There is no doubt that benzene is aromatic. Now we must ask: how can we insert a heteroatom into the ring and retain aromaticity? What kind of atom is needed? If we want to replace one of the carbon atoms of benzene with a heteroatom, we need an atom that can be trigonal to keep the flat hexagonal ring, and that has a p orbital to keep the six delocalized electrons. Nitrogen fits all of these requirements. This is what happens if we replace a CH group in benzene with a nitrogen atom.

The orbitals in the ring have not changed in position or shape and we still have the six electrons from the three double bonds. One obvious difference is that nitrogen is trivalent and thus there is no NH bond. Instead, a lone pair of electrons occupies the space of the C–H bond in benzene. In theory then, pyridine is aromatic. But is it in real life? The most important evidence comes from the proton NMR spectrum. The six protons of benzene resonate at 7.27 ppm, some 2 ppm downfield from the alkene region, clear evidence for a ring current. Pyridine is not as symmetrical as benzene but the three types of proton all resonate in the same region. As we will see, pyridine is also very stable and, by any reasonable assessment, pyridine is aromatic. We could continue the process of replacing, on paper, more CH groups with nitrogen atoms, and would find three new aromatic heterocycles: pyridazine, pyrimidine, and pyrazine:

There is another way in which we might transform benzene into a heterocycle. Instead of using just one electron from N to replace an electron in the π system, we could use nitrogen’s lone pair of electrons to replace two electrons in the π system. We can substitute a CH=CH unit in benzene with a nitrogen atom providing that we can use the lone pair in the delocalized system. This means putting it into a p orbital. We still have the four electrons from the remaining double bonds and, with the two electrons of the lone pair on nitrogen, that makes six in all. The nitrogen atom must still be trigonal with the lone pair in a p orbital so the N–H bond is in the plane of the fi ve-membered ring.

The 1H NMR spectrum of pyrrole is slightly less convincing as the two types of proton on the ring resonate at higher field than those of benzene or pyridine but they still fall in the aromatic rather than the alkene region. Pyrrole is also more reactive towards electro philes than benzene or pyridine, but it does the usual aromatic substitution reactions (Friedel Crafts, nitration, halogenation) rather than addition reactions: pyrrole is also aromatic. Inventing heterocycles by further replacement of CH groups by nitrogen in pyrrole leads to two compounds, pyrazole and imidazole, after one replacement, to two triazoles after two replacements, and to a single tetrazole after three.

All of these compounds are generally accepted as aromatic too as they broadly have the NMR spectra and reactivities expected for aromatic compounds. As you may expect, intro ducing heteroatoms into the aromatic ring and, even more, changing the ring size actually affect the chemistry a great deal. We must now return to pyridine and work our way more slowly through the chemistry of these important heterocycles to establish the principles that govern their behaviour.

اخر الاخبار

اخبار العتبة العباسية المقدسة

مستشفى الكفيل يعلن انضمامه لمشروع الضمان الصحي الحكومي

المجمَع العلمي يقيم مسابقة الكوثر الخاصة بترتيل القرآن الكريم في كربلاء

قسم الشؤون الفكرية والثقافية يُصدر العدد الثالث والعشرين من مجلّة (أوراق معرفية)

قسم التربية والتعليم يُطلق الموسم التدريبيّ الصيفيّ لملاكات مجموعة العميد التربوية

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

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

Double disconnections can be a short cut

1,1C–C disconnections

C–C disconnections

1,2-Disconnections

Two-group disconnections are better than one-group disconnections

Amine synthesis using functional group interconversions

Functional group interconversion

Multiple step syntheses: avoid chemoselectivity problems