Heterocyclic compound, any of a major class of organic chemical compounds characterized by the fact that some or all of the atoms in their molecules are joined in rings containing at least one atom of an element other than carbon c. The novel concept of disk aromaticity proposed as a measure of the aromatic character of planar doublering boron clusters is applied in this context for poly heterocyclic compounds. Many functional groups can be added to aromatic compounds via electrophilic aromatic substitution reactions. Although aromaticity is not an experimentally measurable quantity, many indexes according to different criteria have been. Benzene and aromaticity h h h h h h h h h h h h c 6h 6 2. Heterocycles cyclic organic compounds are carbocycles or heterocycles. Aromaticity in heterocyclic compounds covers hot topics of frontier research summarized by read more.
Keywords polyheterocyclic compounds electron localization. Youll commonly see oxygen, nitrogen, and even sulfur in heterocyclic aromatic compounds. This chapter discusses aromaticity of heterocycles. Dft prediction of multitopic nheterocyclic carbenes using.
C bond formation using enols, enolates, enamines r1 r2 o p2s5 r1 r2 s r sh r1 s r2 thioketones thiols thioethers oh o r h2n o nh2 h2n nh nh2. Heterocyclic compoundsare organic compounds that contain a ring structure containing atoms in addition to carbon, such as sulfur, oxygen or nitrogen, as the heteroatom. Aromatic compounds early in the history of organic chemistry late. Pyridine, for example, is such an aromatic heterocyclic compound. Check the illustrated glossary of organic chemistry available at the course web site for.
After completing this section, you should be able to. Aromatic heterocycles i aromatic compounds organic. Aromaticity in heterocyclic compounds springerlink. The chemistry of heterocyclic compounds is as logical as that of aliphatic or aromatic compounds. Treat it just as you would a carbon atom and follow the checklist. Both compounds have 14 carbon atoms and 10 hydrogen atoms. Pdf this chapter is devoted to a theoretical analysis of the nature of. N bond formation using imines, enamines, acetals, enols, enol ethers. Mar 12, 2008 heteroatoms in organic compounds are usually nitrogen, oxygen, or sulfur atoms. Aromaticity denotes the significant stabilization of a ring compound by a system of alternating single and double bondscalled a cyclic conjugated systemin which six. Heterocyclic compounds have an element other than carbon in the ring. Mar 11, 2019 aromatic antiaromatic nonaromatic pdf admin march 11, 2019 march 11, 2019 no comments on aromatic antiaromatic nonaromatic pdf this study also shows omission behavior of some heterocyclic compounds with respect to their aromaticanti aromaticnon aromatic nature due.
Cyclopentadiene and cycloheptatrienewhich is more acidic. Preparation and reactions of heterocyclic compounds ii fivemembered rings 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. How to use aromaticity to predict the acidity and basicity. Heterocyclic compound the nature of heteroaromaticity.
Note how pyridine the third example has a lone pair but it doesnt count for purposes of aromaticity. Typical hetero atoms include nitrogen, oxygen, and sulfur. About onehalf of all known compounds contain a heterocyclic ring, and many of these, an aromatic heterocyclic ring. In contrast, when comparisons are restricted to some regions or groups of compounds, e. The cyclic part from greek kyklos, meaning circle of. And we have filled the bonding molecular orbitals of benzene, which confers the extra stability that we call aromaticity or aromatic stabilization. Heterocyclic chemistry is the branch of organic chemistry dealing with the synthesis, properties, and applications of these heterocycles. Aromaticity in heterocyclic compounds,download aromaticity in heterocyclic compounds,free download aromaticity in heterocyclic compounds,aromaticity in heterocyclic compounds pdf ebooks, read aromaticity in. Aromatic heterocyclic compounds chemistry libretexts.
However, in the case of heterocyclic systems, the aibic overestimates the aromaticity indicating many of them to be more aromatic than benzene, which seems unlikely. Aromaticity16 heterocyclic aromatic systems2furan,pyrrole and thiophene. Existence of several multitopic n heterocyclic carbene nhc ligands with up to four carbene centers have been predicted on the basis of clars aromatic sextet theory. The data for compounds 1be suggest that the antitb effect depends on the aromaticity of the ring attached to the amide group, particularly if this ring has low aromaticity phenyl thiophene.
When dealing with aromatic compounds, you often need to predict the acidities and the basicities of double bondcontaining rings, including aromatic rings. To determine the stability and reactivity of heterocyclic compounds, it is useful to compare them with their carbocyclic analogs. Electron boronheterocyclic aromatic rings benzene and cyclopentadienyl cp anion are the two most common and simplest 6. Aromaticity in heterocyclic compounds pdf free download. Heterocyclic compound heterocyclic compound the nature of heteroaromaticity. All electrophilic aromatic substitution reactions share a common mechanism. Sep 30, 2019 keeping this in mind, in this article, i have introduced three time economic innovative mnemonics by using two formulae for the prediction of hybridization state of hetero atom of heterocyclic compounds to determine its planarity and aromatic anti aromatic non aromatic nature of heterocyclic compounds containing one, two or more number of. The aromatic heterocycle pyridine is similar to benzene, and is often used as a weak base for scavenging protons. The tricky aspect of heterocyclic compounds arises with lone pairs.
Heterocyclic molecules can also participate in aromaticity. Aromatic compounds heterocyclic compound aromaticity. Aromaticity in heterocyclic compounds tadeusz marek. Aromaticity is a property in organic chemistry of cyclic chainshaped, planar flat structures with a ring of resonance bonds that gives greater stability compared to other geometric or connective arrangements with the same collection of atoms. We also acknowledge previous national science foundation support under grant numbers 1246120.
Wed have to weigh the favorable effect of aromaticity against the unfavorable effect of putting a positive charge into an electronegative ring. The second lone electron pair of oxygen and sulfur in furan and thiophene, respectively, is located in an sp 2 orbital that is parallel to the ring plane. Hence thermodynamic stability was defined as resonance energy 5, 6 and was the first quantitative measure of aromaticity. Hence, broadly considered, the various manifestations of aromaticity are related and aromaticity can be regarded statistically as a onedimensional phenomenon.
Recently, we proposed an aromaticity index based on interaction coordinates aibic j. The concept of aromaticity in heterocyclic chemistry. Heterocyclic compounds occur widely in nature and in a variety of nonnaturally occurring compounds. A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesnt display the bromination of benzene.
It was also quickly discovered that these aromatic compounds did not react like other. Here, we discuss the aromaticity of some of these heteroaromatic compounds using different descriptors of aromaticity, with special emphasis in those measures based on electron delocalization properties of the aromatic rings. Medicinal chemistry heterocyclic compounds free download as powerpoint presentation. These molecules are called heterocyclic compounds because they contain 1 or more different atoms other than carbon in. Chemistry department of chemistry, university of nairobi, p. Medicinal chemistry heterocyclic compounds heterocyclic. A heterocyclic compound is an organic compound in which one or more of the carbon atoms in the backbone of the molecule has been replaced by an atom other than carbon.
If youre seeing this message, it means were having trouble loading external resources on our website. Preparation and reactions of heterocyclic compounds ii. According to the rules for aromaticity, they are aromatic compounds which may additionally be substantiated through the application of the frost circle. Aromatic heterocyclic compounds mcc organic chemistry. A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its rings. Aromaticity in heterocyclic compounds topics in heterocyclic. Organic chemistry ii chem 252 chapter 14 aromatic compounds. So far, you have encountered many carbon homocyclic rings, but compounds with elements other than carbon in the ring can also be aromatic, as long as they fulfill the criteria for aromaticity. Aromaticity is special stability provided to a molecule upon possessing four specific qualities mentioned below.
Again the maps of aelectron densities of typical compounds, derived theoretically, are reproduced without any clear indicationas to how theymaybeused. If youre behind a web filter, please make sure that the domains. Huckels rule is strictly for monocyclic aromatic compound, its application to polycyclic aromatic compounds is tenuous. The most difficult aromaticity topic involves charged compounds cations and anions as well as heterocyclic compounds, molecules that have more than just carbon and oxygen in their ring. Aromaticity also occurs in compounds that are not carbocyclic or heterocyclic.
Journal of heterocyclic chemistry 1968, 5 4, 461465. An introduction to the chemistry of heterocyclic compounds. Heteroaromatics are found in very many of the products of both primary and secondary metabolism as well as in many synthetic compounds of commercial interest such as drugs, pest control agents, co louring agents, flavourings. Aromatic antiaromatic nonaromatic pdf this study also shows omission behavior of some heterocyclic compounds with respect to their aromaticanti aromaticnon aromatic nature due.
Aromaticity is a notion that appeared in the midnineteenth century to differentiate between unsaturated hydrocarbons and formally unsaturated benzene. At the end of the nineteenth century it seemed that cyclicity was a necessary condition for differentiation between the two. Because these compounds are monocyclic aromatic compounds, they must obey huckels rule. Basicity of aromatic heterocycles organic chemistry video. For example, benzene reacts with bromine to form bromobenzene. Prediction of aromatic, anti aromatic and non aromatic. How to use aromaticity to predict the acidity and basicity of. However, unlike phenol were the equilibrium lies far to the aromatic phenol side, these compounds usually it depends on the substituents exist in the keto form.
Furan and pyrrole have heterocyclic fivemembered rings, in which the heteroatom has at least one pair of nonbonding valence shell electrons. Heterocyclic systems are important buildingblocks for new materials possessing. Heterocyclic chemistry is the biggest branch of chemistry covering twothirds of the chemical literature. The aromaticity of heterocycles rings containing one or more noncarbon atoms. This usually originates from the compounds occurrence, its first preparation or its special properties. At the end of the nineteenth century it seemed that cyclicity was a necessary condition. Determining if a molecule is cyclic is pretty straightforward. Heterocyclic compound, also called heterocycle, any of a major class of organic chemical compounds characterized by the fact that some or all of the atoms in their molecules are joined in rings containing at least one atom of an element other than carbon c. For example, borazine is a sixmembered ring composed of alternating boron and nitrogen atoms, each with one hydrogen attached. Aromatic compounds definition, example, properties. This is evident in many biological molecules from amino acid side chains to the nitrogenous bases on dna and rna.
Aromaticity tutorial for cyclic, charged and heterocyclic aromatic. N bond formation using imines, enamines, acetals, enols, enol ethers c. Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved. The journal of physical chemistry a 2014, 118 49, 1159111601. And at the beginning of the present century, the first allmetal heteroaromatic cluster was detected. A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is an atom other than carbon examples of heterocyclic aromatic compounds aromatic heterocyclic compounds a cyclic compound in which one or more of the ring atoms is an atom other than carbon a heteroatom is a ring atoms that is. Aromaticity tutorial for cyclic, charged and heterocyclic. The strength of the keto and amide bonds outweigh the weaker aromaticity of the pyrimidine skeleton and shift the equilibrium to the keto side. Aromatic compound an overview sciencedirect topics.
Quantification of aromaticity of heterocyclic systems. A large number of heterocyclic compounds are essential. As for the second compound, we would need to polarize two carbonyl groups and concentrate two positive charges into the small ring, and that is not happening even with aromaticity. The essential property of b zenelike compounds, often identified with aromatic compounds, was low react ity. These molecules are called heterocyclic compounds because they contain 1 or more different atoms other than carbon in the ring. Pyrrole is a heterocyclic aromatic organic compound, a fivemembered ring with the formula c4h4nh. To determine if a molecule is aromatic, investigate its structure. Pdf chemical bonding and aromaticity in polyheterocyclic. For example, you may need to determine which one of two double bondcontaining rings is more acidic, such as the molecules shown here.
Heterocyclic chemistry university of nairobi personal websites. Assessment on stability and reactivity of nhcs was made by quantifying aromaticity, aromatic stabilization energy earoma, strength of carbene lone pair, proton affinity, and cucl binding energy. The compound with the maximum number of noncumulative double bonds is regarded as the parent compound of the monocyclic systems of a given ring size. Heteroatoms in organic compounds are usually nitrogen, oxygen, or sulfur atoms. In the next few videos, were going to look at several other examples of aromatic compounds and ions. Heterocyclic aromatic compounds the nitrogen in pyrrole is sp 2 hybridized and the lone pair is in the p orbital this p orbital contains two electrons and is in the aromatic system the lone pair of pyrrole is part of the aromatic system and not available for protonation. Their study is of great interest both from the theoretical as well as practical standpoint. Look for the following 4 criteria to identify aromatic compounds.
Bromination studies of certain purines, pyrrolo3,2 d pyrimidines and pyrazolo4,3 d pyrimidines. In case of substituents, the heteroatom is designated number 1, and the substituents around the chain are numbered so as to have the lowest number for the substituents. The rich chemistry of these two rings and their derivatives has driven chemists to explore the possible heteroaromatic rings by substitution of one or. To determine if a molecule is aromatic, investigate its structure for the qualifiers. Chemical bonding and aromaticity in polyheterocyclic. Aromaticity of charged and heterocyclic compounds youtube. Pdf superhyperhalogen aromatic heterocyclic compounds. The 1 h and c nmr spectra of triafulvene 1 both protons and carbon atoms of the 3membered ring moiety display resonances in the region of aromatic compounds evidence a significant contribution of the resonance form 1b aromatic charge separation. This chapter is devoted to a theoretical analysis of the nature of chemical bonding, and in particular the aromaticity, of some intriguing polyheterocyclic compounds which. The cyclic part from greek kyklos, meaning circle of heterocyclic indicates. This index works well for the aromatic hydrocarbons.
Heterocyclic aromatic compounds 26 heterocyclic compounds have an element other than carbon in the ring example of aromatic heterocyclic compounds are shown below numbering always starts at the heteroatom pyridine has an sp 2 hybridized nitrogen the p orbital on nitrogen is part of the aromatic. Pyridine c 5 h 5 n, pyrrole c 4 h 5 n, furan c 4 h 4 o, and thiophene c 4 h 4 s are examples of heteroaromatic compounds. There are three systems for naming heterocyclic compounds. In addition, a recent monograph on general heterocyclic chemistry emphasizes the importance of heterocyclic compounds in the. Aromaticity as a cornerstone of heterocyclic chemistry. So the rule that we are going to use for this is that acids can only react with the lone pairs that are not necessary for aromaticity like we just said if a lone pair is making a molecule stable, why would it make sense for that lone pair to react with an h and then make the molecule non aromatic that molecule wants to stay aromatic so that lone pair that is being donated to the ring is not. In heterocyclic compounds one or more carbon atoms of the ring are displaced by atoms of other elements, which are then called heteroatoms.
Both molecules consist of three fused rings that are shown with a series of alternating single and double bonds. Aromaticity tutorial for cyclic, charged and heterocyclic aromatic compounds february 18, 2016 by leah4sci 10 comments aromaticity is one of the more exciting topics in organic chemistry, typically covered right before electrophilic aromatic substitution. Effect of benzoannelation on local aromaticity in heterocyclic conjugated compounds. Cyclic compounds whose rings do not only contain carbon atoms may be aromatic, as well. It is a colourless volatile liquid that darkens readily upon exposure to air.