Name: cyclopentadienyl anion and aromaticity
Uploaded: 2023-04-30T10:31:23.000Z
Duration: 1 min 18 s
Channel: JoVE

تفتقر الهيدروكربونات غير المشبعة أحادية الحلقة المحايدة التي تحتوي على عدد فردي من ذرات الكربون إلى الروائح بسبب وجود كربون sp3 متداخل في الحلقة.
على سبيل المثال ، السيكلوبنتادين ليس عطريا ، لأنه يحتوي على 4 إلكترونات π فقط وكربون sp3 يعطل التداخل المستمر للمدارات p.
والجدير بالذكر أن إزالة الهيدروجين من مجموعة CH2 مع كل من إلكترونات الترابط أو واحد أو لا شيء منها يحول الكربون sp3 إلى sp2 ، مما يولد كاتيون وجذري وأنيون على التوالي.
بالمقارنة مع الكاتيون والجذر ، فإن الأنيون فقط لديه العدد المطلوب من (4ن + 2) π إلكترونات.
علاوة على ذلك ، فإن توافر مدار 2p في الكربون sp2 يسهل التداخل المستمر للمدارات p وإلغاء تحديد موقع الشحنة السالبة في جميع أنحاء الحلقة ، مما يجعل أنيون سيكلوبنتادينيل عطريا.
بالإضافة إلى ذلك ، فإن الإلكترونات π التي تشغل المدارات الجزيئية المترابطة وهياكل الرنين الخمسة تؤكد بشكل أكبر الاستقرار غير العادي للأنيون العطري.

الأنيونات الهيدروكربونية العطرية: نظرة عامة هيكلية

Aromatic Hydrocarbon Anions: Structural Overview

<div style="direction:rtl">الهيدروكربونات المحايدة مثل السيكلوبنتاديين التي تحتوي على عدد فردي من ذرات الكربون ومجموعة واحدة متداخلة من CH2 في الحلقة ليست عطرية. حلقي البنتادين الذي يحتوي على 4 إلكترونات π لا يحقق قاعدة الإلكترون 4n + 2 π. بالإضافة إلى ذلك، فإن مجموعة CH2 المتداخلة عبارة عن sp3 مهجنة وتفتقر إلى مدار p شاغر، وبالتالي تقطع تداخل مدارات p بطريقة مستمرة ومنع عدم تمركز إلكترونات π في جميع أنحاء الحلقة. ونظرًا لغياب التداخل المستمر للمدارات p، فإن السيكلوبنتادايين لا يتوافق مع معايير العطرية. ومع ذلك، فإن إزالة هيدروجين واحد مع إلكترونين أو واحد أو بدون إلكترونات من مجموعة CH2 يؤدي إلى تحويل كربون sp3 إلى sp2، وبالتالي تحويل سيكلوبنتاديين إلى كاتيون سيكلوبنتاديينيل وجذر وأنيون، على التوالي، مع وجود مدار p شاغر. من بين جميع الأنواع الثلاثة، فإن كاتيون البنتادينيل الحلقي والجذري الذي يحتوي على أربعة وخمسة إلكترونات π، على التوالي، لا يستوفي معايير 4n + 2 π إلكترونات وليست عطرية. من ناحية أخرى، أنيون سيكلوبنتادينيل مع 6 إلكترونات، وحلقة متداخلة مستمرة من مدارات p، وكثافة إلكترون π غير متمركزة، تصبح عطرية. تدعم خريطة الجهد الكهروستاتيكي للأنيون إلغاء تحديد موقع الإلكترونات في جميع أنحاء الحلقة. علاوة على ذلك، فإن هياكل الرنين الخمسة لأنيون سيكلوبنتادينيل وشغل المدارات الجزيئية الرابطة بواسطة جميع الإلكترونات 6 π في مخطط فروست تؤكد صحة الاستقرار غير العادي للأنيون.</div>

الهيدروكربونات المحايدة مثل السيكلوبنتادين مع عدد فردي من ذرات الكربون ومجموعة CH2 متداخلة واحدة في الحلقة ليست عطرية. سيكلوبنتادين ...

aromatic-hydrocarbon-anions-structural-overview

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Organic Chemistry

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Unit 1

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Aromatic Compounds

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KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

cyclopentadienyl anion and aromaticity

Cyclopentadiene is a neutral hydrocarbon that is not aromatic. It contains an odd number of carbon atoms and one CH2 group in the ring. With only 4 pi electrons, it does not follow the 4n + 2 pi electron rule. The CH2 carbon is sp3 hybridized, so it has no vacant p orbital to keep p orbitals overlapped around the ring.

That break in overlap stops pi electrons from spreading across the entire ring. As a result, cyclopentadiene does not meet the usual aromaticity criteria. If one hydrogen is removed from the CH2 group, the carbon changes from sp3 to sp2 hybridization. This creates a vacant p orbital and gives three related species: the cyclopentadienyl cation, radical, and anion.

These three species do not behave the same way. The cyclopentadienyl cation has 4 pi electrons, and the radical has 5 pi electrons. Neither one satisfies the 4n + 2 rule, so neither is aromatic. The cyclopentadienyl anion has 6 pi electrons, a continuous ring of overlapping p orbitals, and delocalized pi electron density. That makes it aromatic and unusually stable.

The electrostatic potential map of the anion supports this electron delocalization across the ring. Its five resonance structures also show how the electrons can be spread out. The frost diagram gives the same result, since all 6 pi electrons fill bonding molecular orbitals. Together, these features explain why the cyclopentadienyl anion is more stable than the neutral ring, the cation, or the radical.

Cyclopentadienyl Anion and Aromaticity

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Frost circles help test whether a cyclic compound is aromatic by using the inscribed polygon method and Hückel’s 4n + 2 rule. A compound is stable and aromatic when every bonding molecular orbital (MO) is completely filled with a pair of electrons. If electrons occupy non-bonding or antibonding orbitals, the compound is unstable and not aromatic. Frost circle diagrams for cycloalkenes with 4 to 8 carbons show a clear pattern. The number of bonding MOs is always odd. The electrons needed to...

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Learn why cyclopentadiene is not aromatic and how the cyclopentadienyl anion becomes stable through 6 pi electrons, resonance, and orbital overlap.

Cyclopentadienyl Anion and Aromaticity | JoVE K12

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JoVE Core Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Cyclopentadienyl Anion and Aromaticity

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