WEEK 5:
NITRATION REACTION
PURPOSE:
This experiment will introduce the student to one of the
important electrophilic aromatic substitution reactions. The
experiment uses concentrated nitric acid and concentrated
sulfuric acid so the student will need to use great care in
handling these reagents.
IMPORTANT REACTIONS:
NO2+ + 2 HSO4- + H3O+
HNO3 + 2 H2SO4
Nitronium ion
CO2CH3
CO2CH3
HNO3 / H2SO4
NO2
Methyl benzoate
(M. Mass 136.16, bp 199.6oC)
Methyl 3-nitrobenzoate
(M. Mass 181.15, mp 78oC)
BACKGROUND INFORMATION:
This experiment and the one next week are examples of
electrophilic aromatic substitution reactions. Aromatic systems
such as the benzene ring have enhanced resistance toward
reactions due to the stability that resonance gives to the
aromatic ring. Addition reactions are highly unlikely as they
would destroy the conjugated aromatic system. Substitutions can
occur and the best reagent to use should have electrophilic
properties as the aromatic ring is rich in electrons. The class
of reactions known as electrophilic aromatic substitution
reactions will be studied extensively in the lecture part of the
course. These two experiments will show how they can be
performed in the laboratory.
There are five reactions that are normally presented when
discussing electrophilic aromatic substitution reactions. They
include:
1. nitration using concentrated nitric acid and concentrated
sulfuric acid to generate the nitronium ion electrophile
(which will be done in this experiment).
2. halogenation using a halogen and a Lewis acid to generate
the halonium ion elctrophile.
3. sulfonation using sulfur trioxide as the electrophile.
This is the only neutral electrophile in this series of
reactions and this reaction is reversible.
4. Friedel Crafts alkylation using some source of a carbenium
ion as the electrophile. This reaction will be done next
week and will use an alcohol and concentrated sulfuric acid
to generate the carbenium ion.
5. Friedel Crafts acylation using an acyl halide and a Lewis
acid to generate the acylium ion electrophile.
In the experiment that will be done today, a substituted
aromatic ring is used which will lead to meta nitration of the
ring. The substituent chosen will moderate the reaction to
avoid di- and trisubstitution of the ring which could lead to
explosive products. Trinitrobenzene is explosive. It is best
not to heat the reaction too high or too long to avoid further
undesired substitution.
Also note that this reaction often shows a delayed start.
It is best to be patient. If the mixture is heated highly or
longer than directed, an uncontrolled reaction may ensue. And,
if the reaction is quickly quenched in an ice bath, it may be
permanently stopped. If brown fumes start to form, the reaction
may be proceeding too quickly and should be moderated.
Concentrated nitric acid and concentrated sulfuric acid
pose a hazard when being handled. Gloves should be worn and
care must be used. If any of the acid mixture gets on the skin,
it must be immediately washed with large amounts of cold water.
Immediately use a water wash if you suspect that acid may be on
your skin. Nitric acid destroys proteins and if some of it
(either as liquid or vapors) gets on the skin, the skin cells
will die over a few hours to days leaving an orange patch on the
surface. This generally wears off in a few days.
This reaction usually goes well if care is exercised. The
final product is a crystalline solid with a low melting point.
EXPERIMENTAL PROCEDURE:
Carefully combine concentrated sulfuric acid (0.4 mL) and
concentrated nitric acid (0.4 mL) in a micro-scale kit reaction
tube and place it in an ice bath. To a second tube, add methyl
benzoate (0.6 g, 4.4 mmoles) and concentrated sulfuric acid (1.2
mL), flick the flask to mix the components of this viscous
mixture, then cool this tube the ice bath as well. Using a glass
Pasteur pipette, carefully add the sulfuric-nitric acid mixture
dropwise to the methyl benzoate solution, using a glass stirring
rod to mix the reaction. On complete addition of the acid
mixture, remove the reaction from the ice bath, allow it to warm
up to room temperature, and let is stand for 15 mins. After this
time, in order to crystallize out the product, pour the mixture
into a small beaker that contains approximately 5g of ice.
Filter the resulting solid using a Hirsch funnel and wash out
the reaction tube with ice-cold water. When all the product has
been transferred to the filtration funnel, wash the product with
ice-cold methanol (0.5 mL). Record the mass and melting point of
this crude product and calculate the percent yield.
Recrystallize the product using an equal mass of methanol and
re-record the mass, melting point and percent yield of the pure
product.
IMPORTANT INFORMATION ABOUT THE REPORT:
The report for this experiment will follow the usual
format. Be sure the percent yield calculation is carefully
done. Also, record the melting point range of the final product
and compare that melting point to the reported melting point of
methyl 3-nitrobenzoate. Using these data, discuss the relative
success on the experiment and discuss whether there is any
evidence polysubstitution occurred.
END OF EXPERIMENT.
© 2007 STEPHEN ANDERSON AND ROBERT SHINE