In the Periodic Table there are only four Groups towards the left of carbon. The noble gases are inert so we are left with three Groups of elements that can bond covalently to carbon, beside those elements beneath carbon in the Periodic Table. The halogens are univalent so the nomenclature does not present a problem. Oxygen, and the other Group VII members, is divalent. When present in straight chain it is an alcohol, an ether or some simple compound and the nomenclature is simple. This is also the case for nitrogen and the other Group VI elements.
However when the hydrocarbon forms a cyclic compound via oxygen and nitrogen the nomenclature is in many cases mind-boggling. It is in this context that we used the term "heterocyclic", meaning hydrocarbons with an element other than hydrogen and carbon in the ring. Example. Oxygen, nitrogen, sulphur, etc.
OXYGEN and SULPHUR
We shall start with oxygen and sulphur, which is below oxygen in the Periodic Table. Also we focus on only saturated cyclic hydrocarbons. The prefix used for oxygen is ox and for sulphur it will be thi. For three-member ring the suffix used is -irane, four-member will be -etane, five-member -olane and six-member -ane.
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| 1,3-dioxolane | 1,4-dithiane |
This system is convenient when there are more than one "foreign" element present in the ring. Example.
Otherwise the common name is more widely used.
| Compound Oxirane Oxetane Oxolane Oxane |
Common Name Ethylene Oxide Propylene Oxide Tetrahydrofuran Tetrahydropyran |
Compound Thiirane Thietane Thiolane Thiane |
Common Name Ethylene Sulphide Propylene Sulphide Tetrahydrothiophene Tetrahydrothiopyran |
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| Furan | Thiophene | pyran-4-H | thiopyran-4-H |
At this juncture it may be enlightening to note the basic names; furan, thiophene, and pyran. The 4-H in pyran denote the position of the sp� carbon (carbon with two hydrogen atoms). So we can also have pyran-2-H. Of course the foreign element is labelled position 1. The same can be applied to thiopyran.
Tutorial 1.3
Can we have more than two pi-bonds in furan, thiophene, pyran and thiopyran? Answer
NITROGEN
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| Aziridine | Azetidine |
The first two members, three- and four-membered rings, showed a slight resemblance to the system used for oxygen. They are known as az-ir-idine and az-et-idine. Az......idine for nitrogen and the middle term for the ring size.
After this there do not appear to be a system for naming the larger compounds
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| Pyrrolidine | Pyrrole | Piperidine | Pyridine |
To complicate matters there can be more than one type of "hetero" elements in the ring. Consider just the three possible combinations with nitrogen.
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| oxazole | thiazole | imidazole | isoxazole | isothiazole | pyazole |
That are five-member rings with nitrogen and another "hetero" element. These are known as azole compounds. It is unreasonable for anyone to expect students to remember the names of most of the heterocyclic compounds, except for the few popular ones.
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On the other hand when you need to name a heterocyclic compound make sure you know what you are doing. Although the heterocyclic compound with at least a nitrogen is considered an amine (azole), the sp� hetero element is still considered to occupy position 1. For example this compound is named 5-methylisoxazole. The position 5 for methyl is considered from oxygen going by way of nitrogen.
