Aromatic Substitutions Flashcards
Aromatic Bromination: Reactants
Br2
———->
FeBr3
–> React together to form Br- and FeBr4 -
–> Bromine from FeBr4 gets attacked by benzene! (has a partial positive due to association with FeBr3)
Aromatic Chlorination: Reactants
Cl2
———->
FeCl3
–> React together to form Cl- and FeCl4 -
–> Chlorine from FeCl4 gets attacked by benzene! (has a partial positive due to association with FeCl3)
Aromatic Iodination: Reactants
I2
———->
CuCI2 (or H2O2)
Aromatic Fluorination: Reactants
F-TEDA-BF4
——————>
TfOH
Aromatic Nitration: Reactants
HNO3
——————>
H2SO4
Aromatic Nitration: Reactants
Aromatic Sulfonation: How is the electrophile formed?
1) HNO3 deprotonates H2SO4
2) Creates water LG on the H2NO3
3) Water spontaneously leaves creating NO2 +
(acts as the electrophile!)
Aromatic Sulfonation: Reactants
SO3
——————>
H2SO4
Aromatic Sulfonation: How is the electrophile formed?
1) SO3 deprotonates H2SO4
2) Forms HSO3 +
(acts as the electrophile!)
What is the general mechanism for aromatic E+ substitution?
1) 1 set of benzene pi electrons attack electrophile
2) Electrophile adds to one side of broken double bond, giving the other carbon a (+) charge
–> The carbon with the added E+ now has the E+ AND Hydrogen on it
3) Some basic component comes in and ABSTRACTS the hydrogen on the same carbon as the E+
4) = Reforms the double bond
Friedel Crafts ALKYLation: Reactants
1°R-Cl
————->
AlCl3
–> The alkyl chloride MUST be primary!!!
Friedel Craft ALKYLation Mechanism
1) AlCl3 “takes” the Cl from1°R-Cl
==> AlCl3 - and CARBOCAT of 1°R (E+)
2) Potential rearrangment of carbocat
3) Pi electrons from benzene attack the carbocation
4) Carbocat gets added to one of the broken-DB carbons, giving (+) charge to the other carbon
5) Basic components abstracts the hydrogen on the carbon with the E+ added
==> DB reforms
What is the E+ in friedel-crafts alkylation?
The carbocation generated from 1°R-Cl
Friedel Crafts ACYLation (not to the alkyl): Reactants
Cl-C=O-R (Acyl chloride)
———————————–>
AlCl3
Friedel Crafts Acylation Mechanism
1) AlCl3 “takes” the Cl from the acyl chloride
==> AlCl3 - and CARBOCAT ON the acyl group (E+) (No rearrange possible)
2) Pi electrons from benzene attack the carbocation (on the acyl carbon)
4) Carbocat gets added to one of the broken-DB carbons, giving (+) charge to the other carbon
5) Basic components abstracts the hydrogen on the carbon with the E+ added
Friedel Crafts Acylation TO the alkyl sub: Reactants
1. Cl-C=O-R (Acyl chloride)
———————————–>
AlCl3
2. H2
————->
Pd/C
What does H2 + Pd/C do to acyl substituted benzene?
Reduces the acyl group! (Removes the double bonded oxygen and replaces with 2 Hydrogens!)
What is a limitation of FC alkylation?
How does FC acylation overcome this?
FC ALKYlation produces a carbocation E+ that is SUBJECT TO REARRANGEMENT
FC ACYlation on the other hand produces a carbocation E+ that is NOT subject to rearrangement!
Reactions conditions for Nucleophilic Aromatic Substitution;
1) Good LG on the ring (halogen)
2) EWG (deactivating grp) in the ortho or para position to the LG
If there are multiple good LGs on the ring with an EWG ortho/para to them, which LG is the one that reacts?
The halogen that is more electronegative!
BECAUSE the rate limiting step is about formation of the most stable carbANION
(a more electronegative LG will produce a more stable carbanion via inductive effects = greater electron distribution!)
Nucleophilic Aromatic Substitution Mechanism
1) Nucleophile attacks the carbon on the ring with the LG on it
2) Electrons get pushed to the carbon with the EWG = (-) charge placed here
==> CARBANION intermediate
3) The electrons on the EWG carbon then push BACK to the LG carbon, pushing the group off!
What defines an activator?
A group that PUSHES electrons INTO the aromatic ring system!
–> Pushes them by resonance and/or inductive effects
What defines a deactivator?
A groups that PULLS electrons OUT of the aromatic ring system!
–> Pulls them out by resonance and/or inductive effects
Resonance vs Inductive Effects
Res = Electron distribution through a pi system (resonance structure charge placement)
Inductive = Electron distribution through a sigma bond (dipoles)
