Dehydration of substituted alcohols produces a mixture of isomeric alkenes. For example, refluxing 2-methylcyclohexanol in the presence of phosphoric acid gives 1-methylcyclohexene as a major product, 3-methylcyclohexene as a minor product, while very little methylenecyclohexane is formed.
The product ratios in this elimination reaction are determined by several factors. According to one model, the thermodynamic stability (heat of formation) of the final alkenes plays a major role. In this model, the higher yield of 1-methylcyclohexene over 3-methylcyclohexene arises from the greater thermodynamic stability of the more highly substituted alkene. The rule that "the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the β-carbon having the fewest hydrogen substituents" is known as the Zaitsev's rule.
However, the Zaitsev's rule is not always followed. For example, the heterogeneous dehydration of 2-methylcyclohexanol on aluminum oxide gives nearly equal amounts of 1-methylcyclohexene and 3-methylcyclohexene. One explanation for this is that the hydrogen at the tertiary carbon is not easily accessible by the surface of the catalyst and the loss of a proton from this position faces a significant kinetic barrier. Thus, the relative energies of the two transition states influence the observed product ratios. Under kinetic control, a significant yield of 3-methylcyclohexene is expected when 2-methylcyclohexanol is dehydrated. The kinetic product is known as the Hofmann product.
A more complex model is required to explain the formation of methylenecyclohexene. This product can form when the proton is abstracted from the tertiary carbocation. The tertiary carbocation forms from the secondary carbocation via 1,2-hydride shift mechanism. Thus, the amount of methylenecyclohexane depends on the relative stabilities of the secondary and tertiary carbocation intermediates, and the ease with which 1,2-hydride shift occurs.
The following tutorial illustrates how quantum chemical methods can be used to predict or rationalize outcomes of chemical reactions when the product ratios are determined by their thermodynamic stability. We will use ab initio quantum chemical calculations to answer the following questions about the dehydration of 2-methylcycloxanol:
The tutorial will cover the following practical skills: