Organic Chemistry - Huckle Rule PDF

Huckle Rule Aromaticity & Hückel (4n + 2) Rule Aromaticity and the Hückel (4n + 2) Rule: In 1931, German chemist and physicist Sir Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties. His rule states that if a cyclic, planar molecule has (4n+2 π)...

Huckle Rule Aromaticity & Hückel (4n + 2) Rule Aromaticity and the Hückel (4n + 2) Rule: In 1931, German chemist and physicist Sir Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties. His rule states that if a cyclic, planar molecule has (4n+2 π) electrons, it is considered aromatic. This rule would come to be known as Hückel's Rule. Aromaticity & Hückel (4n + 2) Rule 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). 3. The molecule is fully conjugated (p orbitals at every atom in the ring). 4. The molecule has (4 n+ 2 π) electrons (n = 0 or any positive integer). Aromaticity & Hückel (4n + 2) Rule To apply the 4n+2 rule, first count the number of π electrons in the molecule. Then, set this number equal to 4n+2 and solve for n. If is 0 or any positive integer (1, 2, 3,...), the rule has been met. Aromaticity & Hückel (4n + 2) Rule 4n + 2 = π 4n + 2 = 6 4n = 6 – 2 4n = 4 n = 4/4 n = 1 For benzene, we find that n = 1, which is a positive integer, so the rule is met. Molecules that have the 3 characteristics listed above (cyclic, conjugated, flat) and have this number of π electrons [4n +2] will be aromatic. The letter “n” is not a characteristic of the molecule because “n” comes from algebra, NOT from chemistry. Aromaticity & Hückel (4n + 2) Rule Aromaticity, Antiaromaticity and the Hückel (4n + 2) Rule: (I) Condition-1: The Molecule Must Be Cyclic Determining if a molecule is cyclic is pretty straight forward. Then, move to condition (2). If there’s no ring, forget it. Because as for example: (Z)-1,3,5 hexatriene has the same number of pi bonds (and pi electrons) as benzene, but isn’t aromatic. No ring, no aromaticity. NOTE BY: If an Atom has 1 or more lone pair electron and is attached to an sp2 atom then this atom is also sp2 atom. Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule (II) Condition-2: Every atom in the ring must be conjugated Obviously, being cyclic isn’t a sufficient condition for aromaticity. “Every atom in the ring must have an available p orbital”, or “Every atom in the ring must be able to participate in resonance”. In order for aromaticty to exist, there must also be a continuous ring of p-orbitals around the ring that build up into a larger cyclic “pi (π) system”. Aromaticity & Hückel (4n + 2) Rule Remember that the “available p orbital” condition applies not just to atoms that are part of a pi (π) bond, but also atoms bearing a lone pair, a radical, or an empty p orbital (e.g. carbocations). Note By: The key thing that “kills” conjugation is a Sp3 hybridized atom with four bonds to atoms. Such an atom cannot participate in resonance. Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule (III) Condition-3: The Molecule Must Have [4n+2] Pi (π) Electrons The third condition is that the cyclic, conjugated molecule must have the correct number of pi (π) electrons. Benzene and Cyclooctatetraene are both cyclic and conjugated, but benzene is aromatic and Cyclooctatetraene is not. The difference is that benzene has 6 pi (π) electrons, and Cyclooctatetraene has 8 pi (π) electrons. Aromaticity & Hückel (4n + 2) Rule 4n+2 are not a formula which applies to see the molecule is aromatic. It is a formula that tells what numbers are in the magic series. If pi (π) electron value matches any number in this series then they have the capacity for aromaticity.” Aromaticity & Hückel (4n + 2) Rule The “magic series” is: 2, 6, 10, 14, 18, 22….. So for n = 0 , we have [4 (0) + 2] = 2 for n = 1 , we have [4 (1) + 2 ] = 6 for n = 2, we have [4 (2) + 2 ] = 10 for n = 3, we have [4 (3) +2 ] = 14 The condition that aromatic molecules must have [4n+2] pi (π) electrons is sometimes called “Huckel’s rule”. Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule (IV) Condition-4: The Molecule Must Be Flat The fourth condition for aromaticity is that the molecule must be flat (planar). Aromaticity is such a stabilizing property (worth 20-36 kcal/mol) that generally a molecule that is cyclic conjugated. Has (4 n+ 2) pi (π) electrons. Aromaticity & Hückel (4n + 2) Rule Aromatic Hydrocarbon: A cyclic, planar, fully conjugated hydrocarbon with 4n+2 pi electrons (2, 6, 10, 14, 18, etc). An aromatic hydrocarbon is especially stable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms. Nonaromatic Hydrocarbon: A cyclic, non-planar, fully conjugated hydrocarbon with 4n+2 pi electrons. A nonaromatic hydrocarbon has similar stability to its open- chain fully conjugated hydrocarbon of the same number of carbon atoms. Aromaticity & Hückel (4n + 2) Rule Antiaromatic hydrocarbon: A monocyclic, planar, fully conjugated hydrocarbon with 4n pi electrons (4, 8, 12, 16, 20...). An antiaromatic hydrocarbon is especially unstable relative to an open-chain fully conjugated hydrocarbon of the same number of carbon atoms. Aromaticity & Hückel (4n + 2) Rule Count pi electrons in aromatic compounds : π- bonds are simply the second bond made in a double bond Any pure double bond is one sigma/σ and one pi/ π bond. Since any one chemical bond (meaning only one line in bond line notation) contains at most two electrons count two π electrons per double bond and ignore the σ electrons. If lone pairs electron present, consider the molecular geometry, and only the π electrons that are in the ring count towards aromaticity. Aromaticity & Hückel (4n + 2) Rule 1- Aromatic: because 4n+2=6 π electrons in the ring (with n=1), planar, fully conjugated all around, and cyclic. Aromaticity & Hückel (4n + 2) Rule 2- Aromatic: Because 4n+2=6 π electrons in the ring (with n=1), planar, fully conjugated all around, and cyclic. The π electrons in the double bond outside of the ring do not count towards the π electrons one considers for aromaticity. Aromaticity & Hückel (4n + 2) Rule 3- Nonaromatic : because 4n+2≠4 π electrons, where n must be an integer. It's also not conjugated all around, so it's not antiaromatic. The π electrons in the double bond outside of the ring do not count towards the π electrons one considers for aromaticity. Aromaticity & Hückel (4n + 2) Rule 4- Aromatic : because 4n+2=6 π electrons in the ring (with n=1), planar, fully conjugated all around, and cyclic. The lone pair is actually in a pure 2p orbital perpendicular to the ring. Aromaticity & Hückel (4n + 2) Rule 5- Aromatic : Because 4n+2=6 π electrons in the ring (with n=1), planar, fully conjugated all around, and cyclic. The lone pair is actually in a pure 2p orbital perpendicular to the ring, which means they count as π electrons. Aromaticity & Hückel (4n + 2) Rule 6- Aromatic : Because 4n+2=6 π electrons in the ring (with n=1), planar, fully conjugated all around, and cyclic. Only one of the lone pairs is actually in a pure 2p orbital perpendicular to the ring, which means those counts as π electrons. The other lone pair is actually in a σ (actually, sp2) orbital, so it doesn't count. Aromaticity & Hückel (4n + 2) Rule Which Electrons Count As π-Electrons and which types lone pair contribute to the pi (π) system? The total number of pi (π) electrons for the Cyclopentadiene anion equals 2 (from the lone pair) plus the 4 electrons in the two pi (π) bonds, giving us a total of 6. This is a Hückel number and the Cyclopentadiene anion is in fact aromatic. Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule For pyrrole the nitrogen bears a lone pair but is not involved in a π bond (unlike pyridine, above). Therefore it can contribute to the pi (π) system and this gives us a total of 6 pi (π) electrons once we account for the 4 electrons from the two pi (π) bonds. Aromaticity & Hückel (4n + 2) Rule In Furan, the oxygen bears two pairs of lone pair’s electron, but it does not means that furan has 8 pi electrons, because each atom can contribute a maximum of one porbital towards the pi (π) system. In furan, one lone pair is in a p-orbital, contributing to the pi (π) system; the other is in the plane of the ring. This gives us a total of 6 pi (π) electrons. Furan is aromatic. (So is thiophene, the sulfur analog of furan). Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule Pyridine and the Benzene Anion : In benzene, each p-orbital is arranged at right angles (90°) to the plane of the ring. Each p-orbital contains a single electron. The total number of pi (π) electrons in Benzene by counting the pi bonds: 3 pi (π) bonds times two electrons = 6 pi (π) electrons total. Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule The Benzene Anion has a lone pair on one of the carbons. This lone pair can’t be in a p-orbital, since the p-orbital is participating in the pi (π) system. Instead, it’s at 90 degrees to the pi (π) system, in the plane of the ring. This lone pair electron on carbon doesn’t count as a pair of pi (π) electrons since it can’t overlap with the pi (π) system. Pyridine, where the lone pair is also at right angles to the pi (π) system, but it is not participating in the pi (π) system. Aromaticity & Hückel (4n + 2) Rule Aromaticity & Hückel (4n + 2) Rule Molecules with 4n p electrons (4, 8, 12, 16,...) can’t be aromatic, even though they may be cyclic, planar, and apparently conjugated. In fact, planar, conjugated molecules with 4n p electrons are said to be antiaromatic because delocalization of their p electrons would lead to their destabilization. Aromaticity & Hückel (4n + 2) Rule Cyclobutadiene has four p electrons and is antiaromatic. The p electrons are localized in two double bonds rather than delocalized around the ring, as indicated by an electrostatic potential map. Aromaticity & Hückel (4n + 2) Rule Cyclooctatetraene has eight p electrons and is not aromatic. The p electrons are localized into four double bonds rather than delocalized around the ring, and the molecule is tub-shaped rather than planar. Cyclooctatetraene Four double bonds; eight π electron Aromaticity & Hückel (4n + 2) Rule In fact, cyclooctatetraene is not even conjugated because neighboring p orbitals don’t have the necessary parallel alignment for overlap. The p electrons are localized in four discrete C=C Aromaticity & Hückel (4n + 2) Rule