Essay About Nitric Acid And Nitration Step

Preparation of P-Nitroaniline from P-Nitroacetanilide
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Step 1: Acetylation of Aniline
In the first step we need to put the removable acetyl protecting group on the
nitrogen of aniline. The acetyl group is electron withdrawing and it therefore makes the
lone pair on the nitrogen less reactive either in an oxidation reaction or a protonation
reaction. The nitric acid used in the nitration step is a fairly strongly oxidizing agent and
we could form an N-oxide. The nitration conditions (nitric acid and sulfuric acid) are
strongly acidic. Protonation of the nitrogen of aniline makes it a very strong deactivating
group, making the aromatic ring less susceptible to reaction with the nitrating agent and
the NH3
+ group would be a meta director. Another value of the acetyl protecting group is
that it is bulky group and preferentially directs the nitration to the para position rather
than the ortho position.
The full mechanism for the reaction is given in Figure 14.2. Acetic anhydride is
partially protonated by the acetic acid. This makes the anhydride an even better
electrophile for the nucleophilic nitrogen of aniline. This attacks to form the tetrahedral
intermediate, which, after proton transfer, loses acetic acid.
Figure 14.2 Mechanism of Acetylation Reaction
aniline
CH3 +
acetanilide
acetic anhydride acetic acid
H N H
Physical Constants
Compound Mol. Wt
(g/mol)
Density
(g/mL)
b.p. (oC) m.p. (oC)
Aniline 93.13 1.022 184 -6
Acetic anhydride 102.09 1.082 138-142 -73
Acetic acid 60.05 1.049 116-117 15-16
Nitric acid (conc. 15.8
63.01 1.420 – –
Sulfuric acid (conc. 36
98.09 1.840 330 –
Acetanilide 135.17 solid – 113-115
p-nitroacetanilide 180.16 solid – 215-217
p-nitroaniline 138.13 solid – 147-148
Procedure:
The set-up for your apparatus is shown in Figure 14.3. Note that there is NO
STOPPER at the top of the condensing column. This is left open to the atmosphere so as
to avoid a pressure build -up inside the flask when heating. Remember: NEVER HEAT
A CLOSED SYSTEM.
Dissolve 4.0 mL of aniline in 10 mL of acetic acid in a 100 mL round bottom
flask. To this solution, add 5 mL of acetic anhydride and mix well by swirling.
CAUTION: the reaction is exothermic and the flask becomes warm. Add two boiling
chips, attach a condensing column and attach the hoses for water cooling (water-in at the
bottom and water-out at the top). Heat at a gentle reflux for fifteen minutes.
Work up
After fifteen minutes, lower the iron ring under your heating mantle and allow the
flask to cool slightly. CAUTIOUSLY add 5 mL of cold water through the top of the
condenser into