Transcript Experiment 17:

Experiment 15:
SUBSTITUENT EFFECTS ON
THE RATE OF
ELECTROPHILIC AROMATIC
SUBSTITUTION
Objectives:



To perform electrophilic aromatic substitution
reactions on various monosubstituted aromatic
compounds using elemental bromine.
To determine the rate of the substitution
reaction based on a color change in order to
explore how different substituent groups on an
aromatic ring affect the reaction rate.
To purify the acetanilide product using liquid
extraction, then analyze the product using TLC
analysis to explore how the acetamide group
affects the orientation of the reaction.
Before coming to lab…

Review these techniques:

TLC Analysis

Acid-base Extraction
CHEMICAL EQUATION
• The progress of the bromination of an aromatic
ring can be followed easily by a color change.
• The more reactive the aromatic ring is, the
faster the color will disappear.
G
G
+
Br2
(red)
colorless
+
HBr
Br
colorless
colorless
MECHANISM
CH2CH2CH3
+
Br
H
CH2CH2CH3
CH2CH2CH3
Br
Br
An electron pair from
the aromatic ring
attacks Br2, forming a
new C-Br bond…
HBr
H
H
H
+
H
Br
…and leaving a
nonaromatic, carbocation
intermediate.
Br
The carbocation intermediate
loses H+, and the neutral
substitution product forms as
two electrons from the C-H
bond move to regenerate the
aromatic ring. HBr forms as a
byproduct.
REACTIVITY OF AROMATIC
RINGS
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The substituent ALREADY ON the aromatic
ring determines the position and rate of
substitution of the second (INCOMING)
electrophile.
We use the reactivity of BENZENE (no
substituent) as a reference point.
REACTIVITY OF AROMATIC
RINGS

If we use the reactivity of benzene (substituent =
H) as a reference point, activating substituents are
all electron donating groups, and their relative
activation strengths are:
H < Phenyl < CH3 < NHCOCH3 < OCH3 < OH < NH2

Deactivating groups are electron withdrawing groups
and their activities relative to hydrogen are:
NO2 < COR < CHO < I < Br < Cl < F < H
SUBSTITUENT EFFECTS
GENERALIZATIONS
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Substituents in which the atom bonded to
the ring has an unshared pair of electrons,
with the exception of alkyl and phenyl
groups, are ortho-para directing. All other
substituents are meta directing.
All ortho/para directors are activators, with
the exception of halogens. Halogens are
ortho-para deactivators.
All meta directors are deactivators.
Alkyl and Phenyl groups are also ortho-para
directing.
OVERVIEW
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React bromine solution with six monosubstituted
aromatic compounds.
Rank reactivity of aromatic compounds using the
rate of reaction based on color change.
Neutralize and extract acetanilide product to
purify compound for further analysis.
To analyze the bromoacetanilide product by TLC
analysis to identify product substitution.
SYNTHESIS
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Place small amount of
monosubstituted aromatic
compound in small test tube.
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Place test tubes in water bath.
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Add Bromine solution to each.
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Record the approximate amount of
time it takes for the solution to
lose color.
Place in hot water bath if
necessary to complete reaction.
After 1 hour, estimate the reaction
order based on the relative amount
of color lost.
Table 15.1: Reactivity Rates
of Aromatic Compounds
* The substituents can be completed by hand once the
table has been copied into your POST LAB document!
Compound
Structure
OPA,
OPD,
or MD
Reaction
Order
ethyl
benzene
anisole
acetanilide
phenol
benzaldehyde nitrobenzene
***Aromatic ring is given for each compound…remember to
complete structure with appropriate substituent!***
***Identify each substituent as an ortho/para activator (OPA),
ortho/para deactivator (OPD), or a meta deactivator (MD).***
***Simply record the order that the reactions occurred. Actual
reaction times are not required. 1=fastest  6=slowest! ***
PURIFICATION/ISOLATION
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Once pale yellow, remove acetanilide
tube.
Add deionized water to tube.
Add 5 drops of NaOH. Stir with
glass rod. Continue adding 5 drops at
a time until visible precipitate forms.
Add ethyl acetate. Place small cork in
top of test tube, and shake to mix.
Allow layers to separate.
ANALYSIS
• Prepare TLC plate and chamber.
• Apply provided standards to TLC
plate.
• Apply TOP layer from test tube to
TLC plate ( sample solution).
A
B U
C D
• Develop plate and visualize spots
using UV lamp.
filter paper
• Calculate Rf value of all spots and
identify product in sample solution.
A
B U C D
Table 15.2: TLC Results
TLC Diagram
Name of Compound
Rf Values
Standard
Sample
Solvent Front= _________cm
o-bromoacetanilide
TLC developing solvent used:
m-bromoacetanilide
_______________________
p-bromoacetanilide
* The TLC diagram can be completed by hand once the
table has been copied into your POST LAB document!
HINTS…
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Be sure that your test tubes do NOT
contain any acetone. It reacts very
quickly with bromine and can give
inaccurate test results.
If after the one hour period of heating in
the water bath the solutions have not
completely lost their color, estimate by
the relative amount of color lost.
SAFETY CONCERNS
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CAUTION: In this experiment you
are using bromine, which is poisonous
and can cause severe burns!
All aromatic solutions are prepared in
glacial acetic acid which can cause
severe burns!
WASTE MANAGEMENT
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Place ALL liquid waste in container labeled
“LIQUID ORGANIC WASTE”.
Place used TLC plates and filter papers in
yellow trash can.
Place used TLC spotters broken glass box
(NOT TRASHCAN!!!!!!!!!!!).
CLEANING UP…
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TEST TUBES: clean all test tubes with
soap/water/brush and rinse with wash acetone.
Leave inverted in test tube rack to dry.
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TLC CHAMBER: remove filter paper and leave
in drawer with cap off.
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BEAKER: if only used for water, simply dry out
with a paper towel.
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GRADUATED CYLINDER: rinse any excess
bromine solution into wash acetone container.
Clean with soap/water/brush and rinse with
wash acetone.
IN LAB QUESTIONS…
(The following questions should be answered in laboratory notebook.)
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Rate the relative reactivity toward electrophilic
aromatic substitution of benzene substituted with the
following: (1 = most reactive, 6 = least reactive)
CH3
C N
F
CH2CH2CH3
O
OCH 2CH3
OH
H3C N CH3
IN LAB QUESTIONS…
(The following questions should be answered in laboratory notebook.)

Draw the structure of the major product, and a
complete mechanism for its formation, for the
bromination of acetanilide which took place in this
experiment.