Date: Winter 2010
Electrophilic Substitution: Nitration of Benzene Could you
please explain the process in "simple" terms? I know it is on Google
but the wording is too complex and I do not understand why the
electrons have to move.
From your Google search you already know that the catalyst for the
nitration of benzene is sulfuric acid. You also know from the name
that this is an "electrophilic substitution". This means that an
electrophile, in this case the NO2(+) ion is formed in the reaction
of nitric acid and sulfuric acid. The NO2(+) is attracted to the
electron-rich site of benzene. A bond is formed between between one
of the benzene carbons with the NO2(+) and the positive charge is
transferred to the ring which can stabilize the charge better due
to resonance structures. The sulfuric acid catalyst is reformed by
abstracting the H from the C holding the NO2(+), the aromatic
structure of benzene is reformed and the product - nitrobenzene is the result.
Essentially, the NO2 substitutes for the abstracted H by an
electrophilic process of the NO2(+) being attracted to the electron
Greg (Roberto Gregorius)
This is quite hard to explain without diagrams, but here we go!
In the nitration of benzene the first step is to form a bond between
a carbon on the ring and the N of the NO2(+) (nitronium) ion (formed
in situ from H2SO4 and HNO3). The electrons that form this bond come
from the aromatic system, you can think of these as coming from a
double bond within the ring or from the delocalized Pi system,
depending on the model you are working with. These leaves a positive
charge on the carbon atom next to the carbon to which the bond is
formed. This positive charge is actually delocalized around the
ring, specifically onto the 4 and 6 carbons, as well as the 2
carbon. This cationic intermediate is called the Wheland intermediate.
The final step in the electrophilic substitution is to re-establish
the aromatic system by losing H+ from the original carbon that
formed a bond to the nitronium ion. You can think of this as giving
the electron back to the aromatic system that you took away when
forming the first bond to the nitronium ion. In fact the H+ doesn't
just fall off, it is removed by another species donating a pair of
electrons to form a bond to the H and the electrons from the C-H
bond being given to the aromatic system at the same time.
Here is a great web site that talks you through the mechanism:
The author is Jim Clark, and his web site is one of the best there
is for covering general chemistry topics in the most correct way.
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Update: June 2012