Nitration of Methyl Benzoate
Demonstration of the effect of an electron withdrawing
group on a monosubstituted benzene ring on subsequent
substitution of other groups on the Benzene ring
References:
1/11/2017

Pavia, et al.
– pp 338 – 342

Slayden, et al.
– pp 76 – 79
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Nitration of Methyl Benzoate
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Overview:
 Synthesis – Nitration of Methyl Benzoate through an
Electrophilic Aromatic Substitution to form:
Methyl m-Nitrobenzoate (MW – 181.15)
Methyl 3-Nitrobenzoate
(MP – 78oC)
3-Nitrobenzoic Acid Methyl Ester
 Determination of reactant Masses, Moles, Molar Ratio,
Limiting Reagent, Theoretical Yield
 Reaction mixtures must be kept cool
 Separation and Purification of Product by Vacuum
Filtration and Recrystallization from Methyl Alcohol
 Percent Yield
 Melting Point
 Summary of Results
 Analysis and Conclusions
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Nitration of Methyl Benzoate
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The Laboratory Report:
 Synthesis Experiment
 Mass, Moles, Molar Ratio, Limiting Reagent,
Theoretical Yield
 Procedures
 Title – Concise: Mass reagent, Vacuum Filtration,
Recrystallization, etc.
 Materials & Equipment (2 Columns in list (bullet)
form)
Note: include all reagents & principal equipment used
 Description:
 Descriptions must be detailed, but concise
 Use list (bullet) form
 Use your own words – Don’t copy book!!
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Nitration of Methyl Benzoate
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Synthesis Experiment (Con’t)
 Results – Neat, logically designed template to
present results (in box to right of Procedure
description)
 Summary
 Paragraph summarizing experimental
observations and computed results
 Analysis & Conclusions
 Limiting reagent
 What is the nature of the product you expected
and what evidence do you have to indicate you
actually got what you expected?
 What was the yield of your product and what
aspects of your experimental procedure could
be improved?
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Nitration of Methyl Benzoate
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Background:
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Electrophilic Addition vs. Electrophilic Substitution

Alkenes, which contain pi () (-C=C-) bonds, are
electron-rich due to the excess of electrons in the
() bonds

These electrons are susceptible to electron-seeking
(electrophilic) reagents called Electrophiles

In an Electrophilic Addition of an Alkene, the Alkene
acts as an Electron-Rich Nucleophile providing a
source of electrons for the Electrophile, for
example, a proton (H+) from an acid (acting as a
Lewis acid)
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Nitration of Methyl Benzoate
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Example of Electrophilic Addition
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Nitration of Methyl Benzoate
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Background (Con’t)
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Electrophilic Aromatic Substitution

Aromatic compounds also have the electron rich pi
() bonds

The resonance in the Benzene ring, however,
makes the  electrons less susceptible to
Electrophilic Additions

An addition reaction would result in a loss of
resonance stabilization

Aromatic compounds do, however, react with
strong electrophilic reagents at somewhat elevated
temperatures in substitution reactions
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Nitration of Methyl Benzoate
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Background (Con’t)
 Electrophilic Aromatic Substitution
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
In today’s experiment the Benzene ring of Methyl
Benzoate is reacted with a mixture of concentrated
Nitric and Sulfuric acids, i.e., source of Nitronium ion

The positively charged Nitronium ion (NO2+) acts as
the Electrophile, temporarily disrupting the ring
resonance, and adds to the Nucleophilic Benzene Ring
forming the intermediate resonance-stabilized
Arenium ion, an electron deficient positively charged
delocalized Carbocation

The rate of formation of the Arenium ion, which is
somewhat stabilized by ring resonance, determines
the rate of reaction
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Nitration of Methyl Benzoate
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Background (Con’t)
The Carbomethoxy group is electron withdrawing, thus;
it deactivates the ring relative to Benzene.
The resultant “resonance” structures favor “Meta”
substitution over Ortho/Para
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Nitration of Methyl Benzoate
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Nitration of Methyl Benzoate
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Background (Con’t)
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Electrophilic Aromatic Substitution (Con’t)

The Carbomethoxy group is electron withdrawing

It has a moderately “Deactivating” effect on the
ring relative to Benzene itself, thus the transition
state to the Arenium ion is highly unstable,
withdrawing electrons from the developing
carbocation leading to increased positive charge on
the ring

The inductive effect of this electron withdrawal sets
up a dipole with the ring at the positive end

Any resonance form of the Arenium ion that would
enhance this positive charge, e.g., either ortho or
para resonance structures, would further destabilize
the ring
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Nitration of Methyl Benzoate
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Background (Con’t)
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Electrophilic Aromatic Substitution (Con’t)

The resonance forms of the Meta substituted
Arenium ion, however, do not attempt to add
additional positive charge at the carbon atom
attached to the electron-withdrawing carbonyl
group of the Carbomethoxy group (see previous
slide)

Electron withdrawing groups favor Meta
substitution because the Meta resonance structures
are more stable than O/P

Full resonance is restored by eliminating the proton
from the sp3-ring carbon (also containing the
Nitronium ion) to the HSO4- ion

By eliminating the proton, the Nitronium ion is
therefore substituted on the ring
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Nitration of Methyl Benzoate
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Background (Con’t)
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Electrophilic Aromatic Substitution (Con’t)

After the single substitution of the Nitro group to
the first Meta position, the combination of the
Carbomethoxy group and the Meta-Nitro group
further “Deactivates” the ring against additional
substitution

Keeping the reaction mixture temperature low also
inhibits the formation of “Dinitration” products

Small amounts of the Ortho and Para isomers of
Methyl m-Nitrobenzoate and the “Dinitration”
products can be in the reaction mixture

These are removed by recrystallizing the solution
with Methanol
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Nitration of Methyl Benzoate
Methyl & Alkyl groups are activating because
of the stabilizing effect of sp2 hybridization
(hyperconjugation) of an unbonded electron
in methyl radical.
Halogens are o,p directing because the
electron donating resonance effect is more
dominant than the withdrawal inductive
effect of these electro-negative
elements.
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Activators (Donate, Release Electrons)
 Available pair of unbonded electrons to
donate to ring.
 More Shielding of Protons
 Less NMR Chemical Shift downfield
Deactivators (Withdraw, Accept Electrons)
 No unbonded electron pairs
 Less Shielding of Protons
 More NMR Chemical Shift downfield
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Nitration of Methyl Benzoate
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Procedure
 Determine the mass of the Methyl Benzoate to the nearest
0.001g (MW – 136.15)
 Determine the mass of 2.000 mL of Conc HNO3 (MW –
63.01) from the volume, density and % Composition
Den – 1.42 g/mL, % Acid - 70.0%)
(Use a volumetric pipet to obtain the HNO3)
 Limiting Reagent & Theoretical Yield (in your report)
 Calculate the Moles of Methyl Benzoate and Nitric Acid
 Setup the Stoichiometric balanced equation
 Determine the Stoichiometric Molar Ratio
 Determine the limiting reagent from the Stoichiometric
balanced equation and the actual moles of reagents
used
 Compute the theoretical yield
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Nitration of Methyl Benzoate
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Procedure (Con’t)
 Note the following precautions
 Sulfuric Acid protonates Nitric Acid acting as a weak
base to form the Electrophilic Nitronium Ion, which
is added to the resonance disrupted Benzene ring
replacing a Proton
 Use extreme care to avoid adding any additional
water to the reaction mixtures
 Water must be kept to a minimum so as to enhance
the reactivity of the nitrating mixture. Water is a
stronger base than HNO3, which is basic relative to
H2SO4; thus, it would interfere with the protonation
of the Nitric Acid, hence the formation of the
Nitronium ion
 The reaction temperature must be kept low to
prevent the excessive formation of the Dinitration
products
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Nitration of Methyl Benzoate
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Procedure (Con’t)
 Prepare an ice/water bath using a 150 ml beaker
 Place a 50 mL Erlenmeyer Flask containing
approximately 6 mL of concentrated Sulfuric Acid
(H2SO4) into the ice/water base; allow to cool
 Add the Methyl Benzoate to the Sulfuric Acid, swirl the
mixture, and allow to cool
 Prepare another ice/water bath using a 100 mL beaker
 Pipet the 2.00 mL of concentrated Nitric Acid (HNO3)
into a small, clean, dry vial and place the vial in the
new ice/water bath (the sides of the vial should be
immersed in the ice/water bath about half-way
 Measure another portion of about 2 ml (exact volume
not critical) of concentrated sulfuric acid and carefully
and slowly add it to the Nitric Acid in the vial
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Nitration of Methyl Benzoate
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Procedure (Con’t)

Allow the mixture to cool for about 10 minutes

Note: Avoid introducing any water into mixture!

Using a medicine dropper or plastic disposable pipet,
very slowly with continuous gentle swirling, add the
cooled HNO3 / Sulfuric acid mixture to the cooled
Methyl Benzoate / Sulfuric Acid mixture
NOTE: The temperature of the reaction mixture must
be kept below 15 oC
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After the two reaction mixtures have been combined,
keep the combined mixture in the ice/water bath for
at least 5 minutes in order to keep the solution
temperature in a cooled state until the reaction is
complete
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Nitration of Methyl Benzoate
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Procedure (Con’t)

Allow the reaction mixture to warm to room
temperature (10 min or so)

Pour the reaction mixture over approximately 25
grams of ice in a 100 mL or 150 mL beaker

The product precipitates; allow the ice to melt
Note: If you get a small yield, you probably
introduced some water from the ice/water bath
Even a small drop of water is sufficient to shut
down the reaction
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
Pre-weigh the top of the Buckner Funnel

Isolate the precipitated product by Vacuum Filtration
through a Buckner Funnel
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Nitration of Methyl Benzoate
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Procedure (Con’t)

Wash product with two 15 mL portions of cold distilled
water

Dispose of aqueous filtrate down the drain with lots of
water

Wash product again with two 5 mL portions of cold
Methanol

Dispose of the Methanol filtrate in the waste jar in the
hood

Weigh the Buckner funnel again to determine the
weight of the crude product

Transfer the crude product to another clean beaker
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Nitration of Methyl Benzoate
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Procedure (Con’t)

Recrystallization

Determine the starting volume of Methanol required
from the mass of the crude product and the density
of methanol.
Density (g/mL)
= mass (g) / Vol (mL)
Density of Methanol = 0.79 g/mL
 Volume Methanol = mass (sample) / density
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
Add Methanol to Sample beaker

Add enough additional Methanol to just cover the
sample

Place beaker containing sample and Methanol in a
larger beaker ¼ full of water, i.e., a water-bath
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Nitration of Methyl Benzoate
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Procedure (Con’t)

Recrystallization (Con ‘t)

Heat water bath to about 80oC

Swirl contents of sample beaker until product
dissolves
Note: Do not boil contents of sample beaker.
It may be necessary to add small increments
of Methanol to effect solution of product
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Cool solution slowly to room temperature
Set up Buckner Funnel

Pre-weigh the top of the cleaned/dried Buckner
funnel or a weighing tray as directed

Pre-moisten the filter with Methanol
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Nitration of Methyl Benzoate
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Procedure (Con’t)
 Transfer purified crystals to Buckner Funnel using
small amounts of additional cold Methanol to complete
transfer of all solid material to the Buckner Funnel
 Vacuum Filtration, washing with 2 5 mL increments of
cold Methanol, to separate crystals from solution
 Dispose of Methanol filtrate in waste jar in hood
 Place the purified product (Methyl m-Nitro Benzoate)
in a pre-weighed plastic weighing tray and put in your
drawer or instructor’s drawer until next week
 Weigh the Buckner funnel containing the dry, purified
product
 Compute the mass of the product
 Compute the percent yield
 Determine the Melting Point
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