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polynuclear aromatic compound

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الكلية كلية العلوم     القسم قسم الكيمياء     المرحلة 3
أستاذ المادة مهند موسى كريم الحجامي       07/05/2017 18:55:45
Although naphthalene, phenanthrene, and anthracene resemble benzene in many respects, they are more reactive than benzene in both substitution and addition reactions. This increased reactivity is expected on theoretical grounds because quantum-mechanical calculations show that the net loss in stabilization energy for the first step in electrophilic substitution or addition decreases
progressively from benzene to anthracene; therefore the reactivity in substitution and addition reactions should increase from benzene to anthracene.In considering the properties of the polynuclear hydrocarbons relative to benzene, it is important to recognize that we neither expect nor find that all the carbon-carbon bonds in polynuclear hydrocarbons are alike or correspond to benzene bonds in being halfway between single and double bonds.
The 1,2 bonds in both napthalene and anthracene are in fact shorter than the other ring bonds, whereas the 9,10 bond in phenanthrene closely resembles an alkene double bond in both its length and chemical reactivity.
Sometimes, small changes in the reagents and conditions change the pattern
of orientation. One example is sulfonation, in which the orientation changes
with reaction temperature. Another example is Friedel-Crafts acylation; in
carbon disulfide the major product is the 1-isomer, whereas in nitrobenzene
the major product is the 2-isomer.
Substitution usually occurs more readily at the 1 position than at the 2 position
because the intermediate for 1-substitution is more stable than that for
2-substitution. The reason is that the most favorable resonance structures for
either intermediate are those that have one f~illy aromatic ring.

المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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