So if every carbon atom in the cyclic compound is $$sp^2$$ hybridized, this means the molecule is fully conjugated (has 1 p orbital at each atom), and the electrons in these p orbitals are the $$\pi$$ electrons. The molecule is planar (all atoms in the molecule lie in the same plane), The molecule has 4n+2 $$π$$ electrons (n=0 or any positive integer). The oxygen has at least 1 lone electron pair and is attached to an sp2 hybridized atom, so it is sp2 hybridized as well. Unfortunately, reasons for not discussing this too much in an introductory class will also emerge; understanding aromaticity even at an elementary level beyond "like benzene" requires understanding orbitals. These 4 fill in the orbitals of the succeeding energy level. The numeric prefix indicates the ring size. Therefore, carbon 1 has a p orbital. (sometimes referred to as a "closed loop of six electrons", which he also calls the "aromatic sextet"). Legal. Huckel's Rule is a useful first step in evaluating the potential for a ringed molecule to be aromatic. Now, the non-aromatic character of cyclooctatetraene alone is not too hard to explain. A bond is known to be polar when the centers of positive and negative charges of the molecule do not coincide with each other. Benzene, C 6 H 6, is the least complex aromatic hydrocarbon, and it was the first one named as such. The formulas and structures of benzene establish it as a purely aromatic hydrocarbon. They occupy a series of p orbitals, one on each C atom. [ "article:topic", "H\u00fcckel\'s rule", "showtoc:no" ], 17.4: The Molecular Orbital Picture of Cyclobutadiene, Huckel's Rule: Aromatic, Antiaromatic, and Nonaromatic. Whereas benzene is aromatic (6 electrons, from 3 double bonds), cyclobutadiene is antiaromatic, since the number of π delocalized electrons is 4, which of course is a multiple of 4. With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4 electrons fill each subsequent energy level (the number of subsequent energy levels is denoted by n), leaving all bonding orbitals filled and no anti-bonding orbitals occupied. Benzene does not have 3 single and 3 double bonds. The typical qualities of benzene and other aromatic compounds are listed below: They are cyclic compounds that contain a cyclic, uninterrupted π electron system. For furan with two lone pairs on the oxygen atom, if we count electrons from the carbon atoms, we have 4 (one per carbon). The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. However, benzene is not the only known aromatic compound; there are many other compounds that also have a considerably high stability and show a similar chemical behavior such as benzene does. The cyclobutadienide (2−) ion, however, is aromatic (6 electrons). This is entirely wrong. A common example is furan, which contains an oxygen atom. First, why is aromaticity an issue? But now look at the anion derived from this chemical -- the ion that would result if this chemical behaved as an acid, and gave off an H+; this ion is also shown below. Cpd 6: Not aromatic - all atoms are sp2 hybridized, but only 1 of S's lone pairs counts as $$\pi$$ electrons, so there 8 $$\pi$$ electrons, n=1.5, Cpd 7: Not aromatic - there are 4 $$\pi$$ electrons, n=1/2, Cpd 8: Aromatic - only 1 of N's lone pairs counts as $$\pi$$ electrons, so there are 6 $$\pi$$ electrons, n=1, Cpd 9: Not aromatic - not fully conjugated, top C is sp3 hybridized, Cpd 10: Aromatic - O is using its 1 p orbital for the elections in the double bond, so its lone pair of electrons are not $$\pi$$ electrons, there are 6 $$\pi$$ electrons, n=1, Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University), Prof. Steven Farmer (Sonoma State University), William Reusch, Professor Emeritus (Michigan State U. Another simple rule to determine if an atom is sp2 hybridized is if an atom has 1 or more lone pairs and is attached to an sp2 hybridized atom, then that atom is sp2 hybridized also. If is 0 or any positive integer (1, 2, 3,...), the rule has been met. It has 2 double bonds, hence has 4 electrons in p orbitals perpendicular to the ring. Hückel's Rule also applies to ions. The key difference between benzene and cyclohexane is that the benzene is an aromatic compound whereas the cyclohexane is a non-aromatic compound.. A molecule is aromatic if it is cyclic, planar, completely conjugated compound with 4n + 2 π electrons. Ka for cyclopentadiene is about 10-16. ), Virtual Textbook of Organic Chemistry. Definition . 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. Cyclooctatetraene has eight, and is not aromatic; its dianion has ten, and is aromatic. (This is known as Hückel's rule.) Study of many molecules and ions, as well as theoretical work that is well beyond our course, have indicated that a species will be aromatic if there are 4n+2 electrons in a planar π electron loop -- where n is any integer, starting with 0. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Note that it now has six electrons in p orbitals perpendicular to the ring -- very similar to pyrrole, above. $$\pi$$ electrons lie in p orbitals and $$sp^2$$ hybridized atoms have 1 p orbital each. A very common misconception is that hybridization can be used to predict the geometry, or that hybridization somehow involves an energy cost associated with 'promoting' electrons into the hybrid orbitals. Thus, it is a separate field of study in organic chemistry. Planarity 2. In a cyclic hydrocarbon compound with alternating single and double bonds, each carbon is attached to 1 hydrogen and 2 other carbons. Therefore, each carbon is $$sp^2$$ hybridized and has a p orbital. Defining aromatic in terms of benzene is a useful start in an introductory course. But as we continue, we find examples of aromatic compounds that contain heteroatoms, charges, and rings of different sizes. In 1931, German chemist and physicist Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties. These include pyrrole, C4H5N, and furan, C4H4O: What do these compounds share with benzene and pyridine that makes them aromatic? Hybridization is always determined by geometry. How do we know that it is fully conjugated?