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Physical Organic 2015-2016 Coloured Lecture 2 Aromaticity

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Physical Organic 2015-2016 Coloured Lecture 2 Aromaticity DAMIETTA UNIVERSITY CHEM-204: PHYSICAL ORGANIC CHEMISTRY LECTURE 2 Dr Ali El-Agamey Aromaticity—Hückel’s Rule HydrogenationUnusual of just oneStability double bond in benzene is endothermic! => Chapter 163 Resonance Energy • This difference between the predicted and the observed value is called the resonance energy. Chapter 164 Reactivity of naphthalene as compared to benzene Reactivity Stability of the As R.E. increases decreases compound increases 5 The Criteria for Aromaticity—Hückel’s Rule Four structural criteria must be satisfied for a compound to be aromatic. [1] A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms. [2] A molecule must be planar. All adjacent p orbitals must be aligned so that the π electron density can be delocalized. Since cyclooctatetraene is non-planar, it is not aromatic, and it undergoes addition reactions just like those of other alkenes. [3] A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. [4] A molecule must satisfy Hückel’s rule, and contain a particular number of π electrons. Hückel's rule: Benzene is aromatic and especially stable because it contains 6 π electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 π electrons. Considering aromaticity, a compound can be classified in one of three ways: 1. Aromatic—A cyclic, planar, completely conjugated compound with 4n + 2 π electrons. 2. Antiaromatic—A cyclic, planar, completely conjugated compound with 4n π electrons. 3. Not aromatic (nonaromatic)—A compound that lacks one (or more) of the following requirements for aromaticity: being cyclic, planar, and completely conjugated. Pi bond (π) is formed by sideways overlap of two parallel p orbitals p175c Energy Diagram for Benzene • The six electrons fill three bonding pi orbitals. • All bonding orbitals are filled (“closed shell”), an extremely stable arrangement. => Chapter 1614 Energy Diagram for Cyclobutadiene • Following Hund’s rule, two electrons are in separate orbitals. • This diradical would be very reactive. => Chapter 16 15 Examples of Aromatic Rings • Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2 π electrons. • Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes. • To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene. • [10]-Annulene has 10 π electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other. Thus, the ring puckers to relieve this strain. • Since [10]-annulene is not planar, the 10 π electrons can’t delocalize over the entire ring and it is not aromatic. [N]Annulenes • [4]Annulene is antiaromatic (4N e-’s) • [8]Annulene would be antiaromatic, but it’s not planar, so it’s nonaromatic. • [10]Annulene is aromatic except for the isomers that are not planar. • Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. => Chapter 1618 Both negatively and positively charged ions can be aromatic if they possess all the necessary elements. We can draw five equivalent resonance structures for the cyclopentadienyl anion. Cyclopentadienyl Ions • The cation has an empty p orbital, 4 electrons, so it is antiaromatic. • The anion has a nonbonding pair of electrons in a p orbital, 6 electrons, it is aromatic. Chapter 16 20 • Two or more six-membered rings with alternating double and single bonds can be fused together to form polycyclic aromatic hydrocarbons (PAHs). • There are two different ways to join three rings together, forming anthracene and phenanthrene. • As the number of fused rings increases, the number of resonance structures increases. Naphthalene is a hybrid of three resonance structures whereas benzene is a hybrid of two. Indicate which of the following are aromatic? C is aromatic 4(3)+2=14 Which of the following is aromatic? C is aromatic 10 pi electrons, 4(2)+2=10 and completely conjugated b/c lone pair is in a p orbital. Which are antiaromatic? Which of these is antiaromatic? D Pyridine Is Aromatic • Pyrrole is another example of an aromatic heterocycle. It contains a five-membered ring with two π bonds and one nitrogen atom. • Pyrrole has a p orbital on every adjacent atom, so it is completely conjugated. • Pyrrole has six π electrons—four from the π bonds and two from the lone pair. • Pyrrole is cyclic, planar, completely conjugated, and has 4n + 2 π electrons, so it is aromatic. Pyrrole Is Aromatic Allotropes of Carbon • Amorphous: small particles of graphite; charcoal, soot, coal, carbon black. • Diamond: a lattice of tetrahedral C’s. • Graphite: layers of fused aromatic rings Chapter 1628 Some New Allotropes • Fullerenes: 5- and 6-membered rings arranged to form a “soccer ball” structure. • Nanotubes: half of a C60 sphere fused to a cylinder of fused aromatic rings. Chapter 1629 .
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