310_17 - Organic Organic Lecture Series CH 310 N LECTURE 17...

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Unformatted text preview: Organic Organic Lecture Series CH 310 N LECTURE 17 Textbook Assignment: Chapter 21 Benzene & Aromaticity Homework (for credit): Problem Set – None this week Today’s Topics: Benzene & Aromatic Compounds Notice & Announcements: EXAM 2: Grading in Progress 1 Organic Lecture Series Benzene & Aromaticity Chapter 21 2 Organic Organic Lecture Series Benzene - Kekulé • The first structure for benzene was proposed by August Kekulé in 1872: • This structure, however, did not account for the unusual chemical reactivity of benzene 3 Organic Lecture Series Benzene - Resonance Model • Benzene is represented as a hybrid of two equivalent Kekulé structures – each makes an equal contribution to the hybrid and thus the C-C bonds are neither double nor single, but something in between 4 Organic Organic Lecture Series Benzene • The concepts of hybridization of atomic orbitals and the theory of resonance, developed in the 1930s, provided the first adequate description of benzene’s structure – the carbon skeleton is a regular hexagon – all C-C-C and H-C-C bond angles 120° 5 Organic Lecture Series Benzene – Molecular Orbitals The π system of benzene: 1. the carbon framework with the six 2p orbitals 2. overlap of the parallel 2p orbitals forms one torus above the plane of the ring and another below it 3. this orbital represents the lowest-lying pi-bonding molecular orbital 6 Organic Organic Lecture Series Ethylene – Molecular Orbitals 7 Organic Lecture Series Benzene – Molecular Orbitals The molecular orbital representation of the π bonding in benzene: 8 Organic Organic Lecture Series Benzene - Resonance Model • Resonance energy: the difference in energy between a resonance hybrid and the most stable of its hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds – one way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene 9 Organic Lecture Series 3(-119.7 kJ/mol) 10 Concept of Aromaticity Organic Organic Lecture Series The underlying criteria for aromaticity were recognized in the early 1930s by Erich Hückel, based on molecular orbital (MO) calculations To be aromatic, a compound must: 1. be cyclic 2. have one p orbital on each atom of the ring 3. be planar or nearly planar so that there is continuous or nearly continuous overlap of all p orbitals of the ring 4. have a closed loop of (4n + 2) π electrons in the cyclic arrangement of p orbitals 11 Organic Lecture Series Antiaromatic Hydrocarbons • Antiaromatic hydrocarbon: a monocyclic, planar, fully conjugated hydrocarbon with 4n π 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 E.G. Cyclobutadiene is antiaromatic – in the ground-state electron configuration of this molecule, two electrons fill the π1 bonding MO – the remaining two electrons lie in the π2 and π3 nonbonding MOs 12 Organic Organic Lecture Series 13 Organic Lecture Series Pyridine – the nitrogen atom of pyridine is sp2 hybridized – the unshared pair of electrons lies in an sp2 hybrid orbital and is not a part of the six π electrons of the aromatic system – pyridine has a resonance energy of 134 kJ (32 kcal)/mol, slightly less than that of benzene 14 Organic Organic Lecture Series Furan – the oxygen atom of furan is sp2 hybridized – one unshared pairs of electrons on oxygen lies in an unhybridized 2p orbital and is a part of the aromatic sextet – the other unshared pair lies in an sp2 hybrid orbital and is not a part of the aromatic system – the resonance energy of furan is 67 kJ (16 kcal)/mol 15 Organic Lecture Series Cyclopentadienyl Anion • To convert cyclopentadiene to an aromatic ion, it is necessary to convert the CH2 group to a CH group in which carbon becomes sp2 hybridized and has 2 electrons in its unhybridized 2p orbital 16 Organic Organic Lecture Series Cyclopentadienyl Anion • The pKa of cyclopentadiene is 16 – in aqueous NaOH, it is in equilibrium with its sodium salt – it is converted completely to its anion by very strong bases such as NaNH2 , NaH, and LDA 17 Organic Lecture Series Nomenclature-Monosubstituted • Monosubstituted alkylbenzenes are named as derivatives of benzene – many common names are retained Toluene OH Phenol Ethylbenzene NH 2 Aniline (commit to memory) CHO Cumene Styrene COOH Benzaldehyde Benzoic acid OCH 3 Anisole 18 Organic Organic Lecture Series Benzyl and phenyl groups: CH 3 Benzene Phenyl group, PhO Toluene CH 2 Benzyl group, Bn- O H 3 CO Ph 1-Phenyl-1pentanone 4-(3-Methoxyphenyl)2-butanone (Z)-2-Phenyl2-butene 19 Organic Lecture Series Disubstituted Benzenes • Locate two groups by numbers or by the locators ortho (1,2-), meta (1,3-), and para (1,4-) 20 Organic Organic Lecture Series Disubstituted Benzenes – where one group imparts a special name, name the compound as a derivative of that molecule: Toluene is the parent name Aniline is the parent name Benzoic Acid is the parent name 21 Organic Lecture Series Disubstituted Benzenes where neither group imparts a special name, locate the groups and list them in alphabetical order 22 Organic Organic Lecture Series Polysubstituted Derivatives – if one group imparts a special name, name the molecule as a derivative of that compound – if no group imparts a special name, list them in alphabetical order, giving them the lowest set of numbers 12 NO2 OH CH3 NO2 Br 6 1 2 4 Br 2 4 4 1 Br Cl 4-Chloro-2-nitrotoluene Br CH2 CH3 2,4,6-Tribromophenol 2-Bromo-1-ethyl-4nitrobenzene 23 Organic Lecture Series Phenols • The functional group of a phenol is an -OH group bonded to a benzene ring OH OH OH OH OH CH Phenol 3 3-Methylphenol (m- Cresol) OH 1,2-Benzenediol 1,4-Benzenediol (Catechol) (Hydroquinone) 24 Organic Organic Lecture Series Acidity of Phenols • Phenols are significantly more acidic than alcohols, compounds that also contain the OH group Phenoxide: Ethoxide 25 Organic Lecture Series Acidity of Phenols – the greater acidity of phenols compared with alcohols is due to the greater stability of the phenoxide ion relative to an alkoxide ion: 26 Organic Organic Lecture Series Acidity of Phenols Alkyl and halogen substituents effect acidities by inductive effects: – alkyl groups are electron-releasing – halogens are electron-withdrawing 27 Acidity of Phenols Organic Lecture Series – nitro groups increase the acidity of phenols by both an electron-withdrawing inductive effect and a resonance effect 28 Organic Organic Lecture Series Acidity of Phenols – part of the acid-strengthening effect of -NO2 is due to its electron-withdrawing inductive effect – in addition, -NO2 substituents in the ortho and para positions help to delocalize the negative charge 29 ...
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This note was uploaded on 11/21/2011 for the course CH 51455 taught by Professor Colapret during the Spring '11 term at University of Texas at Austin.

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