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32 Cards in this Set
- Front
- Back
Describe generally three types of reactions that can occur on an aromatic ring like a benzene ring?
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1.Electrophilic aromatic substitution: most common type. Benzene ring has a large electron density on it so it want to attack epecies with positive charge (electrophiles) to create a TS with positive charge called sigma complex / 2.Nucleophilic aromatic substitution: on a few cases when ring has powerful electron-withdrawing groups like NO2, nuclephiles attack ring to create a TS with negative charge called Meisenheimer complex / 3.Elimination-addition: rarely on high heat or very strong base, benzyne forms
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Describe electrophilic aromatic substitution? (nature of reaction, TS)
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Benzene ring has a large electron density on it so it want to attack species with positive charge (electrophiles) . the TS is called sigma complex and has positive charge. Postivie would be on ortho and para position in regard to the electrophile
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Describe nucleophilic aromatic substitution? (nature of reaction, TS)
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in a few cases when ring has powerful electron-withdrawing groups like NO2, nuclephiles attack the ring. the TS is called Meisenheimer complex and negative charge on ortho and para position (in regard to the nucleophile)
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Define Elimination-addition reaction on benzene ring? (nature of reaction, TS)
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rarely on high heat or very strong base. TS is called benzyne
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Define sigma complex (or arenium ion)?
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In a typical aromatic substitution reaction, the aromatic moiety functions as a nucleophile and attacks the electrophilic agent, generating a positively charged intermediate called a sigma complex, or arenium ion
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What reaction can be used as blocking reagent in the installation of functional groups on benzene ring?
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Sulfonation. Because it is easily reversible
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How to install nitro group (NO2) on a benzene ring?
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By Nitration (electrophilic aromatic substitution)
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HNO3 and H2SO4 create one electrophile suitable for installation on the benzene ring. Explain mechanism?
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What type of reaction is? What's the product?
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Reduction of NO2 to NH2 (on a benzene ring)
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How to install a NH2 group on benzene ring?
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Two step is necessary: (1) nitration, followed by (2) reduction of the nitro group
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Define a typical Friedel–Crafts alkylation?
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Type of reaction? Product?
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Friedel–Crafts alkylation (rearrangement is possible so major product would form from secondary carbocation not primery)
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Explain three limitations of Friedel–Crafts alkylation?
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1.the carbon atom connected to the halogen must be sp3 hybridized (in alkyl halide, meaning vinyl and aryl halides will not work) / 2.polyalkylations often occur / 3. Ring should not be attached to strong and moderate deactivators (Especially NO2)
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Define Friedel-Crafts Acylation?
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What is acylium ion?
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Define Clemmensen reduction?
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a zinc amalgam and HCl, the carbonyl group is completely reduced and replaced with two hydrogen atoms
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Propose a way to install the indicated alkyl chain on the benzene ring?
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1) Friedel-Crafts Acylation 2) Clemmensen reduction
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Rearrangement and polyalkylation are possible in Friedel-Crafts alkylation. What about Friedel-Crafts Acylation?
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1.Rearrangme is impossible 2.polyacylation is impossible (acyl group is a ring deactivator)
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Strong and moderate deactivating groups prevent Friedel-Crafts alkylation. Discuss the effect of deactivators on Friedel-Crafts acylation
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Any type of deactivator (Strong, Modearate and weak) prevents Friedel-Crafts acylaiton.
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Benzene activating groups can be organized in three categories. What are they?
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Activators increase electron density on benzene ring: 1.Strong (presence of a lone pair immediately adjacent to the aromatic ring e.g OH, OR, NR2) / 2.moderate (a lone pair that is already delocalized outside of the ring e.g. COO, CON) / 3.weak (alkyl chain)
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Benzene deactivating groups can be organized in in three categories. What are they ?
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Deactivators reduce electron density on the benzene ring: 1.Strong (NO2, NR3+, -CX3 when X is a halide) / 2.moderate: pull away electron density by resonance (pi bond adjacent to ring) 3.weak: Halides
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Explain directing effect of substitutions on a benzene for the upcoming substitutions?
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1. Activatos direct to ortho and para (Para is major due to steric effects) / 2.deactivators direct to meta except for Halides which direct to ortho and para
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Why halides direct to ortho and para regardless of the fact that they are deactivating group?
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There two opposite effects resonance and induction: for the ring the induction wins ( => pull away electron density on ring) but for the stability of Positive charge Resonance wins (=> direct to ortho and para)
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What is the results of the nitration (HNO3, H2SO4) of the following compound?
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It does lead to nitration because The reagents for nitration (a mixture of HNO3 and H2SO4) can oxidize the amino group, often leading to a mixture of undesirable products
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What is the product of the reaction of the following compound with CH3Cl, AlCl3?
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A Friedel-Crafts reaction (either alkylation or acylation) cannot be accomplished on rings that are either moderately or strongly deactivated.
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nucleophilic aromatic substitution can occur when three requirements exist. What are they?
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1.The ring must contain a powerful electron-withdrawing group (typically a nitro group which can to stabilize negative charge on the TS by resonance and inductive effect)./ 2.The ring must contain a good leaving group (usually a halide) / 3.The leaving group must be either ortho or para to the electron-withdrawing group (so by resonance the resulting negative charge comes under powerful electron-withdrawing group to get stabalized)
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Snar [1.The ring must contain a powerful electron-withdrawing group (typically a nitro group which can to stabilize negative charge on the TS by resonance and inductive effect)./ 2.The ring must contain a good leaving group (usually a halide) / 3.The leaving group must be either ortho or para to the electron-withdrawing group (so by resonance the resulting negative charge comes under powerful electron-withdrawing group to get stabalized)]
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What is Meisenheimer complex?
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The TS with negative charge resulted from Nucleophilic attack on benzene ring
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No reaction (No e-withdrawing group to undergo SNar / not enough heat to undergo Elimination-addition via benzyne intermediate)
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No e-withdrawing group to undergo SNar but enough heat to undergo Elimination-addition via benzyne intermediate
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No e-withdrawing group to undergo SNar but very strong base (NH2-) make it undergo Elimination-addition via benzyne intermediate even not in high temperature
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Benzyne
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