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Org Chem Ch.18
Reactions of Aromatic Compounds
| Question | Answer |
|---|---|
| What is a characteristic reaction of benzene? | electrophilic aromatic substitution (hydrogen is replaced by an electrophile) |
| since benzene is especially stable, what reactions are favored? | reactions that maintain the aromatic ring |
| Regardless of electrophile E+ used, all electrophilic aromatic substitution reactions occur by the same _#_-step mechanism | two-step |
| What is the general two-step mechanisms of substitution? | (1) addition of E+ to form a resonance-stabilized carbocation (2) deprotonation with base |
| Which step is the rate-determining step? | step [1] addition of E+ is the rate-determining step as its transition state is at higher energy |
| Halogenation of benzene occurs with only two halogens: | Cl2 (in presence of FeCl3) or Br2 (in presence of FeBr3) |
| Halogenation of Benzene: Step [1] | X2 reacts with FeX3 to generate an electrophile (XX+ -FeBr3) |
| Halogenation of Benzene: Step [2] | Addition of the electrophile to form a carbocation (pi bond attacks electrophile and FeX4- is formed) |
| Halogenation of Benzene: Step [3] | Loss of proton to reform aromatic ring (FeX4- takes proton and pi bond reforms on ring) |
| nitration (NO2) and sulfonation (SO3H) of benzene requires what? | a strong acid (H2SO4) |
| formation of the the electrophile (+NO2) for nitration | formed by the protonation of HNO3, followed by the loss of water (HNO3 + H2SO4) |
| formation of the electrophile (+SO3H) for sulfonation | formed by the protonation of SO3 (SO3 + H2SO4) |
| Friedel-Crafts Alkylation | Treatment of benzene with an alkyl halide and a Lewis Acid (AlCl3) forms an alkyl benzene |
| Friedel-Crafts Acylation | treatment of benzene with an acid chloride (RCOCl) and AlCl3 to form a ketone (O=C-R) |
| In order to be reactive in Friedel-Crafts alkylation, a halide must be bonded to a sp3 C. What kind of halides DO NOT react in a Friedel-Crafts alkylation? | vinyl halide and aryl halide (halide bonded to sp2 C) |
| The Friedel-Crafts reaction can yield products having rearranged carbon skeletons when __ or __ alkyl halides are used as starting material | 1 or 2 |
| What does this rearrangement do? | converts a less stable 1 or 2 carbocation to a more stable 3 carbocation |
| what is 'rearranging'? | the positive charge can trade spots with an H (with its bond attached) |
| Other functional groups that form ________ can also be used as starting material | carbocation |
| give two examples of other functions groups forming a carbocation that could be used as an electrophile in Friedel-Crafts alkylation | protonation of an alkene forms a carbocation. and the protonation of an alcohol, followed by the loss of water forms a carbocation (both use H2SO4) |
| starting materials that contain both a benzene ring and an electrophile are capable of what? | intramolecular Friedel-Crafts reactions |
| Many substituted benzene rings undergo electrophilic aromatic substitution. Each substituent can effect the _______ _______ in the benzene ring | electron density (increase or decrease) |
| _________ of electron density to the ring makes benzene more electron rich | donation |
| _________ of electron density from the ring makes benzene less electron rich | withdrawal |
| what makes a substituent on a benzene ring electron donating or electron withdrawing? | inductive effects and resonance effects |
| Inductive effects stem from the ____________ of the atoms in the substituent and the _________ of the substituent group | electronegativity, polarizability |
| atoms more electronegative than carbon (N, O, X) exhibit an electron-_________ inductive effect | withdrawing |
| polarizable alkyl groups donate electron density and exhibit an electron-__________ inductive effect | donating |
| resonance effects can either donate or withdraw electron density, depending on what? | whether they place a (+) or (-) charge on the benzene ring |
| a resonance effect is electron-_________ when resonance structures place a negative charge on the carbons of benzene | donating |
| a resonance effect is electron-_________ when resonance structures place a positive charge on the carbons of benzene | withdrawing |
| General electron-donating groups | R groups and Z groups (N or O) |
| General electron-withdrawing groups | X (halogens) and Y (atom bearing full or partial + charge) |
| a substituent effects two aspects of the electrophilic aromatic substitution reaction | rate of reaction and orientation |
| Ortho, Para Directors are ________ | activators (activate a benzene ring) |
| What is the general structure of ortho,para directors? | R Group or Z: Group (not halogen) |
| rank ortho, para directors in order of increasing activation | R < :NHCOR < OR < OH < NH2, NHR, NR2 |
| ortho,para directors have electron-_________ groups that activates the benzene ring to electrophilic attack | donating |
| Meta Directors are ___________ | deactivators (deactivate a benzene ring) |
| What is the general structure of meta directors? | Y (full or partial + charge) |
| rank meta directors in order of increase DEactivation | +NR2 < NO2 < SO3H < CN < COOH < COOR < COR < CHO |
| meta directors have electron-________ groups that deactivate the benzene ring to electrophilic attack | withdrawing |
| Halogens are in a group by themselves. What do halogens do to benzene ring? | they deactivate a benzene ring and direct substitution ortho and para |
| _________ react faster than benzene while __________ react more slowly than benzene | activators, deactivators |
| To understand why some substituents make a benzene ring react faster or slower, we must evaluate the rate-determining step of the mechanism. What is the rate-determining step of electrophilic aromatic substitution? | the first step: addition of the electrophile to form a resonance-stabilized carbocation |
| the more stable the carbocation, the _______ the reaction | faster (lower in energy the transition state) |
| electron-donating groups _______ the carbocation and activate a benzene ring towards electrophilic attack | stabilize |
| electron-withdrawing groups _______ the carbocation and edactivate a benzene ring towards electrophilic attack | destabilize |
| to determine the directing effects of a given substituent, we can use a stepwise procedure: | (1)draw all resonance structures for the carbocation formed from E+ attack (o,m, and p) (2) evaluate the stability of the intermediate resonance structures. E+ attacks at those positions that give most stable carbocation |
| Benzene rings activated by strong electron-donating groups (OH, NH2) and their derivatives (OR, NHR, and NR2) undergo __________ when treated with X2 and FeX3 | polyhalogenation (every ortho and para H is replaced) |
| monosubstitution of H by X occurs with X2 ALONE without added catalyst. This forms what in the products? | a mixture of ortho and para products |
| Reinforcing Directing Effects: When the directing effects of two groups reinforce, the new substituent is located where? | on the position directed by both groups |
| Opposing Directing Effects: When the directing effects of two groups oppose each other, then what? | the more powerful activator wins |
| No substitution occurs between two meta substituents because of _________ | crowding |
| nucleophilic aromatic substitution results in what? | the substitution of a halogen on a benzene ring by a nucleophile |
| What two different mechanisms are proposed to explain the results of the reaction? | addition-elimination or elimination-addition |
| nucleophilic substitution by addition-elimination mechanism (e-withdrawing groups at o and p positions are required) | (1)addition of nucleophile to form a carbanion (2) loss of the leaving group to reform the aromatic ring |
| Reactivity trends in nucleophilic aromatic subst.: increasing the number of e-withdrawing groups _______ the reactivity of the aryl halide | increases (e-withdrawing groups stabilize the intermediate carbanion) |
| Reactivity trends in nucleophilic aromatic subst.: increasing electronegativity of the halogen ___________ the reactivity of the aryl halide | increases (stabilizes carbanion by inductive effect |
| Nucleophilic Aromatic Subst. by elimination-addition: | (1)elimination of HX to form benzyne (2) nucleophilic addition to form subst. product |
| Nucleophilic aromatic substitution by an elimination-addition mechanism affords substitution on the carbon directly bonded to the leaving group and the carbon adjacent to it. Explain position of product | p and m |
Created by:
cmccartney2