Topic 14: Aromaticity and Electrophilic Aromatic Substiution Flashcards
Kekule structure of benzene year
Correct structure (C6H6)
1865
Incorrect structure of benzene
Triprimane (C6H12)
1825
Faraday
Discovered benzene in 1825
Aryl group (Ar)
Benzyl (Bn)
Benzoyl group
C6H5R
R is alkane in benzyl
R is C=O R in benzoyl
Benzene’s reactivity is … to a typical alkene. It is … and …
different
less reactive (e.g. doesn’t react with X2 (halogenation)
more stable
Hydrogenation of Benzene: Benzene can be … and is … due to ..
reduced
more stable than expected
resonance stabilisation energy
Origin of name for aromatic compounds
They were … than expected
Smell
more stable
Aromaticity comes from …
complete delocalisation of pi electron density around the ring
C-C bonds in Benzne are ..
1.4 angstroms
Benzene has … resonase forms
The resonance hybrid is … for drawing mechanisms
reduced
more inconvenient
Hybridisation of C in benzene
Sp2
unhybridised pz orbitals -orthogonal to the plane
strong overlap of pz orbitals
6 electrons in 6 pz orbitals evenly distributed around the ring
nodal planes
regions where the probability of finding an electron is 0
The chemical shift of a H in benzene is … than a H in cyclohexene because …
higher
the pi cloud electron density sets up a ring current when placed in a magnetic field. The ring current seats up its own magnetic field. The H atoms at the edge of benzene experience a greater field than the applied field alone so a greater frequency is required to bring protons into resonance. Can induce a field to oppose applied field and lower chemical shift.
4 rules of aromaticity
Cyclic ring
Cyclic ring must be planar
Cyclic ring must be fully conjugated (must have sp2 hybridised atoms with unhybridised p orbitals in ring atoms)
Cycic ring must have 4n + 2 pi electrons (Huckel rule) (all pi electrons form p orbitals are in bonding MOs)
In benzene, electrophilic addition is … as it … . … is favoured as it … . The … is attracted to … and the … such as … because … activates the …
unfavourable
loses aromaticity
substitution
doesn’t lose aromaticity
electrophile
electron in aromatic ring
catalyst
Lewis acids like AlCl3
it is electron deficient so attracts electron density from lewis base
electrophile to generate E+
Wheland intermediate is …
It is a … and it’s not …
It has … resonance forms
Positively charged intermediate in the electrophilic aromatic substitution
reactive intermediate
aromatic
3
Describe the reaction coordinate diagram of electrophilic aromatic substitution
2 peaks
TS1 at top of first higher peak
Wheland intermediate between peaks (lower in energy than TS1 + TS2, higher than reactants and products)
TS2 at top of second, lower peak
loses aromaticity slowly, gains quickly
Freidel Crafts Acylation is the reaction between … where a curly arrow goes from … to … .This forms the intermediate … where the curly arrow goes from … into to … which forms … and … .The … is stabilised by … and the 2nd is formed when a curly arrow goes from … to the … which forms … and moves the positive charge to the … .The … resonance form reacts with … . A curly arrow comes from … to the … and from the … to the … .This forms a … intermediate which reacts with a … . A curly arrow comes from … to the … and the … to the … and a … is formed.
acid chloride (R-(C=O)-Cl) and a lewis acid catalyst such as AlCl3
Cl lone pair on acid chloride
Al on AlCl3
[R-(C=O)-Cl]^+[AlCl3]^-
The C-Cl^+ bond
Cl^+
R-C^+=O (acylium cation)
AlCl4^-
R-C^+=O (acylium cation)
2 resonance forms
lone pair on oxygen
C=O bond
a triple C-o bond
O
2nd
benzene
double bond in benzene
C in electrophile
triple bond in the electrophile
O^+ on the electrophile
positively charged
base
base lone pair
H on the same C as C=O-R
The C-H bond (same H as above)
C-C^+ bond
phenyl ketone (C6H5-C(=O)-R)
Freidel Crafts Alkylation is the reaction between … where a curly arrow goes from … to … .This forms the intermediate … where the curly arrow goes from … into to … which forms … and … .The … is stabilised by … .The … reacts with … . A curly arrow comes from … to the … .This forms a … intermediate which reacts with a … . A curly arrow comes from … to the … and the … to the … and … is formed.
an alkyl halide reagent such as tertiary butyl chloride and AlCl3
Cl lone pair on acid chloride
Al on AlCl3
[(CH3)3C-Cl]^+[AlCl3]^-
C-Cl^+
Cl^+
(CH3)3C^+
[AlCl4]^-
(CH3)3C^+
+I inductive effect
(CH3)3C^+
benzene
double bond in benzene
C^+ in (CH3)3C^+
positively charged
base
base lone pair
H on the same C as C(CH3)3
The C-H bond (same H as above)
C-C^+ bond
C6H5C(CH3)3
2 problems of alkylation
scrambling with primary and secondary alkyl halides e.g. formation of secondary or tertiary carbocations
polyalkylation can occur
How to avoid scrambling/polyalkylation
Make phenyl ketone by Fredel Crafts Acylation and reduce carbonyl
Make desired product using Gringard reagent
Halogenation is the reaction between … where a curly arrow goes from … to … .This forms … .This also works with … .
Cl2 and AlCl3
Cl lone pair on Cl2
Al on AlCl3
Cl-Cl^+-Al^–Cl3
Br2 or FeBr2
Nitration forms … from … and continues to form products such as … . … reacts with … .A curly arrow goes from … to the … to form … and … and then … which reacts with … .A curly arrow comes from … to the … .This forms a … intermediate which reacts with a … . A curly arrow comes from … to the … and the … to the … and … is formed.
amine
nitrobenzene
diammonium cation
conc HNO3
conc H2SO4
the lone pair on the O attached to H and N^+ on O^–N^+(=O)-O-H
H on H2SO4
O^–N^+(=O)-O(-H)-H
HSO4^-
O=N^+=O
benzene
double bond in benzene
N^+ in O=N^+=O
positively charged
base
base lone pair
H on the same C as NO2
The C-H bond (same H as above)
C-C^+ bond
C6H5CNO2
Sulphonation is the reaction between … and … to form the electrophile … which reacts with … . A curly arrow comes from … to the … .This forms a … intermediate which reacts with a … . A curly arrow comes from … to the … and the … to the … and … is formed. It can react with … and … under … to reform benzene
H2SO4
SO3
S(=O)2=O^+-H
benzene
double bond in benzene
N^+ in O=N^+=O
positively charged
base
base lone pair
H on the same C as SO3H
The C-H bond (same H as above)
C-C^+ bond
C6H5SO3 (benzene sulphonic acid)
steam
H^+
steam distillation
Formulation is the introduction of … . The … reacts with … under … to form … which reacts with … to form … so can act as a … . The final product when reacting with benzene is …
an aldehyde group
C^-=O^+
HCl
equilibrium
H-C(-Cl)=O (formy chloride)
AlCl3 and CuCl
H-C(triple bond)O^+
benzylaldehyde
Halogenation:
reagents
active species
product
X2, Lewis acid (AlCl3 for Cl2/ FeBr3 for Br2)
Cl-CL^+-[AlCl3]^-
C6H5Cl/Br (chloro/bromobenzene)
Nitration
reagents
active species
product
Conc HNO3 and conc H2SO4 (or Ac2O)
Nitronium (O=N^+=O)
C6H5NO2 (nitrobenzene)
Sulfonation
reagents
active species
product
notes
Oleum (H2SO4 +SO3)
O=S(=O)=O^+-H
C6H5SO3H (benzene sulphonic acid)
Reversible using H^+, H2O under heat
Freidel-Crafts Alkylation
reagents
active species
product
notes
R-Cl, Lewis acid
Alkyl carbocation
Alkyl benzene
(1,2 H-shift polyalkylation, scrambling)
Freidel-Crafts Acylation
reagents
active species
product
notes
ROCl, Lewis acid
Acylation (R-C triple bond O^+)
C6H5C(-R)=O
Benzene ketone
Solution to alkylation problems- reduce carbonyl alkyl
Formylation
reagents
active species
product
CO, HCl, AlCl3, CuCl
Formyl (H-C triple bond O^+)
Benzaldehyde
