Transcript Electrophilic Substitution of Arenes

A2 Chemistry
Chapter 2
Chapter 2 Objectives
Properties of Phenol
Delocalisation of Benzene
Phenol + NaOH
Nitration of Benzene
Phenol + Na(s)
Halogenation of Benzene
Phenol + Br2
SAQ 2.1
SAQ 2.3
SAQ 2.6
A2: Arenes
Chapter 2 Objectives (1-8)
1.
2.
3.
4.
Show understanding of the concept of delocalisation of
electrons as used in a model of benzene
Describe electrophilic substitution of arenes with concentrated
nitric acid in the presence of concentrated sulphuric acid, a
halogen in the presence of a halogen carrier, and a
halogenoalkane such as chloromethane in the presence of a
halogen carrier (Friedel-Crafts reaction).
Describe the mechanism of electrophilic substitution in
arenes, using the mononitration of benzene as an example
Understand that reactions of arenes, such as those in point 2
above, are used by industry during the synthesis of
commercially important materials, for example explosives,
pharmaceuticals and dyes (from nitration), and polymers such
as polystyrene (from alkylation),
Chapter 2 Objectives
(continued)
5.
6.
7.
8.
Explain the relative resistance to bromination of benzene,
compared with cyclohexene, in terms of delocalisation of the
benzene ring.
Describe the reactions of phenol with bases and sodium to form
salts and with bromine to form 2,4,6-tribromophenol
Explain the relative ease of bromination of phenol, compared with
benzene, in terms of activation of the benzene ring
State the uses of phenols in antiseptics and disinfectants
HC
CH
HC
CH
HC
Consider only
the carbon ring
that lies in a
plain.
The structure of benzene
can be represented in a
variety of ways
CH
C
C
C
C
C
C
Add the  bonds noticing they
are adding perpendicular to the
plain of the ring, then taking away the lower lobes for clarity
If you imagine
the lobes
being large
enough to
overlap, the
image
changes to:
Again for clarity, the electron clouds
above and below the plain of the ring
have been reduced in size , the
green being above the plain of
the ring and pink below.
e-
C
e-
C
e-
C
C
e-
e-
C
C
Because of the
delocalisation, a
benzene ring
does not attract
electrophiles with
the same force as
aliphatic double
bond molecules.
e•Three electrons are added to each cloud area.
•Because of the overlapping orbitals, they are able to
move over the entire perimeter of the ring.
•This is known as delocalisation.
Replay
slides
Electrophilic Substitution of Arenes
• Example 1: A mixture of concentrated nitric acid
(HNO3) and concentrated sulphuric acid (H2SO4)
The sulphuric acid catalyst provides protons
H2SO4 (l)  H+ +
HSO4-
Nitric acid has
this spatial
arrangement.
The proton from H2SO4
adds to HNO3 creating a
Rearrangement
new arrangement
then occurs
HNO3
This is the
electrophile
which will add
molecules with
NO2+
double bonds.
Nitronium ion
To replay the sequence, right click on the screen and select previous.
Loss of H2O occurs
Substitution on the benzene ring continues as illustrated
benzene
nitronium ion
NO2+
A pair of electrons
moves from the
ring toward the
nitronium ion…
The nitronium ion
moves toward the
benzene ring
giving the
intermediate
Loss of H+
occurs finally
giving nitro
benzene
To replay the sequence, right click on the screen and select previous.
Electrophilic Substitution of Arenes
• Example 2: A halogen (X2) in the presence of a
Iron (III) chloride arrives
halogen carrier. (Br2 with FeCl3)
as the catalyst to help
the reaction.
Benzene…
Initially, bromine
does not become
polarized enough to
react with benzene.
In the presence of
the catalyst,
bromine’s polarity
changes from…

+
Br
Br
Br Br2Br
…is going to
react with
bromine

_

FeCl
Fe 3Cl
Cl
Cl
Br
Fe
Cl
Cl

_
+
Electon
density
shifts…
Into…
Bromine’s
electron density
is immagined
as…
Cl
_
+ Br+
_
It spatial
arrangement
creates the
following
dipoles…
To
create…
Answer SAQ 2.1, 2.2 & 2.3
Br+ ion is a strong enough
electrophile to add to the benzene
ring
The hydrogen
atom is
substituted by
the bromine
atom
Br+
Br
H
+ HBr
Cl
+
Br FeCl
Fe 3 Cl
Cl
The catalyst is regenerated
To replay the sequence, click 
Phenols and their properties
OH
Phenol occurs widely in
nature but the effects differ
remarkably.
OH
Estradiol
HO
•An important female sex
hormone
•Maintains female sexual
characteristics
•Stimulates RNA synthesis and
therefore promotes growth.
Phenol
OH
O
CH 3
•Found in seed pods of vanilla
orchid
•Used as flavouring additive in ice
cream and chocolate
C
H
H2
C
H2C
O
Cholesterol
Vanillin
CH 2
H2C
CH 3
CH
CH 3
HO
Phenol reacting with Alkali (NaOH)
OH
The ring
draws
electron
density
toward it
This weakens
the O-H bond
allowing it
dissociate more
easily than
other alcohols.
It is therefore
slightly acidic.
pH ~ 6
+
+
-
O Na+
+ OHNa
NaOH
Sodium phenoxide
An alkali will
react with
phenol in the
expected way
producing a salt
and water.
+
H2O
Phenol reacting with sodium metal (Na(s))
OH
+
2Na
2
O
Na
Na(s) reacts similarly with all alcohols…
…but phenol is somewhat more reactive.
Because density is drawn into the ring…
…the hydrogen comes off more readily (H+)
Reduction: gain of electrons
Sodium metal and
hydrogen ions undergo
oxidation and reduction.
Na(s) + 2H+(aq)
Oxidation: loss of electrons
Na+(aq) + H2(g)
+ H2 (g)
OH
OH
Br
1
Br
HBr
Br
Br2 Br
1
H
OH
OH
Br
Br
Br
Br
Br
2
HBr 2
H
Br
OH
OH
Br
Br
Br
3
Br
HBr
Br
Br
H
Br
Br
3
OH
OH
Br
+
phenol
Br
+
3Br2
bromine
Br
2,4,6-tribromophenol
Answer SAQs 2.4, 2.5 & 2.6
3HBr
Hydrogen
bromide
CH 3
CH 3
SAQ 2.1
A. Draw and name three
isomers which might be
produced following
electrophilic substitution of
NO2+ for one hydrogen
atom in methylbenzene.
NO 2
NO 2
2-nitro methyl benzene
3-nitro methyl benzene
CH 3
NO 2
4-nitro methyl benzene
B. TNT has the systematic
name 1-methyl-2,4,6trinitrobenzene. Draw the
structural formula of TNT
CH 3
O2N
NO 2
NO 2
2,4,6-trinitro methyl benzene
SAQ 2.3
A. Suggest a suitable halogen
carrier to use in the
reaction of benzene with
chloromethane.
B. Suggest suitable reactants
which might lead to the
formation of the following
compound in the presence
of a halogen carrier.
Anhydrous FeCl3 or AlCl3
Benzene + 2-chloro-2-methyl propane
CH 3
+ H3C
C
CH 3
Cl
B. Write a balanced equation
using your suggested
reactants
CH 3
+
Cl
C
CH 3
CH 3
CH 3
CH 3
CH 3
SAQ 2.6
Br 
A. How does bromine in
aqueous solution become
sufficiently polar to achieve
electrophilic substitution
_
 Fe
FeCl3 Br 
+
Br
Br
Br
Bromine has a
symmetrical electron
density arrangement
_
_
In the presence of
FeBr3, the electron
density shape
changes