Organic Chemistry Ch 18

Electrophilic Aromatic Substitution (EAS)

reaction where an aromatic proton is replaced by a strong electrophile or a functional group is introduced to the ring. the ring acts as a nucleophile.

EAS mechanism

bromination of benzene reagents

Br2

AlBr3

chlorination of benzene reagents

Cl2

AlCl3

Nitration of benzene reagents

HNO3

H2SO4

sulfonation of benzene reagents

fuming H2SO4

adding methyl group to benzene reagents

CH3Cl

AlCl3

Sulfonation mechanism

last part has SO3H on it

Nitration mechanism

reagents to reduce nitro group to amine (NO2 to NH2)

1) Fe, HCl

2) NaOH

Friedel-Crafts alkylation mechanism

3 limitations of friedel crafts alkylation

1) halide leaving group must be attached to an sp3 C (or no rxn)

2) polyalkylation often occurs

3) some substituted aromatic rings are too deactivated to react

reagents for acylation of benzene ring

most common activating groups

methyl group, methoxy group

usually ortho-para directors

activators

major product of ortho-para directors (usually)

para

why is methyl group an activator

CH3 donates electron to the ring so it acts as a better nucleophile and is more reactive

why is methoxy group an activator

it donates electron density to a ring via resonance (even though it looks like it might remove electron density via induction)

which wins if resonance and induction compete

resonance

most common deactivating group

nitro group, halogens

usually meta directors

deactivating groups

why is nitro group deactivating

inductively electron withdrawing and removes electron density via resonance

why are halogens considered exceptions for deactivating groups

they are ortho-para directors

why are halogens ortho-para directors

they withdraw electrons via induction but donate electron density through resonance

strong activators

lone pair adjacent to aromatic ring

moderate activators

lone pair delocalized outside ring

weak activator

alkyl (hyperconjugation)

weak deactivator

halogens (exception)

moderate deactivators

carbonyls and analogs

strong deactivators

positively charged or -CX3

where does addition go on a disubstituted ring where both substituents direct to the same carbon

to the carbon that it is directed to by both substituents

where does addition go if there are multiple substituents directing it to different carbons

where the stronger activator is directing it

where does addition go if there are multiple substituents directing it to different carbons on 1,4 disubstituted rings

less sterically hindered site

where does addition go if there are multiple substituents directing it to different carbons on 1,3 disubstituted rings

not at site located between the 2 substituents

how to favor ortho over para from ortho-para directors

use a blocking group to block para position

common blocking group

sulfonation

Nucleophilic aromatic substitution abbreviation

SnAR

what occurs in nucleophilic aromatic substitution

a substituted benzene ring is attacked by a nucleophile

SnAR mechanism

3 requirements for SnAR

1) benzene ring must posses strong electron withdrawing grou

2) ring must possess good leaving group

3) leaving group must be ortho or para to electron leaving group (can’t be meta)

what is good & best leaving groups for SnAR

best is -F, halides in general are good

what is typically ewg in SnAR

-NO2

elimination-addition reaction

2 step SnAR reaction, first leaving group is eliminated by strong base which leaves a benzyne intermediate, then a nucleophile is added

benzyne intermediate

elimination-addition mechanism

Clemmenson reduction reagents

Zn(Hg)

Hcl, heat

Clemmenson reduction what it looks like

Alkyl group

acyl group