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CHMB42H3 (16)
Chapter 15

CHAPTER 15 - Textbook Notes.docx

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Lana Mikhaylichenko

CHAPTER 15: AROMATICITY • REACTIONS OF BENZENE  Aromatic Compounds – a cyclic and planar compound with an uninterrupted ring of p orbital-bearing atoms containing an odd number of pairs of π electrons.  Aliphatic Compounds – a nonaromatic organic compound. 15.1 Aromatic Compounds are Unusually Stable  Benzene is a planar, cyclic compound with two cyclic clouds of delocalized π electrons. Because its π electrons are delocalized, all the C–C bonds in benzene have the same length, shorter than a typical single bond but longer than a typical double bond.  Benzene is a particularly stable compound because it has an unusually large delocalization energy.  Delocalization energy (also called resonance energy) tells us how much more stable a compound with delocalized electrons is than it would be if all its electrons were localized.  Compounds with unusually large resonance energies, like benzene, are called aromatic compounds.  Aromatic compounds are particularly stable. 15.2 The Two Criteria for Aromaticity To be classified as aromatic, a compound must meet both of the following criteria: 1. It must have an uninterrupted cyclic cloud of π electrons (called a π cloud) above and below the plane of the molecule. Let's look a little more closely at what this means: For the π cloud to be cyclic, the molecule must be cyclic. For the π cloud to be uninterrupted, every atom in the ring must have a p orbital. For the π cloud to form, each p orbital must overlap with the p orbitals on either side of it. Therefore, the molecule must be planar. 2. The π cloud must contain an odd number of pairs of π electrons.  Therefore, benzene is an aromatic compound because it is cyclic and planar, every carbon in the ring has a p orbital, and the π cloud contains three pairs of π electrons.  For a compound to be aromatic, it must be cyclic and planar and have an uninterrupted cloud of π electrons. The π cloud must contain an odd number of pairs of π electrons.  Huckel’s Rule (4n + 2 rule) – states that for a compound to be aromatic, its cloud of electrons must contain (4n + 2) π electrons, where n is an integer (any whole number). This is the same as saying that the electron cloud must contain an odd number of pairs of π electrons.  Because there are two electrons in a pair, Huckel’s rule requires that an aromatic compound have 1, 3, 5, 7, 9, and so on pairs of π electrons. Thus, Huckel’s rule is just a mathematical way of saying that an aromatic compound must have an odd number of pairs of π electrons. 15.3 Applying the Criteria For Aromaticity  Monocyclic hydrocarbons with alternating single and double bonds are called annulenes.  A prefix in brackets denotes the number of carbons in the ring.  Ex of annulenes are:  Cyclobutadiene has 2 pairs of π electrons, and cyclooctateraene has 4 pairs of π electrons, therefore these compounds are not aromatic because they have an even number of pairs of π electrons. Cyclooctatetraene is also not planar, it is tub-shaped.  Cyclopentadiene is not aromatic because it does not have an uninterrupted ring of p orbital – 3 2 bearing atoms. One of its ring atom is sp hybridized, and only sp and sp carbons have p orbitals.  The cyclopentadienly cation also is not aromatic because, although it has an uninterrupted ring p orbital-bearing atoms, its π cloud has two (an even #) pairs of π electrons.  The cyclopentadienyl anion is aromatic: it has uninterrupted ring of p orbital-bearing atoms, and the π cloud contains three (an odd #) pairs of delocalized π electrons.  The cyclopentadienyl anion’s lone-pair electrons from a π bond in the resonance contributors. Thus, they are π electrons.  The criteria that determine whether a monocyclic hydrocarbon is aromatic can also be used to determine whether a polycyclic hydrocarbon is aromatic. Naphthalene (5 pairs of π electrons), phenanthrene (7 pairs of π electrons), and chrysene (9 pairs of π electrons) are aromatic. 15.4 Aromatic Heterocyclic Compounds  A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is an atom other than carbon. The atom that is not carbon is called a heteroatom.  The most common heteroatoms are N, O, and S.  Heterocyclic Compounds:  Pyridine is an aromatic heterocyclic compound. Each of the six ring atoms of pyridine is sp2 hybridized, which means that each has a p orbital, and the molecule contains three pairs of π electrons. The lone pair electrons on the nitrogen are not π electrons.  The lone-pair electrons on the nitrogen atom of the pyrrole are π electrons. The resonance contributors show that lone-pair electrons form a π bond, thus they are π electrons. Pyrrole, therefore, has 3 pairs of π electrons and is aromatic.  Furan and thiophene are sta2le aromatic compounds. Both the oxygen in f2ran and the sulphur in thiophene are sp hybridized and have one lone pair in an sp orbital.  Quinoline, indole, imidazole, purine, and pyrimidine are other examples of heterocyclic aromatic compounds: 15.5 Some Chemical Consequences of Aromaticity  The strength of an acid is determined by the stability of its conjugate base: the more stable its conjugate base, the stronger is the acid.  Alkyl halides tend to be relatively nonpolar covalent compounds – they are soluble in nonpolar solvents and insoluble in water. 15.6 Antiaromaticity  An aromatic compound is more stable than an analogous cyclic compound with localized electrons.  An antiaromatic compound is less stable than an analogous cyclic compound with localized electrons.  Aromaticity is characterized by stability, whereas an
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