Pyridine is a very unreactive aromatic imine

The nitrogen atom in the pyridine ring is planar and trigonal with the lone pair in the plane of the ring. This makes it an imine. Most of the imines you have met before, have been unstable intermediates in carbonyl group reactions, but in pyridine we have a stable imine—stable because of its aromaticity. All imines are more weakly basic than saturated amines and pyridine is a weak base with a pKa (for its conjugate acid) of 5.5. This means that the pyridinium ion is about as strong an acid as a carboxylic acid.

Pyridine is a reasonable nucleophile for carbonyl groups and is often used as a nucleophilic catalyst in acylation reactions. Esters are often made in pyridine solution from alcohols and acid chlorides .

Pyridine is nucleophilic at the nitrogen atom because the lone pair of electrons on nitrogen cannot be delocalized around the ring. They are in an sp² orbital orthogonal to the p orbitals in the ring and there is no interaction between orthogonal orbitals. Try it for yourself, drawing arrows. All attempts to delocalize the electrons lead to impossible results!

● The lone pair of pyridine’s nitrogen atom is not delocalized. Our main question about the reactivity of pyridine must be this: what does the nitrogen atom do to the rest of the ring? The important orbitals—the p orbitals of the aromatic system— are superficially the same as in benzene, but the more electronegative nitrogen atom will lower the energy of all the orbitals. Lower-energy fi lled orbitals mean a less reactive nucleophile but a lower-energy LUMO means a more reactive electrophile. This is a good guide to the chemistry of pyridine. It is less reactive than benzene in electrophilic aromatic substitution reactions, but nucleophilic substitution, which is difficult for benzene, comes easily to pyridine.

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قسم الشؤون الفكرية يختتم المرحلة الثالثة من مشروع استعادة مكتبة جامعة الموصل

جامعة الكفيل تنظّم دورة حول نظام (كوها) العالمي لإدارة المكتبات الجامعية في النجف

مجموعة مشاتل الكفيل: إضافة نحو 70 نوعًا جديدًا من المغروسات تتناسب مع الظروف المناخية المحلية

المجمَع العلمي يطلق الدورة التطويرية الخامسة لمعلمي التربية الإسلامية ومدرسيها في النجف الأشرف

المزيد

الآخبار الصحية

اختراق علمي.. دواء يؤخر ظهور السكري من النوع الأول لسنوات

هذا ما يحدث لجسمك إن أهملت نظافة فراشك

لماذا تعتبر النساء أكثر عرضة للإصابة بألزهايمر من الرجال؟

لماذا يحذر الخبراء من الإفراط في تناول الفراولة؟

المزيد

الآخبار التكنلوجية

السيارات الكهربائية أنظف بنسبة 73% من سيارات الوقود

سيارة لوسِد الكهربائية تدخل "غينيس" بأطول مدى في العالم

نظرية جديدة تنسف كل ما نعرفه عن تشكل أول قارة في الأرض

منذ 37 ألف عام.. دراسة تكشف "أقدم أمراض" أصابت البشر

المزيد

مواضيع ذات صلة

Furan and thiophene are oxygen and sulfur analogues of pyrrole

Five-membered aromatic heterocycles are good at electrophilic substitution

Pyridine as a catalyst and reagent

Pyridine N-oxides are reactive towards both electrophilic and nucleophilic substitution

Pyridones are good substrates for nucleophilic substitution

Nucleophilic substitution is easy with pyridines

Pyridine is bad at electrophilic aromatic substitution

Aromaticity survives when parts of benzene’s ring are replaced by nitrogen atoms

The Claisen ester disconnection: a 1,3-diO relationship needing two carbonyl groups

Aldol-style disconnections with N and O in a 1,3-relationship: I

Look out for concealed functional group relationships

Donor and acceptor synthons