Hantzsch—Widman nomenclature , also called the extended Hantzsch—Widman system , is a type of systematic chemical nomenclature used for naming heterocyclic parent hydrides having no more than ten ring members. A Hantzsch—Widman name will always contain a prefix, which indicates the type of heteroatom present in the ring, and a stem, which indicates both the total number of atoms and the presence or absence of double bonds. The name may include more than one prefix, if more than one type of heteroatom is present; a multiplicative prefix if there are several heteroatoms of the same type; and locants to indicate the relative positions of the different atoms. Hantzsch—Widman names may be combined with other aspects of organic nomenclature, to indicate substitution or fused-ring systems. The Hantzsch—Widman prefixes indicate the type of heteroatom s present in the ring. They form a priority series: If there is more than one type of heteroatom in the ring, the prefix that is higher on the list comes before the prefix that is lower on the list.
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Heterocyclic Chemistry. Compounds classified as heterocyclic probably constitute the largest and most varied family of organic compounds. After all, every carbocyclic compound, regardless of structure and functionality, may in principle be converted into a collection of heterocyclic analogs by replacing one or more of the ring carbon atoms with a different element. Even if we restrict our consideration to oxygen, nitrogen and sulfur the most common heterocyclic elements , the permutations and combinations of such a replacement are numerous.
Devising a systematic nomenclature system for heterocyclic compounds presented a formidable challenge, which has not been uniformly concluded. Many heterocycles, especially amines, were identified early on, and received trivial names which are still preferred.
Some monocyclic compounds of this kind are shown in the following chart, with the common trivial name in bold and a systematic name based on the Hantzsch-Widman system given beneath it in blue. The rules for using this system will be given later.
For most students, learning these common names will provide an adequate nomenclature background. An easy to remember, but limited, nomenclature system makes use of an elemental prefix for the heteroatom followed by the appropriate carbocyclic name. A short list of some common prefixes is given in the following table, priority order increasing from right to left. The Hantzsch-Widman system provides a more systematic method of naming heterocyclic compounds that is not dependent on prior carbocyclic names.
It makes use of the same hetero atom prefix defined above dropping the final "a" , followed by a suffix designating ring size and saturation. As outlined in the following table, each suffix consists of a ring size root blue and an ending intended to designate the degree of unsaturation in the ring.
In this respect, it is important to recognize that the saturated suffix applies only to completely saturated ring systems , and the unsaturated suffix applies to rings incorporating the maximum number of non-cumulated double bonds. Systems having a lesser degree of unsaturation require an appropriate prefix, such as "dihydro"or "tetrahydro".
Despite the general systematic structure of the Hantzsch-Widman system, several exceptions and modifications have been incorporated to accommodate conflicts with prior usage. Examples of these nomenclature rules are written in blue, both in the previous diagram and that shown below. Note that when a maximally unsaturated ring includes a saturated atom, its location may be designated by a " H " prefix to avoid ambiguity, as in pyran and pyrrole above and several examples below.
When numbering a ring with more than one heteroatom, the highest priority atom is 1 and continues in the direction that gives the next priority atom the lowest number. All the previous examples have been monocyclic compounds. Polycyclic compounds incorporating one or more heterocyclic rings are well known. A few of these are shown in the following diagram. As before, common names are in black and systematic names in blue. The two quinolines illustrate another nuance of heterocyclic nomenclature.
Thus, the location of a fused ring may be indicated by a lowercase letter which designates the edge of the heterocyclic ring involved in the fusion, as shown by the pyridine ring in the green shaded box. Heterocyclic rings are found in many naturally occurring compounds. Most notably, they compose the core structures of mono and polysaccharides , and the four DNA bases that establish the genetic code. By clicking on the above diagram some other examples of heterocyclic natural products will be displayed.
Oxiranes epoxides are the most commonly encountered three-membered heterocycles. Epoxides are easily prepared by reaction of alkenes with peracids, usually with good stereospecificity. Because of the high angle strain of the three-membered ring, epoxides are more reactive that unstrained ethers.
Addition reactions proceeding by electrophilic or nucleophilic opening of the ring constitute the most general reaction class. Example 1 in the following diagram shows one such transformation, which is interesting due to subsequent conversion of the addition intermediate into the corresponding thiirane.
The initial ring opening is stereoelectronically directed in a trans-diaxial fashion, the intermediate relaxing to the diequatorial conformer before cyclizing to a 1,3-oxathiolane intermediate. Other examples show similar addition reactions to thiiranes and aziridines. The acid-catalyzed additions in examples 2 and 3, illustrate the influence of substituents on the regioselectivity of addition. Example 2 reflects the S N 2 character of nucleophile chloride anion attack on the protonated aziridine the less substituted carbon is the site of addition.
The phenyl substituent in example 3 serves to stabilize the developing carbocation to such a degree that S N 1 selectivity is realized. The reduction of thiiranes to alkenes by reaction with phosphite esters example 6 is highly stereospecific, and is believed to take place by an initial bonding of phosphorous to sulfur.
By clicking on the above diagram , four additional example of three-membered heterocycle reactivity or intermediacy will be displayed. Examples 7 and 8 are thermal reactions in which both the heteroatom and the strained ring are important factors. Note that two inversions of configuration at C-2 result in overall retention. Many examples of intramolecular interactions , such as example 10, have been documented. As illustrated below, acid and base-catalyzed reactions normally proceed by 5-exo-substitution reaction 1 , yielding a tetrahydrofuran product.
However, if the oxirane has an unsaturated substituent vinyl or phenyl , the acid-catalyzed opening occurs at the allylic or benzylic carbon reaction 2 in a 6-endo fashion. Preparation Several methods of preparing four-membered heterocyclic compounds are shown in the following diagram.
The simple procedure of treating a 3-halo alcohol, thiol or amine with base is generally effective, but the yields are often mediocre. Dimerization and elimination are common side reactions, and other functions may compete in the reaction. In the case of example 1, cyclization to an oxirane competes with thietane formation, but the greater nucleophilicity of sulfur dominates, especially if a weak base is used. In example 2 both aziridine and azetidine formation are possible, but only the former is observed.
This is a good example of the kinetic advantage of three-membered ring formation. Example 4 demonstrates that this approach to azetidine formation works well in the absence of competition. Indeed, the exceptional yield of this product is attributed to the gem-dimethyl substitution, the Thorpe-Ingold effect , which is believed to favor coiled chain conformations.
The relatively rigid configuration of the substrate in example 3, favors oxetane formation and prevents an oxirane cyclization from occurring. Finally, the Paterno-Buchi photocyclizations in examples 5 and 6 are particularly suited to oxetane formation. Reactions Reactions of four-membered heterocycles also show the influence of ring strain. Some examples are given in the following diagram. In the thietane reaction 2 , the sulfur undergoes electrophilic chlorination to form a chlorosulfonium intermediate followed by a ring-opening chloride ion substitution.
Strong nucleophiles will also open the strained ether, as shown by reaction 3b. Example 5 is an interesting case of intramolecular rearrangement to an ortho-ester.
Such electron pair delocalization is diminished in the penicillins, leaving the nitrogen with a pyramidal configuration and the carbonyl function more reactive toward nucleophiles.
Preparation Commercial preparation of furan proceeds by way of the aldehyde, furfural, which in turn is generated from pentose containing raw materials like corncobs, as shown in the uppermost equation below.
Similar preparations of pyrrole and thiophene are depicted in the second row equations. Equation 1 in the third row illustrates a general preparation of substituted furans, pyrroles and thiophenes from 1,4-dicarbonyl compounds, known as the Paal-Knorr synthesis.
Many other procedures leading to substituted heterocycles of this kind have been devised. Two of these are shown in reactions 2 and 3. Furan is reduced to tetrahydrofuran by palladium-catalyzed hydrogenation. This cyclic ether is not only a valuable solvent, but it is readily converted to 1,4-dihalobutanes or 4-haloalkylsulfonates, which may be used to prepare pyrrolidine and thiolane. Dipolar cycloaddition reactions often lead to more complex five-membered heterocycles.
Indole is probably the most important fused ring heterocycle in this class. By clicking on the above diagram three examples of indole synthesis will be displayed.
The first proceeds by an electrophilic substitution of a nitrogen-activated benzene ring. The second presumably takes place by formation of a dianionic species in which the ArCH 2 — unit bonds to the deactivated carbonyl group. Finally, the Fischer indole synthesis is a remarkable sequence of tautomerism, sigmatropic rearrangement , nucleophilic addition, and elimination reactions occurring subsequent to phenylhydrazone formation.
This interesting transformation involves the oxidation of two carbon atoms and the reduction of one carbon and both nitrogen atoms. These units are commonly used as protective groups for aldehydes and ketones, and may be hydrolyzed by the action of aqueous acid. It is the "aromatic" unsaturated compounds, furan, thiophene and pyrrole that require our attention.
This is illustrated by the resonance description at the top of the following diagram. The heteroatom Y becomes sp 2 -hybridized and acquires a positive charge as its electron pair is delocalized around the ring.
An easily observed consequence of this delocalization is a change in dipole moment compared with the analogous saturated heterocycles, which all have strong dipoles with the heteroatom at the negative end. As expected, the aromatic heterocycles have much smaller dipole moments, or in the case of pyrrole a large dipole in the opposite direction. An important characteristic of aromaticity is enhanced thermodynamic stability , and this is usually demonstrated by relative heats of hydrogenation or heats of combustion measurements.
By this standard, the three aromatic heterocycles under examination are stabilized, but to a lesser degree than benzene. Additional evidence for the aromatic character of pyrrole is found in its exceptionally weak basicity pK a ca.
The corresponding values for the saturated amine pyrrolidine are: basicity Another characteristic of aromatic systems, of particular importance to chemists, is their pattern of reactivity with electrophilic reagents. Whereas simple cycloalkenes generally give addition reactions, aromatic compounds tend to react by substitution.
As noted for benzene and its derivatives, these substitutions take place by an initial electrophile addition, followed by a proton loss from the "onium" intermediate to regenerate the aromatic ring.
The reaction conditions show clearly the greater reactivity of furan compared with thiophene. All these aromatic heterocycles react vigorously with chlorine and bromine, often forming polyhalogenated products together with polymers.
The exceptional reactivity of pyrrole is evidenced by its reaction with iodine bottom left equation , and formation of 2-acetylpyrrole by simply warming it with acetic anhydride no catalyst. Reactions of pyrrole require careful evaluation, since N-protonation destroys its aromatic character. For example, pyrrole reacts with acetic anhydride or acetyl chloride and triethyl amine to give N-acetylpyrrole.
Consequently, the regioselectivity of pyrrole substitution is variable, as noted by the bottom right equation. The intermediate formed by electrophile attack at C-2 is stabilized by charge delocalization to a greater degree than the intermediate from C-3 attack. From the Hammond postulate we may then infer that the activation energy for substitution at the former position is less than the latter substitution. Functional substituents influence the substitution reactions of these heterocycles in much the same fashion as they do for benzene.
Indeed, once one understands the ortho-para and meta-directing character of these substituents, their directing influence on heterocyclic ring substitution is not difficult to predict.
Hantzsch-Widmann nomenclature may be applied in the naming of unsaturated, as well as saturated, monocyclic heterocycles. According to this nomenclature system, the name of a heterocycle is composed of a prefix that denotes the heteroatom and a suffix see table below that determines the ring size and the degree of the ring's saturation. In addition, the suffixes distinguish between nitrogen-containing heterocycles and heterocycles that do not contain a nitrogen ring atom. The prefixes applied in Hantzsch-Widman nomenclature are "aza" for nitrogen, "oxa" for oxygen, and "thia" for sulfur. If the prefixes are combined with the suffixes, the last letter of the prefix is left out.
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