Question

Orgo II: Electrophilic Aromatic Substitution Tutorial

Answer 1

\({\bf{Mechanism~Overview:}}\)

Answer 2

\({\bf{Step-by-Step~Mechanism:}}\)



note the loss of aromaticity (disruption of pi system, higher energy) and the restoration of aromaticity (regaining pi system + stability). we can represent this on an energy diagram like so:

Answer 3

note: step 1 = slow, rate-determining
step 2 = fast

Answer 4

For this tutorial I will be going through the mechanisms for three reactions: halogenation, nitration, and sulfonation. for the other EAS reactions I may make another tutorial.

\({\bf{Halogenation:}}\)

Rxn: benzene + X2 + FeX3 --> halogenated benzene + HX

Chlorine and bromine are your best bets here. Fluorination is too reactive to control the reaction under normal conditions and iodination is unreactive. However, iodination can be done w/ the addition of an oxidizing agent like nitric acid.

Answer 5

Nitration: makes use of HNO3 and H2SO4. note that H2SO4 is the stronger acid and thus will make HNO3 electrophilic by donating an electron. This reaction produces the NO2 upon electron movement, which will then be added to the benzene ring through a similar mechanism as the one drawn above.

Answer 6

electrophilic addition proceeds as normal, followed by attack of water on the hydrogen forming hydronium

Answer 7

\({\bf{Sulfonation:}}\) SO3 + benzene + H2SO4 yields benzenesulfonic acid

Answer 8

note: this rxn can be reversed by using dilute sulfuric acid and high conc. of water to move the equilibrium to the left

sometimes sulfonate groups are used as protecting group if we want to block a certain position (more on ortho/meta/para directors in a later tutorial) which can then be removed once the desired conditions are met

Answer 9

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is Organic Chemistry 12th edition by Solomons, et. al.