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Name: Todd
Status: educator
Grade: 9-12
Country: Canada
Date: Winter 2010


Question:
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.



Replies:
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 rich benzene.

Greg (Roberto Gregorius)
Canisius College


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:

http://www.chemguide.co.uk/mechanisms/elsub/nitrationtt.html#top

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.

Best wishes,

Tom Collins



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