eas on substituted benzenes how do we know where e + will substitute? in general, consider: 1....
TRANSCRIPT
EAS on Substituted Benzenes
• How do we know where E+ will substitute?• In general, consider:
1. Orientation• Z is either an ortho,para-director or a meta-director
2. Rate• Z is either activating (the rate of reaction is faster than benzene) or
deactivating (the rate of reaction is slower than benzene)
Z
E+
Z Z Z
E
E
E
Example 1• Nitration of nitrobenzene
• Is –NO2 (the existing substituent) an ortho,para-director or a meta-director?
• Is –NO2 an activator or a deactivator?
NO2
HNO3
NO2NO2 NO2
NO2
NO2
NO2
H2SO4
100 oC
93%
7%
Example 2• Bromination of anisole
• Is –OCH3 an ortho,para-director or a meta-director?
• Is –OCH3 an activator or a deactivator?
• Why more para product than ortho?
OCH3
Br2
OCH3 OCH3
Br
Br
H2O
4%
96%
Activating vs. Deactivating Substituents
• What structural features do activating substituents have in common?
• What structural features do deactivating substituents have in common?
• Electron-donating groups vs. electron-withdrawing groups
Activating/Deactivating Substituent
Strongly activating -NR2, -OR (R = H or alkyl)
Moderately activating -NHC(O)R, -OC(O)R
Weakly activating -R, -Ph
Weakly deactivating -X
Moderately deactivating -C≡N, -SO3H, -C(O)Z (Z = H, R, N, O)
Strongly deactivating -NO2, -NR3+
Remember, no F-C reactions with these substituents
• Activators• Electron-donating groups• Have lone pair on atom next to
benzene, or can donate electrons through induction (-R, -Ph)
• More electron density on ring makes benzene a better nucleophile
• Deactivators• Electron-withdrawing groups• Have + or d+ on atom next to
benzene• Less electron density on ring
makes benzene less nucleophilic
Activating vs. Deactivating Substituents
• Induction• Electrons moving
toward or away from ring through bonds
• Based on partial charges; electronegativity
• Resonance• Substituent on ring
can participate in resonance structures
Electron Donating and Withdrawing
Directing Effects
• Which of these substituents are ortho,para-directors?• Which of these substituents are meta-directors?• Why?
Activating/Deactivating Substituent
Strongly activating -NR2, -OR (R = H or alkyl)
Moderately activating -NHC(O)R, -OC(O)R
Weakly activating -R, -Ph
Weakly deactivating -X
Moderately deactivating -C≡N, -SO3H, -C(O)Z (Z = H, R, N, O)
Strongly deactivating -NO2, -NR3+
↑ o,p
↓ m
Directing Effects• Relationship between structure and orientation effects?• o,p-Directors
• Electron-donating groups
• m-Directors• Electron-withdrawing groups
• Why?• Look at how the intermediates (o,p and m) are stabilized through
resonance or induction
Example• Nitration of anisole
• See benzene worksheet questions 20-24
• o,p-Intermediates have more resonance structures (4) than m-intermediate (3)
Example• Nitration of
nitrobenzene
• See benzene worksheet questions 25-28
• m-Intermediate has more stable resonance structures (3) than o,p-intermediates (3 structure, but 1 is unlikely to form)
Summary• Electron-donating groups: activators and o,p-directors• Electron-withdrawing groups: deactivators and m-directors• Exception: halogens
• Deactivators (EWG due to electronegativity and inductive effect)• o,p-Directors (EDG through resonance)
Summary
EAS on Disubstituted Benzenes• Example 1:
EAS on Disubstituted Benzenes• Example 2:
EAS on Disubstituted Benzenes• Example 3:
Predict the major product(s)
a)
b)
c)
d)
e)
f)
Br2
FeBr3
Br2
FeBr3
O
C
O
CH3CH2COCl
AlCl3
NO2
CH3Cl
AlCl3
OH
Br2
FeBr3
NO2
HNO3
H2SO4
Synthesis Example 1• How would you prepare m-bromonitrobenzene from
benzene?• Process:
1. Identify steps that need to occur• Bromination • Nitration
2. Figure out the order in which the steps need to occur• Which reaction comes first?• Look at directing effects of the two substituents to decide
• Synthetic scheme:
Synthesis Example 2• Target molecule:
• Look at possible precursors
Synthesis Example 2, cont.• Then, look at possible precursors to that target
Synthesis Example 2, cont.• Finally, create synthesis scheme:
Synthesis Example 3From McMurry, Organic Chemistry, 8th ed., 2012
Synthesis Example 3, cont.
Synthesis Example 3, cont.
Synthesis Example 3, cont.
Synthesis Problems
a)
b)
c)
Cl
C(O)CH3
CO2H
Cl
CH3
NO2
Cl
C
O
HO