2009 Practice Final Q6A (m)

[Andy Liao 1B]

Problem: (m) Which reactions could be described as kinetically controlled?

I was wondering why (2) and (4) could be described as kinetically controlled. What about these two reactions makes them kinetically controlled? Why aren't (1) and (3) kinetically controlled? Can someone please explain?

[Nicole Anisgard Parra 2H]

Numbers 2 and 4 are kinetically controlled because they have a large activation energy barrier, even though the products have a lower standard Gibbs Free energy. Because the reaction would have to overcome this barrier, the reaction is governed by kinetics.
This is much like the reaction of diamond turning into graphite: even though graphite is the more stable form of carbon, the reaction does not occur at ambient conditions because the activation energy barrier for diamond to turn into graphite is too high. Thus, that is an example of a kinetically controlled reaction.