Editing Conjugated system (section)

== Generalizations and related concepts ==
[[File:Homoconjugation.png|class=skin-invert-image|left|thumb|Homoconjugation weakens the double bond character of the C=O bond, resulting in a lower IR frequency.]]There 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 σ bonds can also delocalize into a low-lying unoccupied orbital of a π system or an unoccupied p orbital. [[Hyperconjugation]] is commonly invoked to explain the stability of alkyl substituted radicals and carbocations. Hyperconjugation is less important for species in which all atoms satisfy the octet rule, but a recent computational study supports hyperconjugation as the origin of the increased stability of alkenes with a higher degree of substitution ([[Zaitsev's rule]]).<ref>{{Cite journal|last1=Braida|first1=Benoit|last2=Prana|first2=Vinca|last3=Hiberty|first3=Philippe C.|date=2009-07-20|title=The Physical Origin of Saytzeff's Rule|journal=Angewandte Chemie International Edition|language=en|volume=48|issue=31|pages=5724–5728|doi=10.1002/anie.200901923|pmid=19562814|issn=1433-7851}}</ref>

''Homoconjugation''<ref name="IUPAC">{{GoldBookRef |title=homoconjugation |file=H02842}}</ref> is an overlap of two π-systems separated by a non-conjugating group, such as CH<sub>2</sub>. Unambiguous examples are comparatively rare in neutral systems, due to a comparatively minor energetic benefit that is easily overridden by a variety of other factors; however, they are common in cationic systems in which a large energetic benefit can be derived from delocalization of positive charge (''see the article on [[homoaromaticity]] for details.'').<ref>Some orbital overlap is possible even between bonds separated by one (or more) CH<sub>2</sub> 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.</ref> Neutral systems generally require constrained geometries favoring interaction to produce significant degrees of homoconjugation.<ref>{{Cite journal|last=Scott|first=L. T.|date=1986-01-01|title=Cyclic homoconjugation in neutral organic molecules|journal=Pure and Applied Chemistry|language=en|volume=58|issue=1|pages=105–110|doi=10.1351/pac198658010105|issn=1365-3075|citeseerx=10.1.1.562.8748| s2cid=98131188 }}</ref> In the example below, the carbonyl stretching frequencies of the IR spectra of the respective compounds demonstrate homoconjugation, or lack thereof, in the neutral ground state molecules.

Due to the partial π character of formally σ bonds in a cyclopropane ring, evidence for transmission of "conjugation" through cyclopropanes has also been obtained.<ref>{{Cite journal|last1=Stewart|first1=John Mathews|last2=Pagenkopf|first2=Gordon K.|date=January 1969|title=Transmission of conjugation by the cyclopropane ring|journal=The Journal of Organic Chemistry|language=EN|volume=34|issue=1|pages=7–11|doi=10.1021/jo00838a003|issn=0022-3263}}</ref>

Two appropriately aligned π systems whose ends meet at right angles can engage in spiroconjugation<ref>{{Cite journal|last=Maslak|first=Przemyslaw|date=May 1994|title=Spiroconjugation: An added dimension in the design of organic molecular materials|journal=Advanced Materials|volume=6|issue=5|pages=405–407|doi=10.1002/adma.19940060515|bibcode=1994AdM.....6..405M |issn=0935-9648}}</ref> or in homoconjugation across the spiro atom.  

[[Image:Vinylogous.png|class=skin-invert-image|300px|thumb|Delocalization of negative charge in a generic carboxylate anion, derived from an organic carboxylic acid (cf. [[acetic acid]]), and the corresponding vinylogous carboxylate anion (the "vinylog/vinylogue" of the carboxylate anion), where a [[Vinyl group|vinyl]] group now separates the charged oxygen from the [[carbonyl]] ({{chem2|C\dO}}) group. The validity of the theoretical concept of vinylogy is supported by the [[pKa]] of such vinylogs, which approach that of the analogous carboxylic acid.]]
[[Vinylogy]] is the extension of a functional group through a conjugated organic bonding system, which transmits [[electronic effects]].<ref>''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}}</ref>