Organic Chemistry - Heterocyclic Compounds - Nomenclature PDF

Organic Chemistry Heterocyclic Compounds Nomenclature Nomenclature of Heterocyclic compounds Contents : Nomenclature of Heterocyclic compounds 3 Replacement nomenclature 4 Hantzsch-Widman nomenclature (IUPAC) 10 Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds: I. Common...

Organic Chemistry Heterocyclic Compounds Nomenclature Nomenclature of Heterocyclic compounds Contents : Nomenclature of Heterocyclic compounds 3 Replacement nomenclature 4 Hantzsch-Widman nomenclature (IUPAC) 10 Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds: I. Common Nomenclature II. Replacement nomenclature III. Hantzsch-Widman nomenclature (IUPAC) Nomenclature of Heterocyclic compounds II- Replacement nomenclature: In replacement nomenclature, the heterocycle’s name is composed of the corresponding carbocycle’s name and an elemental prefix for the heteroatom introduced (if more than one heteroatom is present they should be listed according to the priority order shown in (table 1). According to this nomenclature, tetrahydrofuran, for instance, is called oxacyclopentane. Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds To name this compound by the replacement method: 1. See the corresponding carbocyclic name. 2. Add the elemental prefixes for the heteroatoms introduced (in this case O=oxa, N=aza). 3. Number all the heteroatoms and double bonds Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds Hantzsch-Widman nomenclature Nomenclature of Heterocyclic compounds III- Hantzsch-Widman nomenclature (IUPAC): According to this system three to ten-membered rings are named by combining the appropriate prefix (or prefixes) that denotes the type and position of the heteroatom present in the ring with suffix that determines both the ring size (depending on the total number of atoms in the ring) and the degree of unsaturation (note that fully saturated and fully unsaturated have certain rules for nomenclature while partially unsaturation will be indicated in certain ways). In addition, the suffixes distinguish between nitrogen-containing heterocycles and heterocycles that do not contain nitrogen. IUPAC name = locants + Prefix + suffix Nomenclature of Heterocyclic compounds Hantzsch-Widman rules for fully saturated an fully unsaturated heterocycles. 1. Identify the hetroatom present in the ring and choose from (table.1) the corresponding prefix (e.g. thia for sulfur, aza for nitrogen and oxa for oxygen). 2. The position of a single heteroatom control the numbering in a monocyclic compound. The heteroatom is always assigned position 1 and if substituents present are then counted around the ring in a manner so as to take the lowest possible numbers. For example: Nomenclature of Heterocyclic compounds 3. A (di, tri, ect.) and locants are used when two or more similar heteroatoms contained in the ring ( two nitrogen indicated by diaza) and the numbering preferably commenced at a saturated rather than an unsaturated atom, as depicted in the following example: 1,3-diaza.... Nomenclature of Heterocyclic compounds 4. If more than one type of hetroatoms present in the ring the name will include more than one prefix with locants to indicate the relative position of the heteroatoms. Atom prefixes have a strict order of priority (preference) in which they are to be listed. For example: a) ‘’Oxa’’(for oxygen) always comes before ‘’aza’’ (for nitrogen) in a name (see table 1). b) When combining the prefixes (e.g. oxa and aza) two vowels may end up together, therefore the vowel on the end of the first part should be omitted (oxaza). Nomenclature of Heterocyclic compounds c) The numbering is started from the heteroatom of the highest priority in such a way so as to give the smallest possible numbers to the other heteroatoms in the ring (the substituents are irrelevant). For example the prefix corresponding to the following compound is 4-Methyl1,3-Thiaza.... Nomenclature of Heterocyclic compounds 5. Choose the appropriate suffix from (table 2) depending on whether or not nitrogen atom is present in the ring, the size of the ring and presence or absence of any double bonds. 6. Combine the prefix (s) and suffix together and drop the first vowel if two vowels came together. Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds Example.1 : This ring contains (N) → Prefix is aza. The ring is 3-membered and fully saturated → suffix is iridine. By combining the prefix and suffix, two vowels ended up together (azairidine), therefore the vowel on the end of the first part should be dropped. This gives the correct name: Aziridine. Nomenclature of Heterocyclic compounds Example 2: This ring contains nitrogen = azaAnd oxygen = oxa- And is a fully saturated four-membered ring = -etidine Drop the vowels in oxa & aza The name = 1,2-Oxazetidine Nomenclature of Heterocyclic compounds Example 3: This ring contains nitrogen = azaAnd oxygen = oxa- And Unsaturated five-membered rings with nitrogen= -ole Oxygen is higher priority than nitrogen, so it is in position 1. The two nitrogens are therefore at positions 2 and 5. The name = 1,2,5-Oxadiazole Nomenclature of Heterocyclic compounds Example 4: This ring contains sulpher= thiaAnd oxygen = oxa- And saturated five-membered rings without nitrogen = -olane Oxygen is higher priority than sulpher, so it is in position 1. Drop the vowel in thia The name = 1,3-Oxathiolane Nomenclature of Heterocyclic compounds Hantzsch-Widman rules for partially unsaturated heterocycles Partial unsaturation in heterocyclic compounds can be indicated by one of the following methods: a) The position of nitrogen or carbon atoms which bear extra hydrogen atoms must be indicated by numbers and italic capital H (e.g. 1H, 2H, etc.) followed by the name of maximally unsaturated ring. Nomenclature of Heterocyclic compounds b) The words dihydro, or trihydro, or tetrahydro are used if two or three or four atoms are saturated. These words are preceded by numbers indicate the position of saturated atoms as low as possible and followed by the corresponding fully unsaturated Hantzsch-Widman name. c) Alternatively, the partially unsaturated 4 and 5 rings (i.e. rings contain one double bond) are given special Hantzsch-Widman suffixes as in table 3 and the double bond is specified as ∆1, ∆2, ∆3, etc.. Which indicates 1 and ; 2 and 3; 3 and 4 atoms respectively have a double bond (i.e. Name: ∆x + Prefix + special suffix ) ( x= locant of the double bond) Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds Nomenclature of Heterocyclic compounds Example: