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

يتم تمثيل البنزين والسيكلوبوتادين والسيكلوكتاتيترين على أنها هجين من هيكلين متكافئين.
ومع ذلك ، على عكس البنزين ، فإن السيكلوبوتادين والسيكلوكتاتيترين غير مستقرين وغير عطريين.
لتفسير هذه الاختلافات ، حدد إريك هوكل مجموعة من السمات الهيكلية الكامنة وراء الروائح ، والمعروفة باسم قاعدة هوكل أو قاعدة 4ن + 2.
تنص قاعدة هوكل على أن المركب العطري يجب أن يكون دوريا ومستويا ، مع مدار p غير مهجن على كل كربون.
يجب أن تتداخل المدارات p لتشكيل حلقة مستمرة من (4n + 2) π الإلكترونات ، حيث n هو عدد صحيح موجب.
يفي البنزين بجميع معايير حكم هوكل ، مما يثبت رائحته.
في المقابل ، يفترض cyclooctatetraene تشكل حوض غير مستوي مع عدم وجود تداخل بين المدارات p المجاورة. بالإضافة إلى ذلك ، فإنه يفتقر إلى (4ن + 2) π الإلكترونات وبالتالي فهو ليس عطريا.
وبالمثل ، فإن سيكلوبوتادين دوري ، مستو ، وله تداخل مستمر بين المدارات p. ومع ذلك ، فإنه يفتقر إلى (4ن + 2) π الإلكترونات ويفقد الرائحة.

معايير العطرية وقاعدة hückel 4n + 2

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

<div style="direction:rtl">مثل البنزين، فإن البيوتادايين الحلقي والسيكلو أوكتاتاترين عبارة عن مركبات حلقية لها روابط أحادية ومزدوجة متناوبة. ومع ذلك، فإن سلوكها الكيميائي يختلف عن البنزين، حيث أنها غير مستقرة وليست عطرية. إذن، ما هي الخصائص التركيبية للمركبات غير المشبعة المصنفة ضمن المركبات العطرية؟ ولأول مرة، استخرج إريك هوكل، عالم الفيزياء الكيميائية الألماني، مجموعة من السمات الهيكلية لمركب ليتم تصنيفه على أنه عطري. يُعرف هذا الآن باسم قاعدة هوكل أو قاعدة 4n + 2. وفقًا لهذه القاعدة، يجب أن يكون المركب العطري حلقيًا ومستويًا ومقترنًا بشكل مستمر، ويحتوي على عدد فردي من أزواج إلكترونات π أو 4n + 2  π إلكترونات، حيث n عدد صحيح (n = 0, 1, 2...). لذلك، يجب أن يحتوي المركب العطري على 2 (n = 0)، 6 (n = 1)، 10 (n = 2)، 14 (n = 3)، وما إلى ذلك، عدد الإلكترونات π أو 1، 3، 5، 7 الخ، أزواج من الإلكترونات π. على سبيل المثال، يحتوي 1،3،5-هكساترين والبنزين على 6 إلكترونات؛ وحتى في هذه الحالة، فإن 1،3،5-هكساترين ليس عطريًا لأن المدارات p الموجودة على ذرتي الكربون الطرفيتين لا تتداخل لأن الرابطة لا تربط ذرات الكربون هذه. لذلك يجب أن تكون المركبات حلقية لتكون عطرية. <img alt="Figure1" src="/files/ftp_upload/12453/12453_PT-Figure_1.svg" style="text-align: center; height: 150px;"> على الرغم من أنه مركب حلقي مكون من ثمانية أعضاء مع روابط متصلة بأربعة π، إلا أن سيكلو أوكتاتاترين غير مستقر وليس عطريًا. وذلك لأن سيكلوكتاتترين غير مستوي؛ انه يعتمد تشكل الحوض. وبالتالي، فإن التداخل الفعال للمدارات p لتكوين حلقة مغلقة من السحابة الإلكترونية π ليس ممكنًا. والجدير بالذكر أنه، على عكس البنزين، يخضع لتفاعلات الإضافة. لكي يكون المركب مقترنًا بشكل مستمر، يجب أن يحتوي على جميع ذرات كربون التهجين sp2 مع مدار p غير مهجن نصف مملوء على كل منها. على سبيل المثال، 1،3-سيكلوهيكساديين و1،3،5-سيكلوهيباتاترين لديهما كربونات مهجنة sp3 لا تحتوي على مدار p غير مهجن، لذلك لا يمكن تحقيق التداخل المستمر. لذلك، كلاهما ليسا عطريين. <img alt="Figure2" src="/files/ftp_upload/12453/12453_PT-Figure_2.svg" style="text-align: center; height: 85px;"> من ناحية أخرى، فإن البيوتادايين الحلقي، وهو مركب حلقي مستو مع اقتران مستمر، غير مستقر وليس عطريًا. يحتوي على زوجين من إلكترونات π أو 4 إلكترونات π، وهو ما لا يستوفي معايير وجود 4n + 2  π إلكترونات. لذلك فهي ليست عطرية.</div>

معايير العطرية وقاعدة Hückel 4n + 2

مثل البنزين ، السيكلوبوتادين والسيكلوكتاتيترين هي مركبات دورية ذات روابط مفردة ومزدوجة بديلة. ومع ذلك ، يختلف سلوكها الكيميائي ...

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

Education

JoVE Core

Organic Chemistry

الكيمياء العضوية

Unit 1

الوحدة الأولى

Aromatic Compounds

المركبات العطرية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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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criteria for aromaticity and the h&uuml;ckel 4n + 2 rule

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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Cycloheptatrienyl Cation and Aromaticity

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Learn the H&uuml;ckel 4n + 2 rule and the key features of aromatic rings, including cyclic shape, planarity, continuous conjugation, and pi electrons.

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Hückel Rule and Aromatic Ring Criteria

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