Work, Energy, and Power
The Chemical Bond
Chemical Thermodynamics and the Equilibrium State
Reactions of Alkyl Halides
Reactions of Alcohols and Ethers
|The dehydration of an alcohol is an E1 elimination reaction. Therefore, acid-catalyzed dehydration of alcohols involves a carbocation intermediate, and rearrangement to form more stable carbocation can occur. Acid catalyzed dehydration of alcohols is both regioselective (Zaitsev's Rule favoring the highly substituted alkene) and stereoselective (favoring the trans product). The basis for both the regioselectivity and stereoselectivity is that the most probable state involves minimizing internal energy (minimum enthalpy, minimum free energy).|
Highly substituted alkenes are more stable because carbon-carbon bonds in sp3 hybridized alkyl groups release electron density inductively to sp2 hybridized carbons which are more electron withdrawing than alkyl groups. The greater the number of alkyl substituents of the double bond, the more stabilization can occur. (The movement of electron density to satisfy an electron withdrawing substituent corresponds to a decrease in electric potential energy. In terms of thermochemistry, this means lowered internal energy, lowered enthalpy, and, in terms of thermodynamics, lower free energy (i.e. favored by equilibrium)).
Similar reasoning explains the stereoselective preference with elimination reactions to form the trans isomer in greater yield than the cis. Here the decisive electrostatic interaction at the molecular level involves van der Waals repulsion between alkyl groups on the same side of the double bond. Thermodynamics favors the lower energy trans form which minimizes these repulsions.