Reaction Reagents 7 Flashcards
What makes benzene so unreactive?
stabilized by resonance of 3 pi bonds, so not e- rich = AROMATIC STABILITY
What are the 2 criteria for aromatic compounds?
- fully conjugated ring with overlapping p orbitals (every atom sp2 in ring or has resonance structure with anion/cation)
- have 4n+2 = total pi electrons
if a lone pair in a ring is needed for aromaticity then…
the lone pair is not as basic
what are the positions for derivatives of benzene that contain 2 substituents called?
1, 2 substituent position = ortho
1,3 = meta
1,4 = para
What is the benzylic position? How is it stabilized?
one C away from benzene
charge delocalized and stabilized if anion or cation, or group by resonance = more acidic
benzene with benzylic C + chromic acid (H2CrO4)
benzylic position C with at least 1H mixed with chromic acid oxidizes the C
= carboxylic acid
** not H on benzylic C = NR
benzene + H2CrO4
no rxn (not easily oxidized)
alkane/alkyl group + H2CrO4
no rxn (not easily oxidized)
benzene + KMnO4
no reaction
benzene with benzylic C + KMNO4 + H2O
MnO4- oxidizes benzylic C
acidic water protonates
carboxyl group
KMnO4 and H2CrO4 are oxidizing agents
The p bonds of aromatic rings are not easily oxidized, but
benzylic carbons, are oxidized to carboxylic acids if they are bonded to one or more H.
benzene + E+ (electrophilic aromatic substitution)
pi bond nuc attacks electrophile (one C has E+ and other C is carbocation)
base takes on H that was on the C of the C=C (elimination rxn)
reform double bond/reformed aromatic ring
halogenation of benzylic C on a benzene?
MAKE E+(Br+):
Br2 + FeBr3:
one Br attacks Fe making intermediate
middle Br is EWG, other Br becomes Br+
RXN:
pi bond attacks Br in intermedate (Br+)
carbocation rerrangement in sigma complex intermediate
base (remaining intermediate: Br-FeBr3) attacks H making HBr and FeBr3
aromatcity restored
FeBr3 is a Lewis acid, could also be AlBr3
this could also be cholrination with Cl2 and AlCl3
sulfonation of benzylic C on a benzene?
Make E+ (SO3)
fuming H2SO4: base sulfuric acid attacks acidic sulfuric acid
RXN
pi bond attacks S of SO3 making carbocation
carbocation rearrangement in sigma complex
H2O formed from making E+ deprotonates benzylic C making H3O+
aromaticity restored
SO3- group’s O on benzene attacks H on H3O+ making SO3H group on benzene
what is unique about sulfonation?
reversible:
conc H2So4 = SO3H group
dilute H2So4 = benzene
nitration of benzylic C on benzene?
Make E+ (NO2):
HNO3 + H2SO4 makes NO2+ = E+
RXN
pi bond attacks N on NO2
carbocation rearrangement in sigma complex
water formed from making E+ basic attacks benzylic C’s H reforming aromaticity
nitration can be reduced to make…
amines (aniline)
benzene + NO2 -> benzene + NH2
aniline rxn of benzylic C on benzene?
benzene + 1)HNO3/H2SO4 to make nitration (NO2)
2) Fe or Zn, HCl to make benzene _ NH3+
3) NaOH base takes H
benzene + NH2
What is alkylation?
benzene + alkyl halide + lewis acid = alkyl group on benzene
Friedel-Crafts Alkylation mechanism? What is necessary for this reaction to occur?
make E+ (carbocation):
alkyl halide + AlCl3: X attacks Al making intermediate with +ve charged Cl which is EWG -> carbocation group leaves = E+
(also makes AlCl4- base)
this carbocation can be rearranged
RXN:
pi attack carbocation E+
sigma complex carbocation rearrangement
Cl of AlCl4- base attacks H in benzylic position reforming aromatic ring
necessary: halide LG is attached to sp3 carbon
alkylation of alkene to a benzylic position C?
make E+:
- alkene + H3PO4 (atrong non-nucleophilic acid)
- alkene attacks one H of H3Po4
- carbocation rearrangement
RXN
- pi bond attacks carbocation
- H2PO4- deprotonates to reform aromaticity
Friedel-Crafts Acylation benzylic position C rxn?
benzene + lewis acid + acyl halide (ketone with R grp and X)
Acylation and alkylation both…
for new C-C bonds
Friedel-Crafts Acylation benzylic position C rxn mechanism?
make E+
- acyl halide X attacks Al of AlCl3
- intermediate forms, Cl is EWG and +ve charge
- R-C(+)=O –> R-C≅O(+) acylium ion = E+
RXN:
- pi bond attacks C of acylium ion
- sigma complex carbocation rearrangement
- Cl of AlCl4- from E+ formation attacks H to reform aromaticity
forms aryl ketone/aldehyde
