Compuestos Aromáticos

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Transcripción de la presentación:

Compuestos Aromáticos Organic Chemistry, 6th Edition L. G. Wade, Jr. Compuestos Aromáticos Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2006, Prentice Hall

Descubrimiento de Benceno Michael Faraday lo aisló en el 1825 y determinó su fórmula empírica (CH). Eilhard Mitscherlich lo sintetizó en el 1834 y determinó su F.M. C6H6. Otros compuestos con baja razón de C/H olían bien, por lo tanto se les llamó aromáticos. => Chapter 16

Estructura Kekulé La propuso Friedrich Kekulé en 1866. Los 3 dobles enlaces se alternan con los 3 enlaces simples. No explica la existencia de un sólo isómero para el 1,2-dichlorobenceno. => Chapter 16

(Problemas, Explicaciones y Antecedentes) Estructura Kekulé (Problemas, Explicaciones y Antecedentes) Chapter 16

(Comportamiento químico Anormal) Estructura Kekulé (Comportamiento químico Anormal) Chapter 16

Estructura de Resonancia Los 6 C tienen hibridación sp2 y tienen un orbital p no hibridado perpendicular al anillo. => Chapter 16

Estabilidad de Benceno => Chapter 16

Anulenos Se pensó que todos los HC cíclicos conjugados eran aromáticos. Ciclobutadieno es tan reactivo que no se puede aislar porque dimeriza rápidamente. Cicloctatetraeno reacciona con Br2. Chapter 16

Orbitales Moleculares de Benceno Interacción (solapamiento) de 6 orbitales p deben generar 6 orbitales moleculares. 3 enlazantes y 3 antienlazantes. Chapter 16

Orbitales Moleculares de Benceno => Chapter 16

Diagrana de Energía para Benceno Los seis electrones ocupan tres orbitales π enlazantes. Todos los orbitales enlazantes están completos formando una estructura extremadamente estable. => Chapter 16

OM para Ciclobutadieno => Chapter 16

Diagrama de Energía para Ciclobutadieno De acuerdo a la Regla de Hund. Este diradicalThis diradical would be very reactive. => Chapter 16

Regla del Polígono El diagrama de energía de un anuleno tiene la misma forma que el compuesto cíclico con uno de los vértices al fondo. => Chapter 16

Requisitos de Aromaticidad Estructura cíclica con enlaces pi conjugados. Cada átomo en el anillo debe tener un orbital p no hibridado. Los orbitales p deben interaccionar alrededor de todo el anillo (estructura planar). El compuesto es mucho más estable que su análogo de cadena abierta. => Chapter 16

Anti- y No-aromático Compuestos Antiaromáticos son sistemas conjugados, cíclicos con solapamiento continuo de orbitales p alrededor del anillo, pero su energía es mayor que su análogo de cadena abierta. Compuestos No-aromáticos no tienen solapamiento continuo de orbitales p y pueden ser no planares. => Chapter 16

Regla de Hückel Si el compuesto tiene solapamiento continuo de orbitales p alrededor del anillo y tiene (4N + 2) electrones, es aromático. Si el compuesto tiene solapamiento continuo de orbitales p alrededor del anillo y tiene (4N ) electrones, es antiaromático. => Chapter 16

[N]Anulenos [4]Anuleno es antiaromático (4N e-’s) [8]Anuleno debiera ser antiaromático, pero no es planar, por lo tanto es no-aromático. [10]Anuleno es aromático excepto para los isómeros que son no planares. Los anulenos más grandes 4N no son antiaromáticos porque tienen la flexibilidad suficiente para ser no planares. => Chapter 16

MO Derivation of Hückel’s Rule Lowest energy MO has 2 electrons. Each filled shell has 4 electrons. => Chapter 16

Cyclopentadienyl Ions The cation has an empty p orbital, 4 electrons, so antiaromatic. The anion has a nonbonding pair of electrons in a p orbital, 6 e-’s, aromatic. => Chapter 16

Acidity of Cyclopentadiene pKa of cyclopentadiene is 16, much more acidic than other hydrocarbons. => Chapter 16

Tropylium Ion The cycloheptatrienyl cation has 6 p electrons and an empty p orbital. Aromatic: more stable than open chain ion. => Chapter 16

Dianion of [8]Annulene Cyclooctatetraene easily forms a -2 ion. Ten electrons, continuous overlapping p orbitals, so it is aromatic. => Chapter 16

Pyridine Heterocyclic aromatic compound. Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8. => Chapter 16

Pyrrole Also aromatic, but lone pair of electrons is delocalized, so much weaker base. => Chapter 16

Basic or Nonbasic? Pyrimidine has two basic nitrogens. Imidazole has one basic nitrogen and one nonbasic. Purine? => Chapter 16

Other Heterocyclics => Chapter 16

Fused Ring Hydrocarbons Naphthalene Anthracene => Phenanthrene Chapter 16

Reactivity of Polynuclear Hydrocarbons As the number of aromatic rings increases, the resonance energy per ring decreases, so larger PAH’s will add Br2. (mixture of cis and trans isomers) => Chapter 16

Larger Polynuclear Aromatic Hydrocarbons Formed in combustion (tobacco smoke). Many are carcinogenic. Epoxides form, combine with DNA base. => pyrene Chapter 16

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 16

Diamond One giant molecule. Tetrahedral carbons. Sigma bonds, 1.54 Å. Electrical insulator. => Chapter 16

Graphite Planar layered structure. Layer of fused benzene rings, bonds: 1.415 Å. Only van der Waals forces between layers. Conducts electrical current parallel to layers. => Chapter 16

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 16

Fused Heterocyclic Compounds Common in nature, synthesized for drugs. => Chapter 16

Common Names of Benzene Derivatives => Chapter 16

Disubstituted Benzenes The prefixes ortho-, meta-, and para- are commonly used for the 1,2-, 1,3-, and 1,4- positions, respectively. => Chapter 16

3 or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1. => Chapter 16

Common Names for Disubstituted Benzenes => Chapter 16

Phenyl and Benzyl Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon. => Chapter 16

Physical Properties Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. Density: More dense than nonaromatics, less dense than water. Solubility: Generally insoluble in water. => Chapter 16

IR and NMR Spectroscopy C=C stretch absorption at 1600 cm-1. sp2 C-H stretch just above 3000 cm-1. 1H NMR at 7-8 for H’s on aromatic ring. 13C NMR at 120-150, similar to alkene carbons. => Chapter 16

Mass Spectrometry => Chapter 16

UV Spectroscopy => Chapter 16

End of Chapter 16 Chapter 16