Difficulty: Intermediate | Prerequisites: General chemistry (stoichiometry, molecular weights), introductory organic chemistry (functional groups, nomenclature, carbocations)
Dehydration of alcohols is one of the first elimination reactions you meet in organic chemistry lab. This experiment takes 2-methyl-2-butanol (a tertiary alcohol) and, using sulfuric acid as catalyst and heat, removes water to form a mixture of alkene products. The lab ties together several core skills: E1 mechanism reasoning, fractional distillation for separation, liquid-liquid extraction for purification, and gas chromatography for product analysis. If you are comfortable drawing carbocations and understand why tertiary centres are more stable, you are ready for this material.
2-Methyl-2-butanol loses water (via an E1 mechanism catalysed by H₂SO₄ and heat) to give a mixture of 2-methyl-1-butene and 2-methyl-2-butene. The more substituted alkene (2-methyl-2-butene) is the major product, as predicted by Zaitsev's rule. The lab practises fractional distillation, separatory-funnel washes, drying, and GC analysis to identify and quantify the products.
Dehydration (of an alcohol)
An elimination reaction in which an alcohol loses water (H₂O) to form an alkene. The hydroxyl group and a hydrogen on an adjacent carbon are removed. In simple terms, the molecule "dries out" and a double bond forms where the water left.
E1 mechanism (unimolecular elimination)
A two-step elimination: the leaving group departs first to form a carbocation intermediate, then a base removes a proton from an adjacent carbon to form the alkene. Think of it as "leaving group leaves, then proton leaves" in two separate steps, with rate depending only on the substrate concentration.
Carbocation
A positively charged carbon intermediate. Stability order: tertiary > secondary > primary > methyl. In simple terms, the more alkyl groups attached to the positive carbon, the more stable it is (hyperconjugation and inductive effects spread the charge).
Zaitsev's rule (Saytzeff's rule)
In elimination reactions, the major product is the more substituted alkene (the alkene with more alkyl groups on the double-bond carbons). Think of it as "the most stable alkene wins."
Fractional distillation
A separation technique using a fractionating column that provides multiple vaporisation-condensation cycles (theoretical plates), allowing separation of liquids with closer boiling points than simple distillation can handle.
Separatory funnel (sep funnel)
A piece of glassware used to separate immiscible liquid layers. You shake the two layers together, let them settle, and drain off the denser layer from the bottom.
Drying agent
An anhydrous salt (here, CaCl₂) added to an organic layer to absorb residual water. In simple terms, it soaks up trace water like a chemical sponge.
Gas chromatography (GC)
An analytical technique that separates volatile compounds by passing them through a heated column in a stream of carrier gas. Each compound exits at a characteristic retention time, allowing identification and quantification.
Substrate: 2-methyl-2-butanol (a tertiary alcohol, MW 88.15 g/mol)
Reagent/catalyst: concentrated sulfuric acid (H₂SO₄)
Conditions: heat
Products: a mixture of 2-methyl-1-butene and 2-methyl-2-butene, plus water
The dehydration proceeds via E1 because the substrate is tertiary, meaning the carbocation intermediate it forms is relatively stable.
Step 1, protonation: the hydroxyl group on 2-methyl-2-butanol is protonated by H₂SO₄, converting the poor leaving group (OH) into a good one (H₂O).
Step 2, loss of water: H₂O departs, generating a tertiary carbocation at C-2. This is the rate-determining step.
Step 3a, forming 2-methyl-2-butene (Zaitsev product): a base (water or bisulfate) removes a proton from C-3. The double bond forms between C-2 and C-3. This is the more substituted alkene (trisubstituted).
Step 3b, forming 2-methyl-1-butene (Hofmann product): a base removes a proton from one of the methyl groups at C-1. The double bond forms between C-1 and C-2. This is the less substituted alkene (disubstituted).
Steps 1 and 2 are the same for both products. It is only Step 3 that diverges, depending on which proton is removed.
Zaitsev's rule predicts that the more substituted alkene forms preferentially. 2-Methyl-2-butene is trisubstituted while 2-methyl-1-butene is only disubstituted. The trisubstituted alkene is more thermodynamically stable (greater hyperconjugation), so it dominates the product mixture. Typical ratios are roughly 2:1 in favour of the Zaitsev product, though exact values depend on conditions.
After the acid-catalysed reaction, the product mixture is separated from the reaction pot by fractional distillation. A fractionating column sits between the pot and the condenser, providing multiple theoretical plates so that the low-boiling alkene products (bp ~31 to 38 °C) are collected while the higher-boiling starting alcohol (bp 102 °C), water, and sulfuric acid stay behind.
After distillation, the chemicals remaining in the pot typically include unreacted 2-methyl-2-butanol, water, sulfuric acid, and possibly polymeric by-products formed from acid-catalysed side reactions.
10% aqueous NaOH wash: removes acidic impurities. Any sulfuric acid or sulfonate esters carried over from the distillation are neutralised and pulled into the aqueous layer.
Aqueous NaCl wash (brine wash): reduces the solubility of the organic product in water (salting-out effect), helping to remove residual water from the organic layer and improving the phase separation.
CaCl₂ drying: anhydrous calcium chloride is added to the organic layer to absorb remaining traces of water. You add small portions and swirl: if the granules clump together and the liquid stays cloudy, more is needed. When freshly added granules remain free-flowing and the liquid is clear, drying is complete.
Compound | Molecular Formula | MW (g/mol) | Boiling Point (°C) |
|---|---|---|---|
2-Methyl-2-butanol | C₅H₁₂O | 88.15 | 102 |
2-Methyl-1-butene | C₅H₁₀ | 70.13 | 31 |
2-Methyl-2-butene | C₅H₁₀ | 70.13 | 38 |
Water | H₂O | 18.02 | 100 |
Balanced reaction:
C₅H₁₂O → C₅H₁₀ + H₂O
Theoretical yield calculation:
moles of alcohol = mass of alcohol / 88.15
Theoretical mass of alkene = moles of alcohol × 70.13
Percent yield = (actual yield / theoretical yield) × 100
Atom economy = (MW of desired product / MW of all products) × 100 = (70.13 / (70.13 + 18.02)) × 100 = 79.6%
Acid-catalysed dehydration of alcohols is an industrial route to alkenes used as monomers (e.g., ethanol dehydration to ethylene for polyethylene production). The same E1 logic applies in biochemistry: terpene biosynthesis involves carbocation intermediates and elimination steps closely analogous to what you see in this lab.
Students often think the acid (H₂SO₄) is consumed in the reaction. It is not. It acts as a catalyst: it protonates the alcohol, water leaves, and the acid is regenerated when the proton is lost in the final step.
Students sometimes confuse E1 with E2. E1 is two steps (leaving group departs, then proton removed) and proceeds through a carbocation. E2 is a single concerted step with no carbocation. Tertiary alcohols in acidic conditions favour E1.
A common error is thinking the NaOH wash removes water. It does not. The NaOH wash removes acidic impurities. The NaCl wash and CaCl₂ are the water-removal steps.
Students sometimes assume the Hofmann (less substituted) product should be major because more hydrogens are available on that side. Zaitsev's rule overrides that statistical argument: thermodynamic stability of the alkene matters more than the number of available protons.
⚠️ Be able to draw the full E1 mechanism, including the divergent Step 3 for each product. This is a very common exam question.
⚠️ Know Zaitsev's rule and be ready to predict the major product for any alcohol dehydration.
⚠️ Understand the purpose of each purification step (NaOH wash, NaCl wash, CaCl₂ drying). Exam questions often ask "what does this step remove?" or "why was this reagent used?"
⚠️ Be able to calculate theoretical yield, percent yield, and atom economy from molecular weights and experimental masses.
True or false: the rate-determining step of E1 dehydration is the removal of a proton by a base. (False: it is the departure of water to form the carbocation.)
Fill in the blank: the more substituted alkene product predicted by ______ rule is 2-methyl-2-butene. (Zaitsev's)
True or false: NaOH wash in the separatory funnel removes water from the product. (False: it removes acidic impurities.)
Fill in the blank: CaCl₂ is used as a ______ to remove residual water from the organic layer. (drying agent)
True or false: 2-methyl-2-butanol follows an E2 mechanism because it is a tertiary alcohol. (False: tertiary alcohols in acidic conditions favour E1, not E2.)
Q: Draw the complete E1 mechanism for the dehydration of 2-methyl-2-butanol to form 2-methyl-2-butene. Label each step.
A: Step 1, protonation of the OH by H₂SO₄ to form an oxonium ion. Step 2, departure of water to give a tertiary carbocation at C-2 (rate-determining step). Step 3, a base (H₂O or HSO₄⁻) removes a proton from C-3, and the electron pair forms a pi bond between C-2 and C-3, giving 2-methyl-2-butene.
Q: What differentiates the mechanism leading to 2-methyl-1-butene from the one leading to 2-methyl-2-butene?
A: Only the final step differs. For 2-methyl-1-butene, the base removes a proton from a methyl group at C-1 (double bond forms between C-1 and C-2). For 2-methyl-2-butene, the proton is removed from C-3 (double bond between C-2 and C-3).
Q: What was the purpose of washing the distillate with 10% aqueous NaOH?
A: To neutralise and remove acidic impurities, principally any sulfuric acid or sulfonate esters that co-distilled with the alkene products.
Q: After fractional distillation, what chemicals likely remained in the distilling pot?
A: Unreacted 2-methyl-2-butanol, water, sulfuric acid, and possibly polymeric by-products.
Q: Explain why the more substituted alkene is the major product of this reaction.
A: Zaitsev's rule. The more substituted alkene (2-methyl-2-butene, trisubstituted) is more thermodynamically stable than the less substituted one (2-methyl-1-butene, disubstituted), so it forms preferentially in an E1 elimination where the transition state resembles the product.
Q: How did you decide how much CaCl₂ to use and whether you had used enough?
A: Add CaCl₂ in small portions and swirl. If granules clump and the liquid remains cloudy, more drying agent is needed. When freshly added granules stay free-flowing and the liquid appears clear, drying is sufficient.
This material connects directly to elimination vs substitution (E1/E2/SN1/SN2), which is a central framework in CHEM 2311. Understanding the E1 mechanism here helps with predicting outcomes for other tertiary substrates and with distinguishing when elimination vs substitution will dominate. The purification techniques (distillation, extraction, drying) recur in nearly every subsequent lab. Gas chromatography analysis from this experiment carries over to any lab that requires product identification or purity assessment.
Dehydration of alcohols, elimination reaction, E1 mechanism, unimolecular elimination, 2-methyl-2-butanol, tert-amyl alcohol, 2-methyl-1-butene, 2-methyl-2-butene, Zaitsev's rule, Saytzeff's rule, carbocation, tertiary carbocation, fractional distillation, separatory funnel, liquid-liquid extraction, NaOH wash, brine wash, NaCl wash, drying agent, CaCl₂, anhydrous calcium chloride, percent yield, theoretical yield, atom economy, CHEM 2311, organic chemistry lab