Thermodynamic and Kinetic
Control of Organic reactions
By
Dr. Mehboob Peeran
Multiple products in organic reactions are a common feature.
What governs the product proportions and how? is an interesting and
fascinating feature. Among others, concentration, temperature and even
time can be factors, which can influence the product ratio.
Sulphonation
of Naphthalene
Consider the sulphonation of naphthalene. There are two
isomeric products possible. 1-Naphthalene sulphonic acid (A)
and its 2-isomer (B).
Experimental observations
At lower temperatures (800C) it is found (A) is the
major product and at higher temperatures (1600C) (B) is the major product.
Mechanistic explanation
Facts
1-Naphthalene sulphonic acid is less stable while the other isomer is
more stable.
The large sulphonic acid group encounters steric hindrance from the
hydrogen in the adjacent aromatic ring. Such hindrance is almost absent
in its isomer, 2-Naphthalene sulphonic acid.
Features
The activation energy barrier for the formation of (A)
is less than for (B) Hence (A) is
formed faster.
At low temperatures there isn’t sufficient energy to over come
the activation barrier for the reverse process that is the desulphonation
of (A). Hence (A) that is formed faster remains as
the major product.
At higher temperatures there is sufficient energy to over come this
energy barrier, the product can revert back to the reactant and go in
the direction of the more stable product, making 2-naphthalene sulphonic
acid as the major product.
Also the equilibrium is fairly and rapidly established between the product
and reactant and gradually most of naphthalene is converted to (B) |