone pair]]s, [[radical (chemistry)|radicals]] or [[carbenium ion]]s may be part of the system, which may be [[Cyclic molecule|cyclic]], acyclic, [[Linear molecular geometry|linear]] or mixed.  The term "conjugated" was coined in 1899 by the German chemist [[Johannes Thiele (chemist)|Johannes Thiele]].<ref>{{cite journal|last1=Thiele|first1=Johannes|title=Zur Kenntnis der ungesättigten Verbindungen|journal=Justus Liebig's Annalen der Chemie|date=1899|volume=306|pages=87–142|url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015026322779;view=1up;seq=473|trans-title=[Contribution] to our knowledge of unsaturated compounds|language=de|doi=10.1002/jlac.18993060107}} On p. 90, Thiele coined the term "conjugated":  {{lang|de|"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".)</ref>

'''Conjugation''' is the [[orbital overlap|overlap]] of one p-orbital with another across an adjacent [[Sigma bond|σ bond]] (in [[transition metal]]s, d-orbitals can be involved).<ref>{{GoldBookRef |title=conjugated system (conjugation) |file=C01267}}</ref>{{efn|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 to π (and not σ) delocalization. Canonical molecular orbitals are fully delocalized, so in a sense, all electrons involved in bonding, including ones making up the σ bonds and lone pairs, are delocalized throughout the molecule. However, while treating π electrons as delocalized yields many useful insights into chemical reactivity, treatment of σ and nonbonding electrons in the same way is generally less profitable, except in cases of multicenter σ-bonding as found in cluster compounds of Li and B. Moreover, the added complexity tends to impede chemical intuition.  Hence, for most [[organic molecule]]s, chemists commonly use a localized orbital model to describe the σ-bonds and lone pairs, while superimposing delocalized molecular orbitals to describe the π-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.}}

A conjugated system has a region of overlapping p-orbitals, bridging the interjacent locations that simple diagrams illustrate as not having a π bond. They allow a delocalization<!-- the link [[delocalized electron]]s is already present, don't link the potentially ambiguous title please --> of [[pi electrons|π electrons]] across all the adjacent aligned p-orbitals.<ref>{{cite book|last=March|first=Jerry|year=1985|title=Advanced Organic Chemistry reactions, mechanisms and structure|edition=3rd|location=New York|publisher=John Wiley & Sons, Inc.|isbn=0-471-85472-7}}</ref>
The π electrons do not belong to a single bond or [[atom]], but rather to a group of atoms.

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 chemistry|organic]] conjugated molecules are 1,3-butadiene, benzene, and [[allylic]] carbocations.<ref>{{cite book |title=Organic Chemistry |publisher=[[Iswar Chandra Vidyasagar College]] |location=Belonia, South Tripura, India |edition=3rd |url=http://icvcollege.edu.in/sites/default/files/Conjugation%2C%20Resonance%2C.pdf |access-date=19 April 2022 |chapter=16 Conjugation, Resonance, and Dienes}}</ref> The largest conjugated systems are found in [[graphene]], [[graphite]], [[conductive polymer]]s and [[carbon nanotube]]s.