Aromatic Compounds - Inorganic & Organic Chemistry - PDF

AROMATIC COMPOUNDS INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) bromobenzene, and hydrogen bromide are OUTLINE produced IV. AROMATIC COM...

AROMATIC COMPOUNDS INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) bromobenzene, and hydrogen bromide are OUTLINE produced IV. AROMATIC COMPOUNDS a. Some Facts About Benzene b. The Kekule Structure of Benzene c. Resonance Model For Benzene d. Symbols for Benzene e. Nomenclature of Aromatic Compounds f. The Resonance Energy of Benzene g. Electrophilic Aromatic Substitution one H atom is being replaced by Br ( a Halogen ) h. The Mechanism of Electrophilic Aromatic ➔ only one monobromobenzene is isolated; no Substitution isomers i. Ring-Activating and Ring-Deactivating Substituents ➔ 17 billion pounds are produced annually in j. Ortho, Para-Directing, and Meta-Directing the United States Groups ➔ obtained mostly from petroleum by catalytic k. The Importance of Directing Effects in Synthesis reforming of alkanes and cycloalkanes or by l. Polycyclic Aromatic Hydrocarbons cracking certain gasoline fractions AROMATIC COMPOUNDS The Kekule Structure of Benzene BENZENE FRIEDRICH AUGUST KEKULE (1829-1896) ➔ proposed a reasonable structure for ➔ the parent hydrocarbon of a class substance benzene (1865) known as aromatic compounds ➔ pioneer in the development of structural ➔ first isolated from compressed illuminating formulas in organic chemistry; viewed gas by Michael Faraday (1825) chemistry as molecular architecture ➔ C6H6 (1:1 ratio) ➔ first to recognize the tetracovalence of ➔ special chemical property: stability carbon and the importance of carbon chains ➔ used to make styrene, phenol, acetone, in organic structures cyclohexane, and other industrial chemicals ➔ to have a valence of 4, he suggested that single and double bonds alternate around the Some Facts About Benzene ring (conjugated; highly unsaturated) ➔ carbon-to-hydrogen ratio suggests a highly ➔ Negative test for unsaturation: single and unsaturated structure (1:1 ratio = C6H6) double bonds exchange position so quickly ➔ benzene rings with -OH: phenols that the typical reactions of alkenes cannot take place ➔ Benzene does not decolorize bromine Resonance Model for Benzene solutions ➔ Kekule’s structure is the requirement for ➔ Benzene is not easily oxidized by potassium resonance permanganate ➔ two identical contributing structures = single ➔ reacts mainly by Substitution resonance hybrid structure of benzene example: benzene, when treated with bromine ➔ nearly, but not entirely correct (Br2), with ferric bromide as a catalyst, Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 1  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) ➔ two structures differ only in the arrangement Symbols for Benzene of the electrons; all the atoms occupy the ★ Kekule structure (one carbon in each corner; same position in both structures reminds us that there are six pi electrons in ➔ use the double-headed arrow to indicate benzene; helps in keeping track of the resonance hybrid valence electrons in chemical reactions) ★ Hexagon with an inscribed circle (to ➔ resonance hybrid = most stable represent the idea of a delocalized ➔ no single or double bonds in benzene-only pi-electron cloud; electrons distributed one type of carbon-carbon bond: evenly around the ring; more accurate) intermediate type ➔ Benzene is planar ➔ six H atoms are located at the corners of a regular hexagon (1 hydrogen atom for each carbon) ➔ all carbon-carbon bond lengths are identical: Nomenclature of Aromatic Compounds 1.39 Å, intermediate between typical single (1.54 Å) and double (1.34 Å) carbon-carbon benzene toluene cumene bond lengths Orbital Model for Benzene ➔ each C atom in benzene is connected to only three other atoms (two Carbons and a Hydrogen) ➔ each carbon is sp2 hybridized (ethylene) styrene phenol anisole ➔ two sp2 orbitals of each atom overlap with similar orbitals of adjacent carbon atoms to form the sigma bond of the hexagonal ring ➔ the 3rd sp2 orbital of each carbon overlaps with a hydrogen 1s orbital to form the C–H sigma bonds ➔ the 4th valence electron, perpendicular to benzaldehyde acetophenone the plane of the three sp2 orbitals at each carbon, is a p orbital containing one electron ➔ p orbitals on each carbon atom overlap laterally to form pi orbitals that create a ring/ cloud of electrons above and below the plane of the ring aniline ➔ H–C–C and C–C–C bonds have angles of 10° ➔ double bonds are not fixed ➔ monosubstituted benzenes (mono: one) that do not have common names accepted by IUPAC are named as derivatives of benzene Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 2  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) bromobenzene chlorobenzene AROMATIC HYDROCARBONS ➔ also called as arenes ➔ Ar (aryl group) - (aromatic substituent) ➔ Ar–R: arylalkane ➔ phenyl group (abbreviated as Ph): benzene substituent (benzene na ready magconnect) nitrobenzene ethylbenzene ➔ benzyl group (benzene with methyl) propylbenzene ➔ Cyclohexane: most stable ➔ dienes: two C==C double bonds ➔ trienes: three C==C double bonds ➔ when two substituents are present = three isomeric structures are possible The Resonance Energy of Benzene ➔ -ortho (-o), -meta (-m), -para (-p): can be ➔ IUPAC name for Kekule structure: used even when compounds are not identical 1,3,5-cyclohexatriene (benzene ring) ➔ hydrogenation of a C–C double bond is an exothermic reaction ➔ resonance hybrid: always more stable than any of its contributing structures ➔ stabilization/ resonance energy: difference some examples: between the energy of the real molecule and the calculated energy of the most stable ortho-dichlorobenzene contributing structure ➔ benzene is more difficult to hydrogenate than any ordinary alkanes ➔ hypothetical molecule: 1,3,5-cyclohexatriene by: 86 - 50 = 36 kcal/mol meta-dichlorobenzene ➔ benzene and other aromatic compounds react in such a way as to preserve their aromatic structure and retain their resonance energy Electrophilic Aromatic Substitution para-dichlorobenzene ★ electrophile: loves electrons; lacks electrons ★ nucleophile: gives electrons; many electrons ★ benzene: ➔ chemical reactivity: electrophilic substitution ➔ ortho-, para-, and meta- can also be used when there are 2 different substituents ➔ more than two substituents are present = numberings are used in naming Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 3  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) as opposed to electrophilic addition The Mechanism of Electrophilic Aromatic Substitution Lewis Definition: ★ Acid: electron pair (lone pair) acceptor ★ Base: electron pair donor ➔ involve substitution of other atoms or groups for ring hydrogen on the aromatic unit ➔ most are carried out at temperatures between about 0°C and 50°C ➔ initial attack on the benzene ring by an ➔ conditions may have to be milder or electrophile more severe if other substituents are ➔ catalyst acts as a Lewis acid and converts already present on the benzene ring chlorine to a strong electrophile by forming a ➔ conditions can usually be adjusted to complex and polarizing the Cl-Cl bond introduce more than one substituent if desired ★ Chlorination ➔ electrophile bonds to one carbon atom of the benzene ring, using two of the pi electrons ★ Bromination from the pi cloud to form a sigma bond with a ring carbon atom ➔ the carbon atom becomes sp3 -hybridized ➔ benzene ring acts as a pi-electron donor, or nucleophile, toward the electrophilic reagent ★ Nitration Benzenonium ion - a carbocation ➔ involve substitution of atoms or groups for a ring hydrogen on the aromatic unit ➔ resonance-stabilized is the intermediate in ★ Sulfonation electrophilic aromatic substitution reactions ➔ the positive charge of benzenonium ion is delocalized by resonance to the carbon atoms ortho and para to the carbon to which the chlorine atom became attached ➔ ortho and para to the sp3 carbon atom ★ Alkylation ➔ benzenonium ion - similar to an allylic carbocation, but the positive charge is Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 4  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) delocalized over three carbon atoms HALOGENATION instead of only two ➔ resonance energy is much less than that of the starting benzene ring SUBSTITUTION ➔ completed by loss of a proton from the sp3 carbon atom, the same atom to which the ➔ iron halide as a catalyst electrophile became attached ➔ carried out by adding the halogen slowly to a mixture of the aromatic compound and iron filings ➔ iron reacts with the halogen to form the iron halide, which then catalyzes the halogenation ➔ direct fluorination or iodination of aromatic rings is also possible but requires special STEP 1: the stabilization energy (resonance energy) of methods the aromatic ring is lost due to disruption of the aromatic pi system (caused by addition of the NITRATION electrophile to one of the ring carbons, requires energy and a strong electrophile) STEP 2: the aromatic resonance energy is regained by loss of a proton + ➔ nitronium ion (NO2 ) – the electrophile in the nitration (will attack the benzene ring) ➔ sulfuric acid catalyst - protonates the nitric acid, which then loses water to generate the nitronium ion (NO2+), which contains a positively charged nitrogen atom + ➔ NO2 - a strong electrophile, is then attacked Step 1: usually slow or rate-determining because it by the aromatic ring requires substantial activation energy to disrupt the aromatic system SULFONATION ➔ either concentrated or fuming sulfuric acid, Step 2: has a low activation energy and is usually fast and the electrophile may be sulfur trioxide because it regenerates the aromatic system (SO3), or protonated sulfur trioxide, +SO3H ➔ sulfonic acids - are strong organic acids ➔ can be converted to phenols by reaction with base at high temperatures Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 5  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) Ring-Activating & Ring-Deactivating Substituents ALKYLATION AND ACYLATION ➔ OH and CH3 speed up the reaction, and Cl and NO2 retard (slow down) the reaction Friedel–Crafts reaction – Alkylation ➔ Charles Friedel (French) and James Mason Crafts (American) ➔ support the electrophilic mechanism for ➔ discovered the reaction in 1877 substitution ➔ has some limitations; cannot be applied to an ➔ electrophilic (that is, electron seeking) attack aromatic ring that already has on it a nitro or on the aromatic ring sulfonic acid group because these groups form complexes with and deactivate the Ring-Activating Substituents aluminum chloride catalyst ➔ electrophile - a carbocation, which can be donate electrons; increase its electron formed either by removing a halide ion from density an alkyl halide with a Lewis acid catalyst increase reactivity of aromatic rings example - AlCl3 more electrons mean the ring is more attractive to other chemicals (electrophiles) that want to react with it "helpers" that make the ring more welcoming; speed up the reaction -OH (like in alcohols), -NH₂ (like in amines), -OCH₃ (methoxy), -CH₃ (methyl) hydroxyl and methyl groups - more electron donating than hydrogen ortho/para-directing Friedel–Crafts reaction – Acylation ➔ electrophile is an acyl cation generated from Ring-Deactivating Substituents an acid derivative, usually an acyl halide ➔ provides a useful general route to aromatic pulls away electrons; decrease electron ketones density decrease reactivity of aromatic rings withdraw electrons through resonance effects, where electron-withdrawing groups (EWGs) pull electrons away from the ring "blockers" that make the ring less inviting; slow down the reaction Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 6  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) -NO₂ (nitro), -CN (cyano), -CF₃ ➔ positive charge is on the methyl-bearing (trifluoromethyl), -COOH (carboxyl) carbon chloro and nitro groups - more electron ➔ a tertiary carbocation and more stable than withdrawing than hydrogen the other contributors meta-directing Ortho/Para-Directing and Meta-Directing Groups ➔ all of the resonance contributors are secondary carbocations ➔ the positive charge in the intermediate benzenonium ion is never adjacent to the methyl substituent ➔ the methyl group is ortho, para directing so that the reaction can proceed via the most stable carbocation intermediate ➔ similarly, all other alkyl groups are ortho, ORTHO/PARA-DIRECTING SUBSTITUENTS ON para-directing BENZENE RING ➔ groups with unshared electrons on the atom ➔ directs a second electrophile to positions attached to the ring are ortho/para-directing ortho (next to) and para (opposite) to it on the ring META-DIRECTING SUBSTITUENTS ON BENZENE ➔ usually donate electrons to the ring, RING increasing electron density at the ortho and ➔ directs a second electrophile to a position para positions meta to it on the ring ➔ electron-donating groups (EDGs) ➔ usually withdraw electrons from the ring, ➔ make the ring more reactive at positions decreasing electron density at the ortho and next to or opposite the group para positions ➔ electron-withdrawing groups (EWGs) ➔ make the ring less reactive at nearby positions 7% ortho isomer 4% meta isomer ➔ has two adjacent positive charges ➔ a highly undesirable arrangement because like charges repel each other ➔ no such intermediate is present for meta-substitution ➔ meta-substitution is preferred Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 7  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) substituted benzene than for unsubstituted benzene ortho, para-directing groups - supply electrons to the ring and are therefore ring-activating electron-donating groups (EDGs) Deactivating - if the rate of reaction is slower than for benzene meta-directing groups - the atom connected ➔ groups in which the atom directly attached to to the ring carries a full or partial positive the aromatic ring is positively charged or is charge and will, therefore, withdraw part of a multiple bond to a more electrons from the ring; therefore, electronegative element will be ring-deactivating groups meta-directing electron-withdrawing groups (EWGs) Halogens (F, Cl, Br, and I) - two opposing effects bring about the only important exception to these rules - are strongly electron-withdrawing - the halogens are ring-deactivating, but because they have unshared electron pairs, they are ortho, para directing The Importance of Directing Effects in Synthesis Bromination and nitration of benzene to make bromonitrobenzene if we brominate first and then nitrate, we will get a mixture of the ortho and para isomers if we nitrate first and then brominate, we will get mainly the meta isomer because the nitro group is meta-directing Substituent Effects on Reactivity (SUMMARY) Activating - if the rate of electrophilic aromatic substitution is faster for the Transcribed By: Tessa Jane G. Aranjuez | MLS 1-4 | 8  MT-MLS104: INORGANIC & ORGANIC CHEMISTRY (LEC/LAB) The sequence in which we carry out the prebiotic chemistry (see “A Word about reactions is, therefore, very important Methane, Marsh Gas, and Miller’s It determines which type of product is formed Experiment,” page 60) Polycyclic Aromatic Hydrocarbons Reaction Summary AROMATICITY The unusual stability of certain fully conjugated cyclic systems HÜCKEL’S RULE In general, ring systems containing 4n + 2 pi electrons (n = 0, 1, 2,...) in adjacent p orbitals are aromatic 4n + 2 = 6 4n = 4 n=1 FUSED POLYCYLIC HYDROCARBONS Contains at least two benzene rings; each ring shares two carbon atoms with at least one other ring. ➔ comprise a large percentage of the carbon found in interstellar space ➔ observed in interstellar ice (Halley’s comet) ➔ has been shown that ultraviolet irradiation of PAHs in ice produces aromatic ketones (Chapter 9), alcohols (Chapter 7), and other compounds, suggesting a role of PAHs in Transcribed By: Tessa Jane G. 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