Chapter 15: Benzene, Aromatics PDF

Chapter 15: Benzene; aromatics First 7 slides from Chapter 14: Kinetic, thermodynamic products 1  Kinetic vs Thermodynamic Products see chapter 14 pgs 663-666 2 situations may occur with the formation of “major” products...

Chapter 15: Benzene; aromatics First 7 slides from Chapter 14: Kinetic, thermodynamic products 1  Kinetic vs Thermodynamic Products see chapter 14 pgs 663-666 2 situations may occur with the formation of “major” products If more than one product can form: The product that is formed faster at lower temperature is the kinetic product The product that predominates at equilibrium is the more stable product and is the thermodynamic product What does this mean? Kinetic product: the reaction is irreversible; depends on the rate Thermodynamic product: the reaction is reversible; depends on the stabilities of the product 2  Kinetic vs Thermodynamic Products see chapter 14 pgs 661-666 What does this mean? Kinetic product: the reaction is irreversible; depends on the rate If A can produce B or C; then the major product is the one that forms faster; this means the reaction is under “kinetic control” B A C Thermodynamic product: the reaction is reversible; depends on the stabilities of the product Reaction is under “thermodynamic control” (equilibrium control) and eventually the more stable product will be formed B A A forms B first but equilibrates eventually to form C C 3  Example; Kinetic vs Thermodynamic Electrophilic addition of one equivalent of HBr to an isolated diene (the double bonds are not directly connected) yields one product and Markovnikov’s rule is followed. Electrophilic addition in conjugated dienes (double or multiple bonds are connected directly) gives a mixture of two products, called the 1,2- and 1,4- addition products. 4 Note this mechanism…….between step 1 and 2 5  Reaction Conditions and Addition Products The amount of 1,2- and 1,4-addition products formed in electrophilic addition reactions of conjugated dienes depends greatly on the reaction conditions. important Kinetic Thermodynamic 6  Kinetic vs Thermodynamic Products NOTE: the isolated kinetic product can become the thermodynamic product When a mixture containing predominantly the 1,2-product is heated, the 1,4- addition product becomes the major product at equilibrium. 7 Why does this occur? The 1,4-product (1-bromo-2-butene) is more stable because it has two alkyl groups bonded to the carbon–carbon double bond, whereas the 1,2-product (3- bromo-1-butene) has only one. 8 Chapter 15 starts here 9  Benzene Benzene is the simplest aromatic hydrocarbon C6H6 four degrees of unsaturation: Cyclic, 3 double bonds Does not undergo addition reactions like other double bonds The degree of unsaturation indicates the total number of pi bonds and rings within a molecule 10  Structure of Benzene Any structure for benzene must account for the following facts: 1. It contains a six-membered ring and three additional degrees of unsaturation. 2. It is planar. 3. All bond lengths are equal. Resonance structures 11  Resonance Hybrid of Benzene resonance description of benzene consists of two equivalent Lewis structures, The true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the bonds. one of the two Lewis structures and not the hybrid in drawing benzene. This will make it easier to keep track of the electron pairs in the bonds (the electrons). 12  Representations of Benzene Because each π bond has two electrons, benzene has six π electrons. Benzene is sometimes drawn this way the actual bond length (1.39 Å) is intermediate between the carbon–carbon single bond (1.53 Å) and the carbon–carbon double bond (1.34 Å). 13 Each carbon atom in a benzene ring is surrounded by three atoms and no lone pairs of electrons, making it hybridized. Each carbon is then trigonal planar with bond angles. Each carbon also has a p orbital with one electron that extends above and below the plane of the molecule. There is a high degree of electron density above and below the ring Note: PDF on schoology had a slide that says “Later slide Reactivity of Benzene” that slide has been removed 14  Naming Substituted Benzenes To name a benzene ring with one substituent, name the substituent and add the word benzene. Many monosubstituted benzenes have common names 15  Nomenclature of Benzene Derivatives three different ways that two groups can be attached to a benzene ring, so a prefix—ortho, meta, or para—can be used to designate the relative position of the two substituents. This nomenclature is important (in this class)…. 16  Naming Benzene as a Substituent A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as “Ph−”. benzene can be represented as PhH, and phenol would be PhOH. 17  Benzyl and Aryl Groups The benzyl group, another common substituent that contains a benzene ring, differs from a phenyl group. Substituents derived from other substituted aromatic rings are collectively known as aryl groups. 18  Unusual Reactivity of Benzene (because of aromaticity) Lower than expected heat of hydrogenation (alkene to alkane by adding hydrogen) Does not undergo addition reactions typical of other highly unsaturated compounds, including conjugated dienes. Does not react with to yield an addition product as other alkenes Needs Lewis acid catalyst 19  What accounts for Aromaticity 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. 20  2. A molecule must be planar. All adjacent p orbitals must be aligned so that the electron density can be delocalized. Since cyclooctatetraene is nonplanar and not aromatic, it undergoes addition reactions just like those of other alkenes. 21 3. A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. 22 4. A molecule must satisfy Hückel’s rule, and contain a particular number of electrons. Hückel's rule: Benzene is aromatic and especially stable because it contains 6 electrons. Cyclobutadiene is antiaromatic and especially unstable because it contains 4 electrons. 23  Hückel’s Rule and Number of π Electrons Hückel’s rule refers to the number of electrons, not the number of atoms in a particular ring. 24  Larger Aromatic Rings Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2 π electrons, called annulenes. Naming: indicate the number of atoms in the ring in brackets and add the word annulene. 25 Example of not aromatic ring because of strain -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. ring puckers to relieve this strain; …not planar…no delocalization of electrons …..not aromatic. 26  Fused Ring Aromatics Two or more six-membered rings with alternating double and single bonds can be fused together to form Poly Aromatic Hydrocarbons (PAH) Meet criteria for aromaticity…….. 27  Resonance Structures of Fused Ring Aromatics resonance structures increase as more fused rings exist in compounds Naphthalene is a hybrid of three resonance structures (remember: benzene is a hybrid of two resonance structures) Note that all three have the double bonds arranged differently 28 Heterocycles containing oxygen, nitrogen or sulfur, can also be aromatic. the lone pair can be localized on the heteroatom or part of the delocalized system….example of an aromatic heterocycle is pyridine. 29 30 Pyrrole: example where the lone pair is part of the “aromatic” system contains a five-membered ring with two bonds and one nitrogen atom. has a p orbital on every adjacent atom……conjugated. has six electrons—four from the bonds and two from the lone pair. Pyrrole is cyclic, planar, completely conjugated, and has 6 pi electrons, so it is aromatic. 31 Effect of delocalization of the lone pair of electrons pyridine, nonbonded electron pair is localized on the N atom in an hybridized orbital: high electron density on N. pyrrole, nonbonded electron pair is in a p orbital and is delocalized over the ring: entire ring is electron rich. 32  Ionic Aromatic Compounds Both negatively and positively charged ions can be aromatic…must meet the criteria We can draw five equivalent resonance structures for the cyclopentadienyl anion. 33  Cyclopentadienyl examples Having the “right” number of electrons is necessary for a species to be unusually stable due to aromaticity. five resonance structures can also be drawn for the cyclopentadienyl cation and radical, only the cyclopentadienyl anion has 6 electrons and satisfies Hückel’s rule. Aromatic molecules are cyclic, conjugated, have (4n+2) pi electrons, and are flat. Anti-aromatic molecules are cyclic, conjugated, have (4n) pi electrons, and are flat. Non-aromatic molecules are every other molecule that fails one of these 34 Cyclopentadiene itself is not aromatic because it is not fully conjugated. cyclopentadiene is much more acidic (pKa=15) than most hydrocarbons because its conjugate base is aromatic….makes it very stable. Loss of a proton causes the cyclopentadienyl anion to become fully conjugated and consequently aromatic. 35 Aromaticity in a cation: The tropylium cation is a planer carbocation with three double bonds and a positive charge contained in a seven-membered ring. Because the tropylium cation has three π bonds and no other nonbonded electron pairs, it contains six π electrons and is fully conjugated, thereby satisfying Hückel’s rule….aromatic 36 We will deal with Molecular Orbital Theory (last part of Chapter) as a separate lecture 37 Remember: 1. Kinetic vs Thermodynamic (control) reactions 2. Aromaticity: what is it 3.. Aromatic Criteria: how is it determined 38 Chapt 15 HW (carefully think through these) 26, 27, 28 a, b; 30, 32, 38 (note the products carefully and explain or show why they form) 39