The compound must contain sp2 hybridised carbon atoms. Criteria for aromaticity 1 the molecule is cyclic a ring of atoms 2 the molecule is planar all atoms in the molecule lie in the same plane. It also has to have 4n plus 2 pi electrons in the ring, where n is equal to 0, 1 2, or any other positive integer. Polycyclic aromatic hydrocarbons aromatic compounds can have rings that share a set of carbon atoms fused rings compounds from fused benzene or aromatic heterocyclic rings are themselves aromatic naphthalene. Conjugation a the special stability provided by three or more adjacent, parallel, overlapping p orbitals. After completing this section, you should be able to. We also acknowledge previous national science foundation support under grant numbers 1246120, 1525057, and 14739. Use these assessments to practice identifying aromatic compounds and understanding of huckel s rule.
Examples of molecules following huckels rule have only been established for values of n ranging from zero to six. Be planar, in an sp2 hybridized orbital, over every atom of the ring. Aromatic, antiaromatic, or nonaromatic, huckels rule, 4n. Organic chemistry notes aromatic compounds and aromaticity. If and only if the molecule in question meets the criteria for being either aromatic or antiaromatic i. Pdf aromaticity, antiaromaticity, homoaromaticity and. Aromaticity of the most stable adenine and purine tautomers. Join indias leading elearning platform for smarter and faster exam preparation. Huckel arrived at this rule by performing molecular orbital calculations on cyclic systems containing x carbon atoms, and with each carbon atom supplying one pi electron.
Section 14 aromatic compounds and aromaticity 141 definition of aromaticity cyclic, planar, fully conjugated array of porbitals. And so if the molecule is planar, thats what allows the p orbitals to overlap. Jan 04, 2015 they probably have a fully conjugated polyunsaturated ring system. A pedestrian approach to the aromaticity of graphene and nanographene. In 1931, german chemist and physicist erich huckel proposed a rule to determine if a planar ring molecule would have aromatic properties. All the carboncarbon bonds are of equal length, and all the bond angles are 120. Indeed, winstein noticed that the only fundamental difference between this aromatic propenyl cation and his nonclassical hexyl cation was the fact that, in the latter ion, conjugation is interrupted by three ch 2 units. Molecules which fully obey the criteria for aromaticity as well as huckel s rule are said to be aromatic and those which follow those rules partially fall in the category of anti aromatic and non aromatic compounds. Non aromatic compounds, are not aromatic due to reasons such as lack of planarity or disruption of delocalization. Sep 03, 2012 aromaticity the term aromaticity is used to describe aromatic compounds properties. Might want to follow that link for more detailed information.
Huckels rule is strictly for monocyclic aromatic compound, its application to polycyclic aromatic compounds is tenuous. For example, the cyclopentadienyl anion and the cycloheptatrienylium cation are both considered to be aromatic ions, and the azulene molecule can be approximated as a combination of both. Any compound that satisfies the first three rules and contains one of these numbers of pi electrons should be aromatic. Huckel aromaticity and frost circles organic chemistry help. Pdf aromaticity, antiaromaticity, homoaromaticity and the. Antiaromaticity is a characteristic of a cyclic molecule with a. Thus, the concept of aromaticity extended to allmetal clusters and a new era in aromatic metal clusters began. Aromaticity means cyclic planar conjugated and huckel s rule for a stable pi system. Aug 07, 2014 all aromatic compounds follow huckels rule. The conditions that lead to aromaticity are as follows.
Chemistry aromatic compounds and huckels rule youtube. Aromatic compounds and aromaticity section 14 of organic chemistry notes is 11 pages in length page 141 through page 1411 and covers all youll need to know on the following lecturebook topics. In organic chemistry, huckels rule estimates whether a planar ring molecule will have aromatic. Huckel s rule for aromaticity states that a molecule must meet all of the following four conditions in order to be aromatic. Molecules which obey these rules are aromatic and those which follow these rules. This includes multiresonating systems, charged and heterocyclic aromatic compounds. Aromaticity, as it turns out, just has to do with these rings as well as huckel s rule. Aromaticity plays a major role in the field of biochemistry of all the living structures. Huckels rule is applied to all molecules which obeys the criteria of aromaticity in cyclopentadienyl anion there are two double bonds which contributes 4. Aromaticity tutorial for cyclic, charged and heterocyclic aromatic compounds.
Aromatic compounds are highly unsaturated, cyclic molecules that show extra stability as compared to their open chain counterpart. Mar 12, 2019 for any system to be aromatic, it has to follow huckels rule according to which. If we look at the frost circle and we look at the molecular orbitals, we can understand huckel s rule a little bit better visually. It consists of a set of 6 sp 2 c arranged in a ring. The obtained aromaticity indices are in line with these results. In particular, consider the molecular orbitals on the two atoms involved in the alkynelike bond. Undergoes substitution reaction rather than addition reaction. Complete delocalization of pie electrons should take place in the atom. Huckels rule is a set of algorithms that combine the number of pi electrons n and the physical structure of the ring system to determine whether the molecule is aromatic, antiaromatic, or nonaromatic. In 2000, andreas hirsch and coworkers in erlangen, germany, formulated a rule to determine when a fullerene would be aromatic. Antiaromaticity antiaromaticity is a characteristic of a cyclic molecule with a.
From these models, rules of aromaticity 11 have been obtained that are commonly used in assessing the aromatic character of molecules. Aromaticity in molecules with more than one ring page 3 rings together, note that the molecule has eight pi electrons within the closed loop, and conclude correctly that the molecule is antiaromatic. How do the huckel and baird rules fade away in annulenes. Review the answer to part b above if youve forgotten how to treat fused ring cases. The most important and well known aromatic system is benzene. For example, cyclopentadienyl anion is an aromatic ion. Huckels rule for aromaticity 6 electrons aromatic heterocycles also can be aromatic. Benzene and aromaticity h h h h h h h h h h h h c 6h 6 2. Aromaticity a much more thorough explanation of what makes a molecule aromatic. In order to be aromatic, a molecule must have a certain number of pi electrons electrons with pi bonds, or lone pairs within p orbitals within a closed loop of parallel, adjacent p. Huckel s rule is used in order to estimate the aromatic qualities of any planar ringshaped molecule in the field of organic chemistry. So a compound is aromatic if it contains a ring of continuously overlapping p orbitals. Thus, the aromaticity of the benzene molecule is established since it obeys the.
Although the number of pi electrons fits, this compound is not aromatic because it is not closed. Doering 1951, although several authors were using this form at around the same time. Aromaticity a extra stability possessed by a molecule that meets specific criteria. Aromatic compounds exhibit ring current induced magnetic shielding, but the reverse conclusion that ring current induced magnetic shielding identifies aromaticity is not justified. Chemist erich huckel was the first one to recognize that an aromatic. Compounds with n 1 the most common examples have n 1. Therefore the lone pair contributes to the aromaticity. Aromaticity definition, example benzene, aromaticity rules. Pdf aromaticity, antiaromaticity, homoaromaticity and the huckel. Aromaticity general organic chemistry chemistry notes. Organic chemistry ii chem 252 chapter 14 aromatic compounds.
Aromaticity tutorial for cyclic, charged and heterocyclic. Thats a rough example, but the gist of huckel s rule is to look for the electron magic numbers 2, 6, 10, 14, etc. Molecules which fully obey the criteria for aromaticity as well as huckels rule are said to be aromatic and those which follow those rules partially fall in the category of antiaromatic and non aromatic compounds. As it takes a minimum of three, adjacent, overlapping p orbitals for planarity, aromaticity. Huckels rule was originally based on calculations using the huckel method, although it can also be justified by considering a particle in a ring system, by the lcao method and by the pariserparrpople method. The following rule s explain the concept of aromaticity.
The planarity of the ring means that the interaction of the adjacent p orbitals is optimal. In order to be aromatic, a molecule must have a certain number of pi electrons electrons with pi bonds, or lone pairs within p orbitals within a closed loop of parallel, adjacent p orbitals. Huckel s rules are a set of few rules to identify an aromatic compound as benzenec6h6. Unhybridized p orbital in continuous cyclic system 3. Spherical aromaticity is aromaticity that occurs in fullerenes.
Economou the journal of physical chemistry c 2015 119 29, 1699117003. The last one is that the organic compound has to be flat. Huckel s rule is a set of algorithms that combine the number of pi electrons n and the physical structure of the ring system to determine whether the molecule is aromatic, antiaromatic, or nonaromatic. Huckel aromaticity and frost circles posted on january 29th, 2019 as orgo 2 lectures start up, there will be a number of concepts that will be covered quickly to speed up the class so you can learn lots of functional group transformations this semester. The supporting quantum mechanics required for the formulation of this rule was solved first by the german physical chemist and physicist erich armand arthur joseph huckel in the year 1931. Aromaticity rules and definition organic chemistry help. This creates a set of 6 p orbitals, no gaps in the. Huckel s rule is a simple rule that sometimes fails.
406 1574 26 1419 51 232 1113 312 1429 877 1144 1498 1060 532 238 1069 610 404 1492 736 536 1320 107 1257 1170 1437 947 283 238 1092 1617 479 1613 62 1229 1374 871 59 1406 90 929 631 713 989 420