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Chem Chapter 17
| Question | Answer |
|---|---|
| Why was benzene challenging for scientists? | its high degree of unsaturation could not be reconciled with its low reactivity toward many reagents that readily transformed alkenes and alkynes into other types of molecules. |
| What's special about benzene? | All six pi bonds are equal, all sp2, can be made into resonance, delocalization |
| Heat of hydrogenation and resonance energy | Benzene has a lower height of hydrogenation than 1,3,5-cyclohextriene (isolated bonds) and a high resonance energy. This is cause of its conjugated bonds and aromaticity. |
| Huckel's Rule (4n+2) | a molecule is aromatic when it has a planar, uninterrupted, no sp3, and cyclic π system that contains 4n+2 electrons, when n = 0,1,2,3,4, and so on. |
| Aromaticity | Amount of resonance you have, resonance ability |
| Numbers for Huckel's Rule | 2,6,10,14 electrons |
| Perpendicular Benzenes | Pyridine, thiopene, and pyrrole are have perpendicular electrons, meaning only one pair will be counted. |
| Frost Circle | Draw the cyclic molecule with one atom pointing down. Then, for every vertex, draw a horizontal line that corresponds to an energy level. ψ2 and ψ3 are degenerate bonding orbitals. Upper half are anti bonding, lower half are bonding. |
| Antiaromatic Frost Circle | Lower half will be bonding, middle with be nonbonding, and upper will be anti bonding. All the bonds will NOT be fully filled -> instability |
| Cations and Anions Tip | Anions will have two more electrons than what you see, cations will have the same amount |
| pKa and lone pairs | aromaticity in a molecule leads to a lower pKa value, meaning a stronger acid; because the delocalization of electrons allows it to more easily accept a proton. Delocalization leads to more acidity. |
| Antiaromatic | cyclic, no sp3, conjugated, planar, doesn’t have to follow Huckel’s rule, only 4n (4,8,12). UNSTABLE! |
| Relative Stabilities of Each of the Types | Aromatic (filled orbitals) > nonaromatic > antiaromatic (not filled) |
| Nonaromatic | noncyclic, sp3, nonplanar, odd numbers (3,5,7,9). Fails any one of the four criteria for the other ones. NORMAL! |
| Substituted Derivatives of Benzene | Their prefixes identify the substitutions attached to ring and numerals for the positions of the substituents and their root is benz followed by a suffix that denotes the principal functional group. |
| Exception to Benzene Root Word | the names of most methylated benzene derivatives do not include the root word benz. The common root word for the isomers of dimethylbenzene is xylene, which requires use of o, m, or p. |
| Ortho, or o: | 1,2-disubstitution |
| Meta, or m: | 1,3-disubstitution |
| Para, or p: | 1,4-disubstitution |
| Fancy a: | for benzene derivatives with common root names, substitution on a carbon atom adjacent to the ring is indicated by “a.” |
| Aniline: | Benzene + NH2 |
| Phenol: | Benzene + OH |
| Anisole: | Benzene + OCH3 |
| Toluene: | Benzene + CH3 |
| o-Xylene: | Benzene with two methyls, 1-2 |
| m-Xylene: | Benzene with two methyls, 1-3 |
| p-Xylene: | Benzene with two methyls, 1-4 |
| Mesitylene: | Benzene with three methyls |
| Durene: | Benzene with four methyls |
| Amino: | -NH2 |
| Bromo: | -Br |
| Carboxyl: | -COOH |
| Chloro: | -Cl |
| Cyano: | -CN |
| Fluoro: | -F |
| Formyl: | -C=O-H |
| Hydroxy: | -OH |
| Iodo: | -I |
| Nitro: | -NO2 |
| Mercapto: | -SH |
| Oxo: | =O |
| Phenoxy: | -O-benzene |
| Ortho Coupling | coupling between protons that are ortho to each other. The magnitude of this coupling constant, J, is 7-8 Hz. |
| Meta-Coupling: | engages in long-range coupling and tends to have J values that are less than 2, so they are easy to spot. |
| Draw The Electrophilic Aromatic Substitution of Benzene | One of the benzene pi bonds attacks EX, separating E from X and forming a carbocation. Then use X to attack the H next to the E and attack the carbocation with H’s bond. You should have HX alone at the end and E on the benzene with a new double bond. |
| Exergonic or Endergonic? EAS | The overall reaction is exergonic (products have less energy than reactants). The carbocation intermediate (the part with both E and H) is resonance stabilized. |
| Draw the Mechanism for chlorobenzene, bromobenzene: | the electrophiles would be Cl+ and Br+ and the reagents would be Cl2, Fe-Cl3, Br2, Fe-Br3. |
| Draw the Mechanism for Nitrobenzene: | the electrophiles would be NO2+ and the reagents would be HNO3, H2SO4. |
| Draw the Mechanism for benzenesulfonic Acid: | the electrophiles would be HSO3+ and the reagents would be SO3, H2SO4. |
| Alkylbenzenes: | the electrophiles would be R+ and the reagents would be RCl, AlCl3. |
| Acylbenzenes: | the electrophiles would be RCO+ and the reagents would be RCOCl, AlCl3. |
| BrFrBr4 is made from... | Br-Br and FeBr3 |
| NO2 and OSO3H is made from... | HNO3 and H2SO4 |
| Stuff that converts nitrobenzene (NO2 + Benzene) to aniline... | H2, Pd/C or SnCl2, HCl or Zn, HoAc |
| HSO3+ (electrophile) and OSO3H comes from... | sulfuric acid or from fuming sulfuric acid, SO3/H2SO4 |
| Reversal of Benzenesulfonic acid (Benzene + SO3H) | Heating with H2SO4, H2O, heat turns it back to regular benzene |
| Friedel-Crafts Alkylation: | RX over AlCl3. R attaches to benzene. When RX is a primary alkyl chloride, rearrangement is common even with secondary or tertiary alkyl chlorides. Therefore, isopropyl and tert-butyl will often be the major products. |
| Problems with Friedel-Crafts Alkylation: | Polyalkylation (alkyl group makes the ring more reactive than benzene itself), highly deactivated benzenes (NO2) will NOT be able to react, alkylation is reversible, so an alkyl group can sometimes migrate from one place to another. |
| Friedel-Crafts Acylation | eliminates some problems from alkylation. Lewis acid, AlCl3, is required. Electrophile is acyl cation. Rearrangement and polymerization do not occur, but no heavily deactivated groups react. Benzene reacts with ROCl, AlCl3 over H3O+ and makes COR. |
| How to Turn an Acyl Group to an Alkyl (CH2) Group | 1. H2, Pd/C 2. Zn(Hg), HCl 3. NH2NH2, OH-, heat 4. HSCH2CH2SH, then Raney Ni |
| Activating: | compounds react faster than benzene, usually add to ortho and para |
| Deactivating: | compounds react more slowly than benzene, usually only add to meta |
| Three factors influencing the transition state of electrophilic substitution reactions: | steric effects, inductive effects, and resonance effects. |
| Steric Effects: | most important for substituents that generate ortho/para substitution products. If the substituent is bulky, the para-substituted product will be formed in relative amounts to the ortho one. |
| Inductive Effects: | related primarily to differences in electronegativity values. A less nucleophilic ring will react more slowly than benzene towards electrophiles. |
| Resonance Effects: | can stabilize the pi system or can influence stabilization/destabilization. |
| –NH2 | strongly activating, resonance (donating) > inductive (withdrawing). |
| –NHR | strongly activating, resonance (donating) > inductive (withdrawing). |
| –NHR2 | strongly activating, resonance (donating) > inductive (withdrawing). |
| –OH | strongly activating, resonance (donating) > inductive (withdrawing). |
| –OR | strongly activating, resonance (donating) > inductive (withdrawing). |
| –NHR + Carbonyl | moderately activating, resonance (donating) > inductive (withdrawing). |
| –NHAr + Carbonyl | moderately activating, resonance (donating) > inductive (withdrawing). |
| –OHR + Carbonyl | moderately activating, resonance (donating) > inductive (withdrawing). |
| –OHAr + Carbonyl | moderately activating, resonance (donating) > inductive (withdrawing). |
| Halogens (F, Cl, Br, I) | weakly deactivating, inductive (withdrawing) > resonance (donating). |
| Benzene: | in the middle of weakly deactivating and moderately activating. |
| CHO: | moderately deactivating, resonance = inductive (all withdrawing) |
| CHR (or Ar): | moderately deactivating, resonance = inductive (all withdrawing) |
| COOH: | moderately deactivating, resonance = inductive (all withdrawing) |
| COOR: | moderately deactivating, resonance = inductive (all withdrawing) |
| CONH2 (or NR2): | moderately deactivating, resonance = inductive (all withdrawing) |
| –NO2: | strong deactivating, inductive only (withdrawing) |
| –CN: | strong deactivating, inductive only (withdrawing) |
| -SO3H: | strong deactivating, inductive only (withdrawing) |
| –NH3+: | strong deactivating, inductive only (withdrawing) |
| –NR3+: | strong deactivating, inductive only (withdrawing) |
| –CF3: | strong deactivating, inductive only (withdrawing) |
| –CCl3: | strong deactivating, inductive only (withdrawing) |
| Ortho/Para Directing (rings are more activated and more reactive than benzene: | strongly activating, moderately activating, weakly activating, and weakly deactivating. |
| Meta Directing (rings are more deactivated and less reactive than benzene): | moderately deactivating, strongly deactivating. |
| Electron Donating: | strongly activating, moderately activating, weakly activating. They speed up reactions with benzene. |
| Electron Withdrawing: | strong deactivating, moderately deactivating, weakly deactivating. They slow down reactions with benzene. |
| Mega Tip: | basically, when you move bonds around to form resonance structures, the carbocation will go to the place of the bond that attached it (further end). |
| The stability of the intermediates: | ortho and para > meta |
| Delta G and k | Ortho and para have less activation energy and more k (rate constant for electrophilic substitution of benzene derivatives). |
| Substituents that have an electron pair adjacent to the ring are activators and ortho/para directors | The resonance contributors show that the o- and p- positions are more electron rich (negatively charged), and the incoming E+ will preferably attack on the o- or p- position. THIS HAPPENS WITH ALOXYL AND AMINO GROUPS! |
| Most Stable Resonance Contributor (Either cannot or is usually not done by meta) | The one where the octet is full and the pos charge is outside the ring. |
| Resonance > Inductive | Inductive effects would make the ring less nucleophilic than benzene itself (cause the oxygen is highly electronegative and pulling electrons away) while resonance means the pi system is more nucleophilic due to formal charges). |
| Halogen atoms are deactivators and ortho/para directors | Halogen atoms withdraw electrons inductively and donate electrons by resonance. Since its inductive effects is more significant than resonance effects, the ring is deactivated. The resonance effects lead to substitution at ortho and para positions. |
| Substituents with positive charge or partial positive charge adjacent to the ring are deactivators and meta directors... | This means the substituent itself has a positive charge (NO2, SO3H, etc). These groups withdraw electrons inductively and by resonance. |
| If two or more groups are attached to a ring, electrophilic substitution will be influenced by a combination of three factors: | the positions to which the electrophile will be directed by each substituent (ortho/para versus meta), the relative strength of activation or deactivation by each substituent, steric effects. |
| Upfield and Downfield Ortho/P | The chemical shift for a response that is o/p to an activating group is upfield from 7.25 (benzene) and downfield when deactivating group. The closer you are to deactivating groups + the more # of groups means chemical shift increases. |
| Ortho Effect | Being ortho to a deactivating group is more substantial than being para. |
| Blocking Para Position Super Tip! | Sulfonation can be used to block the para position, so the incoming group goes into ortho position (SO3, H2SO4). The sulfonyl group can be removed by heating in aqueous sulfuric acid (heat, H2SO4 or H3O). |
| When you have competing substituents... | See which is stronger and if they're going to the same positions |
| KMnO4 and Na2Cr2O7 | Turns substituents into COOH. 3o alkylbenzene cannot be oxidized because there is no Hat benzylic position. Methyl, 1o, and 2o alkylbenzene can be oxidized to benzoic acid derivatives. IF YOU HAVE NA2CRO7 (XS), IT'LL TURN BOTH SUBSTITUENTS. |
| HONO (NaNO2, HCl) SUPER IMPORTANT, SERVES AS THE BASIS FOR OTHER REACTIONS! | Formed from HCl and NaNO2. Turns NH2 to N2. |
| When N2 reacts with H2SO4 and heat... | The N2 turns into OH. H2SO4 is used in the diazonium formation step so that no Cl- is present to react and form chlorobenzene. |
| When N2 with KI... | The N2 turns into I. Direct iodination is not as facile as chlorination and bromination, so this is a good way to prepare aryl iodide. |
| When N2 reacts with KX, Cu2X2... | The N2 turns into X. Cl-, Br-, and CN- can replace the N2 leaving group, but Cu(I) salt , as a catalyst, is required. |
| When N2 reacts with H3PO2... | The N2 is removed from the ring and replaced with H. |
| Nucleophilic Aromatic Substitution | If electron-withdrawing NO2 is in the ortho, or para (or both) position to a halogen atom, the halogen atom can be replaced by a nucleophile. The reagent will look something like this: Na + OCH3, and then you'd replace the halogen with OCH3. |
| Fluoride Reactivity in NAS | F ion is not a good leaving group for SN1 and SN2, but in SNAr reactions, aryl F's are more reactive than Cl. F- makes its attached C more susceptible to reaction with a Nu. The loss of F- occurs as a fast step, so it does not affect the reaction rate. |
| Benzyne | You can turn bromobenzene into benzyne using highly basic conditions, like K+ NH2-. Benzyne is highly reactive intermediate. It can be trapped by 1,3-butadiene in a Diels-Alder reaction. |
| Tip for Cyclobutadiene | A "square" cyclobutadiene is classified as anti aromatic because you would view it as a diamond in a Frost Circle but an actual cyclobutadiene would be nonautomatic (see slide 11). |
| How does cycloocctraene go from anti aromatic to nonromantic? | It molds itself to a tub shape for the Frost circle (see slide 12). |
| Tip for H3O+ | Fridel Acylation needs it, but friedel alkylation doesn't! |
| How to convert NH2 to deactivating NH4+? | You can use H2O4. And if you want to make a para-sulfonation product instead, use acetanilide first to turn NH2 into COCH3NH2. |
| When N2 reacts with heat dried BF4 salt... | The N2 turns into F |
| Draw the typical mechanism for NAS | -- |
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smurtab