Organic Chemistry, "Change", Part 20 - Dehydration of a Second Degree Alcohol (2-Butanol to 2-Butene Mechanism)

 Greetings,  

The past few posts have discussed dehydration of 2-butanol, in some detail. The reaction is acid catalyzed, which works because of the formation of a good leaving group, namely water. This post focuses on the kinetics of the reaction, with an illustration of the mechanism. The mechanism consists of three basic steps, with the overall rate equal to the slowest step; Step 2 - the rate determining step.

The reaction proceeds at an elevated temperature to help overcome the activation energy of the rate-determining step. The rate determining step consists of the departure of water, a good leaving group, but this results in the formation of the unstable reaction carbocation intermediate. Even with the energetically-favored loss of a water molecule, it's not enough to overcome the high activation energy barrier of carbocation formation, so an increased temperature is required to drive this reaction (Otherwise, the overall rate of reaction would be too slow to be practical).

The following diagram shows the three mechanism steps, complete with structural formulas of all entities involved. 

Note: The catalyst (aka, Hydrogen Ion) and intermediate entities cancel, leaving only the reactants and products of the overall reaction.

Cis and Trans isomers are formed because of the free rotation around the central carbocation single (sigma) bond.

That's all for this post.  As always, thank you for reading!