Q.8-8-68E - Organic Chemistry (Mcmurry)

Question

Evidence that cleavage of 1,2-diols by ${\mathbf{HIO}}_{\mathbf{4}}$occurs through a five membered cyclic periodate intermediate is based on kinetic data—the measurement of reaction rates. When diols A and B were prepared and the rates of their reaction with ${\mathbf{HIO}}_{\mathbf{4}}$were measured, it was found that diol A cleaved approximately 1 million times faster than diol B. Make molecular models of A and B and of potential cyclic periodate intermediates, and then explain the kinetic results

       

Step-by-Step Solution

Answer

Cis 1,2 diol easily forms five-membered cyclic periodate than trans 1,2 diol.

Reaction with HIO 4

Treat the diol with HIO4. If OH groups are in an open chain, it results in two carbonyl compounds. If the two OH groups are on a ring, then an open-chain dicarbonyl compound is formed.

                

Formation of cyclic periodate

Cis 1,2 diol easily form the five members cyclic periodate than trans 1,2 diol, because of the steric hindrance, the energy gets lower in the cis as compare to trans. So, cis is easily broken than trans change to the five members cyclic periodate easily. Hence, the Gibbs energy of the reaction decreases and increase the rate of the reaction.