THE CONCEPT
Two Alcohol Molecules Combining Instead of One Falling Apart
You already know intramolecular dehydration from the Alcohol Dehydration to Alkenes lesson — one alcohol molecule loses water to form an alkene. Intermolecular dehydration is a related but structurally different reaction: TWO separate alcohol molecules combine with each other, losing one molecule of water between them, to form a symmetrical ether. Both reactions start from the same protonation step (activating the alcohol's oxygen with H₂SO₄), but what happens after that first step diverges completely.
In intermolecular dehydration, one protonated alcohol molecule loses water to become a good electrophile (essentially the same first step as the intramolecular pathway), but instead of eliminating a beta-hydrogen from within the same molecule, a SECOND, unreacted alcohol molecule attacks that electrophilic carbon as a nucleophile, forming a new C-O-C ether linkage between the two original molecules.
💡 Memory Trick
The hub's trick states the reaction and its key condition directly: alcohol + alcohol, with H₂SO₄ at 140°C, gives a symmetrical ether. The single most important detail to memorize alongside the reaction itself: 140°C specifically favors the ether, a noticeably LOWER temperature than the 180°C that favors dehydration to the alkene instead. The hub's blunt summary of this temperature dependence is worth holding onto as a simple two-number rule: 140°C → ether, 180°C → alkene.
WHY TEMPERATURE ALONE FLIPS THE PRODUCT
Kinetics vs. Thermodynamics, and a Substrate Restriction
The reason temperature alone can flip which product dominates comes down to a kinetics-versus-thermodynamics distinction: at the lower temperature (140°C), there isn't quite enough thermal energy to drive the intramolecular elimination pathway efficiently, so the intermolecular substitution pathway (a second alcohol acting as nucleophile) dominates instead, since it has a comparatively lower activation energy under these milder conditions. At the higher temperature (180°C), there's enough energy to favor the elimination pathway, and elimination reactions are also generally favored thermodynamically at higher temperature (more separate product molecules, higher entropy) — so alkene formation takes over as the major pathway.
This reaction is also restricted by substrate class in a way worth remembering directly: it only works well for primary alcohols. Secondary and tertiary alcohols, once protonated, ionize too readily (forming comparatively stable secondary or tertiary carbocations) and undergo elimination (E1) long before a second alcohol molecule has any real chance to act as a competing nucleophile — meaning intermolecular ether formation is essentially a primary-alcohol-only reaction, exactly paralleling the same substrate-class logic that's shown up repeatedly across this entire subject.
🧪 Lab Application
You need to synthesize diethyl ether from ethanol and must select the correct temperature to favor the ether product over the competing alkene (ethylene) product.
1
Confirm the substrate is suitable. Ethanol is a primary alcohol, satisfying the substrate requirement for intermolecular ether formation to compete successfully against elimination.
2
Select the correct temperature. Heat the ethanol with concentrated H₂SO₄ at 140°C specifically, rather than the higher 180°C that would favor dehydration to ethylene instead.
3
Predict the mechanism at this temperature. One ethanol molecule is protonated and loses water to become an electrophile; a second, unreacted ethanol molecule then attacks that electrophilic carbon as a nucleophile, forming the new C-O-C linkage.
4
Confirm the product. The result is diethyl ether (CH₃CH₂-O-CH₂CH₃), the symmetrical ether formed from two combined ethanol molecules, with water as the byproduct.
📌 Exam Application
Exams frequently test this exact temperature-dependence directly — expect a question giving the same primary alcohol and asking for the product at two different temperatures, testing whether you remember that 140°C favors the ether while 180°C favors the alkene, rather than assuming a single fixed product regardless of conditions.
⚠️ Most Common Intermolecular Dehydration to Ethers Mistakes
The most common mistake is forgetting the substrate restriction and attempting this reaction with a secondary or tertiary alcohol, which will simply dehydrate to the alkene via E1 long before intermolecular ether formation has a chance to compete. The other frequent trap is reversing the two temperatures — always double check that the LOWER temperature (140°C) favors the ether, and the HIGHER temperature (180°C) favors the alkene, since this can feel counterintuitive on a rushed exam.
✓ Quick Self-Test
1) What two alcohol molecules combine to form in intermolecular dehydration? 2) What temperature favors ether formation, and what temperature favors alkene formation instead? 3) Why does the lower temperature favor the ether pathway over elimination? 4) Why does this reaction only work well for primary alcohols? 5) What is the byproduct of intermolecular dehydration to an ether?
Next Lesson
Tollens vs Fehling Test
→
← All Alcohols & Ethers Lessons