Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Dehydration of alcohols involves the elimination of a molecule of water to form an alkene or an ether, typically catalyzed by protic acids like concentrated or .
The reaction follows a carbocation mechanism, and the ease of dehydration follows the order: Tertiary () > Secondary () > Primary () alcohols, due to the relative stability of the intermediate carbocations.
The mechanism consists of three major steps: (1) Formation of protonated alcohol (oxonium ion), (2) Formation of carbocation (slowest, rate-determining step), and (3) Elimination of a proton to form the double bond.
Temperature and reagent concentration determine the product: Ethanol yields ethene at with conc. , but yields ethoxyethane (ether) at with excess alcohol.
According to Saytzeff's Rule, in the dehydration of unsymmetrical secondary or tertiary alcohols, the more highly substituted alkene (the one with the greater number of alkyl groups attached to the doubly bonded carbon atoms) is the preferred product.
📐Formulae
💡Examples
Problem 1:
Explain the dehydration of (Butan-2-ol) and identify the major product.
Solution:
Explanation:
Butan-2-ol forms a secondary carbocation. Deprotonation can occur from either or . According to Saytzeff's rule, the more substituted alkene, But-2-ene, is the major product because it is more stable.
Problem 2:
Write the conditions required for the dehydration of Propan-2-ol to Propene.
Solution:
Explanation:
Secondary alcohols like Propan-2-ol require milder conditions () compared to primary alcohols () because the secondary carbocation intermediate is easier to form.