Name: hückel rule and aromatic ring criteria
Uploaded: 2023-04-30T10:31:08.000Z
Duration: 1 min 20 s
Channel: JoVE

Benzol, Cyclobutadien und Cyclooctatetraen werden als Hybrid aus zwei äquivalenten Strukturen dargestellt.
Im Gegensatz zu Benzol sind Cyclobutadien und Cyclooctatetraen jedoch instabil und nicht aromatisch.
Um diese Unterschiede zu erklären, skizzierte Erich Hückel eine Reihe von strukturellen Merkmalen, die der Aromatizität zugrunde liegen, die als Hückelsche Regel oder 4n + 2-Regel bekannt sind.
Die Hückelsche Regel besagt, dass eine aromatische Verbindung zyklisch und planar sein muss, mit einem unhybridisierten p-Orbital auf jedem Kohlenstoff.
Die p-Orbitale müssen sich überlappen, um eine Endlosschleife von (4n + 2) π Elektronen zu bilden, wobei n eine positive ganze Zahl ist.
Benzol erfüllt alle Kriterien der Hückelschen Regel und legt seine Aromatizität fest.
Im Gegensatz dazu nimmt Cyclooctatetraen eine nicht-planare tub-Konformation ohne Überlappung zwischen benachbarten p-Orbitalen an. Außerdem fehlen ihm (4n + 2) π Elektronen und ist daher nicht aromatisch.
In ähnlicher Weise ist Cyclobutadien zyklisch, planar und weist eine kontinuierliche Überlappung von p-Orbitalen auf. Es fehlen jedoch (4n + 2) π Elektronen und es verliert an Aromatizität.

kriterien für aromatizität und die hückel 4n + 2-regel

Criteria for Aromaticity and the H&uuml;ckel 4n + 2 Rule

Cyclobutadien und Cyclooctatetraen sind wie Benzol zyklische Verbindungen mit abwechselnden Einfach- und Doppelbindungen. Ihr chemisches Verhalten unterscheidet sich jedoch vom Benzol, da sie instabil und nicht aromatisch sind. Was sind also die strukturellen Merkmale ungesättigter Verbindungen, die als aromatisch eingestuft werden?

Eric Hückel, ein deutscher Chemiephysiker, leitete erstmals eine Reihe von Strukturmerkmalen für eine als aromatisch einzustufende Verbindung ab. Dies ist heute als Hückel-Regel oder 4n + 2-Regel bekannt. Nach dieser Regel muss eine aromatische Verbindung zyklisch, planar und kontinuierlich konjugiert sein und eine ungerade Anzahl von π-Elektronenpaaren oder 4n + 2 π-Elektronen haben, wobei n eine ganze Zahl ist (n = 0, 1, 2…). Daher sollte eine aromatische Verbindung 2 (n = 0), 6 (n = 1), 10 (n = 2), 14 (n = 3) usw. π-Elektronen oder 1, 3, 5, 7 usw. π-Elektronenpaare aufweisen.

Beispielsweise haben 1,3,5-Hexatrien und Benzol 6 π-Elektronen; selbst dann ist 1,3,5-Hexatrien nicht aromatisch, da sich die p-Orbitale an den beiden terminalen Kohlenstoffatomen nicht überlappen, da diese Kohlensubstanzen nicht durch eine Bindung verbunden sind. Daher müssen Verbindungen zyklisch sein, um aromatisch zu sein.

<img alt="Figure1" src="/files/ftp_upload/12453/12453_PT-Figure_1.svg" style="text-align: center; height: 150px;">

Obwohl es sich um eine achtgliedrige zyklische Verbindung mit durchgehenden vier π-Bindungen handelt, ist Cyclooctatetraen instabil und nicht aromatisch. Dies liegt daran, dass Cyclooctatetraen nichtplanar ist; es nimmt eine Wannenkonformation an. Folglich ist eine effektive Überlappung der p-Orbitale zur Bildung einer geschlossenen Schleife der π-Elektronenwolke nicht möglich. Bemerkenswert ist, dass es im Gegensatz zu Benzol Additionsreaktionen eingeht.

Damit eine Verbindung kontinuierlich konjugiert werden kann, muss sie alle Kohlenstoffatome der sp^2-Hybridisierung mit jeweils einem halbgefüllten, nicht hybridisierten p-Orbital aufweisen. 1,3-Cyclohexadien und 1,3,5-Cycloheptatrien zum Beispiel haben sp^3-hybridisierte Kohlensubstanzen, die kein unhybridisiertes p-Orbital besitzen, so dass eine kontinuierliche Überlappung nicht möglich ist. Daher sind beide nicht aromatisch.

<img alt="Figure2" src="/files/ftp_upload/12453/12453_PT-Figure_2.svg" style="text-align: center; height: 85px;">

Andererseits ist Cyclobutadien, eine zyklische, planare Verbindung mit kontinuierlicher Konjugation, instabil und nicht aromatisch. Es hat 2 Paare von π-Elektronen oder 4 π-Elektronen, wodurch das Kriterium von 4n + 2  π-Elektronen nicht erfüllt wird. Daher ist es nicht aromatisch.

Kriterien für Aromatizität und die Hückel 4n + 2-Regel

Cyclobutadien und Cyclooctatetraen sind wie Benzol zyklische Verbindungen mit abwechselnden Einfach- und Doppelbindungen. Ihr chemisches Verhalten unterscheidet sich jedoch ...

criteria-for-aromaticity-and-the-hckel-4n-2-rule

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Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Aromatic Compounds

Aromatische Verbindungen

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

h&uuml;ckel rule and aromatic ring criteria

Aromatic rings follow H&amp;uuml;ckel’s rule, also called the 4n + 2 rule. This rule helps identify which unsaturated cyclic compounds are aromatic and why some ring systems are unstable instead. Benzene is the classic example, while cyclobutadiene and cyclooctatetraene show different behavior and are not aromatic.

Eric H&amp;uuml;ckel, a German chemical physicist, first described the structural features needed for aromaticity. An aromatic compound must be cyclic, planar, and continuously conjugated. It must also contain an odd number of pairs of pi electrons, which means it has 4n + 2 pi electrons, where n is an integer.

That electron pattern gives possible counts of 2, 6, 10, 14, and so on pi electrons. These are the same as 1, 3, 5, 7, and so on pairs of pi electrons. Benzene fits this pattern, and it is aromatic. Cyclobutadiene does not, even though it is cyclic and planar with continuous conjugation. It has 4 pi electrons, or 2 pairs of pi electrons, so it fails the 4n + 2 rule and is not aromatic.

A ring must also stay closed to count as aromatic. 1,3,5-Hexatriene has 6 pi electrons, like benzene, but it is not aromatic because it is not cyclic. The p orbitals on the two terminal carbon atoms do not overlap, since no bond connects those carbons.

Planarity is another key requirement. Cyclooctatetraene has four pi bonds in an eight-membered ring, but it is nonplanar and adopts a tub shape. Because of that shape, its p orbitals cannot overlap well enough to form a closed loop of pi electrons. It is unstable, not aromatic, and unlike benzene it undergoes addition reactions.

Continuous conjugation also matters. Every carbon in the ring must be sp2 hybridized and have a half-filled unhybridized p orbital. In 1,3-cyclohexadiene and 1,3,5-cycloheptatriene, some carbons are sp3 hybridized, so they do not have the needed p orbital. As a result, continuous overlap cannot happen, and both compounds are not aromatic.

H&uuml;ckel Rule and Aromatic Ring Criteria

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Aromatic rings follow H&amp;uuml;ckel’s rule, also called the 4n + 2 rule. This rule helps identify which unsaturated cyclic compounds are aromatic and why some ring systems are unstable instead. Benzene is the classic example, while cyclobutadiene and cyclooctatetraene show different behavior and are not aromatic. Eric H&amp;uuml;ckel, a German chemical physicist, first described the structural features needed for aromaticity. An aromatic compound must be cyclic, planar, and continuously...

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Hückel Rule and Aromatic Ring Criteria

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