The dehydration of tertiary alcohols converts an alcohol into an alkene, a molecule containing a carbon-carbon double bond. This process involves the removal of a water molecule, facilitated by an acid catalyst and heat, driving the elimination of the hydroxyl (\(\text{OH}\)) group and a hydrogen atom from an adjacent carbon. Tertiary alcohols proceed through a specific, highly efficient pathway known as the \(\text{E}1\) mechanism. The unique arrangement of carbon atoms around the functional group in these alcohols allows for a rapid, multi-step reaction that contrasts sharply with the conditions needed for other alcohol types.
Reaction Conditions and Tertiary Alcohol Stability
The conditions required to dehydrate a tertiary alcohol are mild compared to those needed for primary or secondary alcohols. Tertiary alcohols react readily with dilute strong acids, such as \(20\%\) phosphoric acid (\(\text{H}_3\text{PO}_4\)), at lower temperatures (sometimes \(25^\circ\text{C}\) to \(80^\circ\text{C}\)). This lower energy requirement results from the stability of the reaction intermediate that forms. Primary alcohols, in contrast, require much harsher conditions, like concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)) at temperatures exceeding \(170^\circ\text{C}\), because they follow a different, higher-energy reaction pathway.
The preference for the \(\text{E}1\) mechanism in tertiary alcohols is due to the ability of the molecule to form a stable intermediate called a tertiary carbocation. A carbocation is a carbon atom carrying a formal positive charge. A tertiary carbocation is one where the positively charged carbon is bonded to three other carbon atoms (or alkyl groups). These surrounding alkyl groups stabilize the positive charge through hyperconjugation.
Hyperconjugation involves the partial sharing of electron density from adjacent sigma bonds (\(\text{C}-\text{H}\) or \(\text{C}-\text{C}\)) into the empty \(p\)-orbital of the positively charged carbon. This electron donation spreads out the charge, which lowers the energy of the intermediate, making it more stable and easier to form. The stability of this intermediate substantially increases the reaction rate, explaining why tertiary alcohols are the most reactive in dehydration reactions.
The Step-by-Step E1 Mechanism
The \(\text{E}1\) mechanism (Elimination, Unimolecular) proceeds in three distinct steps to produce the final alkene product. The overall rate is determined by the slowest step, which involves only one molecule (the alcohol derivative), hence the term “unimolecular.” The mechanism begins with the reaction between the alcohol and the acid catalyst.
The first step is the protonation of the hydroxyl (\(\text{OH}\)) group by the strong acid. The oxygen atom acts as a weak base, accepting a proton (\(\text{H}^+\)). This proton transfer converts the poor leaving group (\(\text{OH}^-\)) into a much better leaving group, a neutral water molecule (\(\text{H}_2\text{O}\)). The resulting alkyloxonium ion is unstable due to the positive charge residing on the oxygen atom.
The second step is the rate-determining step. The carbon-oxygen bond breaks heterolytically, meaning the bond’s electrons move entirely to the oxygen atom, allowing the water molecule to depart. This results in the formation of the tertiary carbocation intermediate. Because this step involves breaking a chemical bond and forming a charge-separated species, it requires the most energy and dictates the overall speed of the reaction. The rate depends solely on the concentration of the protonated alcohol.
The final step is the rapid deprotonation of a hydrogen atom from a carbon adjacent to the carbocation center. This adjacent carbon is referred to as the \(\beta\)-carbon. A weak base, such as water or the conjugate base of the acid catalyst, removes the \(\beta\)-hydrogen. The electrons from the broken carbon-hydrogen bond shift to form the new carbon-carbon double bond, yielding the final alkene product. This step also regenerates the acid catalyst.
Regioselectivity and the Final Alkene Product
When the tertiary alcohol is asymmetric, the final deprotonation step can remove a hydrogen from more than one possible \(\beta\)-carbon, leading to a mixture of alkene products. This phenomenon is known as regioselectivity, the preference for forming one constitutional isomer over others. The outcome is predicted by Zaitsev’s rule.
Zaitsev’s rule dictates that the major product will be the most highly substituted alkene. A more substituted alkene has the largest number of alkyl groups attached directly to the carbons of the double bond. For instance, a double bond with three attached alkyl groups is more stable than one with only two, and this stability difference drives the selectivity.
The preference for the more substituted alkene is related to hyperconjugation. More alkyl groups surrounding the double bond allow for greater stabilization, lowering the energy of the final product. Therefore, the reaction pathway that leads to the most stable alkene isomer is kinetically favored, meaning it forms faster and predominates. The dehydration of a tertiary alcohol typically yields a mixture, but the most substituted alkene is produced as the major product.