Chapter-1-Heterocyclic compounds-nomenclture
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Chapter-1-Heterocyclic compounds-nomenclture
- 1. 1 HETEROCYCLIC COMPOUNDS are cyclic compounds in which an element other than carbon is present in the ring: N H 1 2 3 4 5 S N 1 2 4 3 5 Pyrrole Thiazole Furan
- 2. 2 H H N S Имидазол Тиазол N N Пиразол N N 1. Classification of heterocycles а) Five-membered heterocycles •containing one heteroatom •containing two heteroatoms (azoles) H N 1 2 3 4 5 O 1 2 3 4 5 S 1 2 3 4 5 Furan Pyrrole Thiophene N O S Pyrrole Furan Thiophene N N N N N S Imidazole Pyrazole Thiazole
- 3. 3 N 1 6 2 3 4 5 Пиридин O Пиран c) Six-membered heterocycles: •containing one heteroatom •containing two heteroatoms N 1 6 2 3 4 5 Пиримидин N N 1 6 2 3 4 5 Пиридазин N N 1 6 2 3 4 5 Пиридазин N N O Pyridine Pyrane N N N N Pyrimidine Pyridazine N N Pyrazine
- 4. 4 H H 1 6 2 3 4 5 Индол N 7 8 9 6 4 5 8 9 10 N 1 2 3 7 Хинолин 1 6 2 3 4 5 N 7 8 9 N N N Пурин в) condensed heterocycles: Usually trivial names are used. Numeration of ring atoms begins with heteroatoms. Indole Quinoline Purine 1 N 1 N 1 N N 9 N N
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- 40. 40 AROMATICITY OF HETEROCYCLES Hückel’s rule N = 4n + 2 where is n (number of cycles) = 1,2, etc. N - number of conjugate -electrons
- 41. 41 PYRIDINE NITROGEN N 1 6 2 3 4 5 Пиридин O Пиран Pyridine is basic in nature; it reacts with water and acids. The nitrogen atom is more electronegative as compared with the carbon itself. It pulls electron density from the ring. Therefore, the system with the pyridine nitrogen is called -deficiency. Pyridine N
- 42. 42 Pyridine is -deficiency system p-orbital is involved in conjugation sp2 - orbital lone pair of electrons Pyridine
- 43. 43 • Pyrrole is slightly acidic. • The system with the pyrrole nitrogen is called -excess. H N 1 2 3 4 5 Пиррол PYRROLE NITROGEN Pyrrol N
- 44. 44 Pyrrole is -excess system p orbital lone pair of electrons involved in conjugation sp2 - orbital forms σ- bond Pyrrole
- 45. 45 Pyridine nitrogen (basic properties) Pyrrole nitrogen (acidic properties ) AMPHOTERIC COMPOUNDS Imidazole Pyrazole
Editor's Notes
- Heterocyclic compounds containing nitrogen, oxygen, or sulfur are by far the most common. Tree important examples are given here in their Kekulé forms. These tree compounds are all aromatic
- Heterocyclic compounds have the aromatic properties, because its structure is a planar ring formed -bonds to the number of conjugated -electrons:
- The nitrogen atoms in the molecules of pyridine are sp2 hybridized. In pyridine the sp2–hybridized nitrogen donates one bonding electron located on the p orbital to the π system. This electron, together with one from each of the five carbon atoms, gives pyridine a sextet of electrons like benzene. The two unshared electrons of the nitrogen of pyridine are in an sp2 orbital that lies in the same plane as the atoms of the ring. This sp2 orbital does not overlap with the p orbitals of the ring. The unshared pair on nitrogen is not a part of the π system, and these electrons confer on pyridine the properties of a weak base.
- On the slade you can see electronic structure of pyridine and pair electrons which are outside of the cycle
- In pyrrole the electrons are arranged differently. Because only four π electrons are contributed by the carbon atoms of the pyrrole ring, the sp2–hybridized nitrogen must contribute two electrons to give an aromatic sextet. Because these electrons are a part of the aromatic sextet, they are not available for donation to a proton. Thus, pyrrole is slightly acidic.
- And on the slade you can see two electrons which are a part of the aromatic sextet.
- One nitrogen atom in imidazole provides one electron to the conjugated system and, therefore, this is pyridic nitrogen. It imparts basic properties to imidazole. The second one provides two electrons to the conjugated system and is, therefore, pyrrolic nitrogen. This imparts weak acidic properties to imidazole. So, imidazole is amphoteric in nature.