dehydration of cyclohexanol

What type of reaction converts cyclohexanol to cyclohexene?

An acid-catalyzed dehydration reaction (E1 elimination mechanism).

What is removed during dehydration of an alcohol?

Water (H₂O).

What type of mechanism do secondary and tertiary alcohols follow in dehydration?

The E1 (elimination unimolecular) mechanism.

Why is the E1 mechanism first order?

The rate-determining step involves only one molecule — formation of the carbocation.

What happens in the first step of alcohol dehydration?

Protonation of the hydroxyl group to form an oxonium ion (R-OH₂⁺).

Why is protonation necessary before elimination?

The hydroxyl group is a poor leaving group; protonation makes it a better one (water).

What happens after the oxonium ion forms?

Water leaves, forming a carbocation intermediate.

What happens after the carbocation forms?

A base removes a β-hydrogen, forming a C=C double bond (alkene).

Which carbocations are more stable?

Tertiary > secondary > primary.

Why are tertiary carbocations more stable?

Due to hyperconjugation and inductive effects from alkyl groups.

What rearrangements can occur during dehydration?

Hydride or alkyl shifts to form a more stable carbocation.

Does cyclohexanol undergo rearrangement during dehydration?

No, it forms a single product, cyclohexene.

What acid is commonly used for dehydration in the lab?

Sulfuric acid (H₂SO₄).

Why is sulfuric acid preferred over hydrochloric acid?

HCl can produce alkyl chlorides (undesired substitution).

Why is heat (Δ) required for dehydration?

It's an endothermic reaction that needs energy input.

What two reactions compete under acidic conditions?

Elimination (E1) and substitution (SN1).

How can elimination be favored over substitution?

Use high temperature and strong acid.

What product forms in the competing substitution reaction?

An alkyl halide or alkyl hydrogen sulfate.

What does Zaitsev's rule state?

The more substituted (stable) alkene is the major product.

Which alkene is more stable

E or Z?,The E (trans) isomer is more stable due to less steric strain.

What determines the ratio of alkene products?

The relative energies of transition states.

How can distillation drive the dehydration reaction forward?

Removing the alkene product shifts equilibrium toward more product.

Why does distillation work for dehydration?

Alkenes have lower boiling points than alcohols and can be removed as they form.

What is the general reaction for cyclohexanol dehydration?

C₆H₁₁OH →(H⁺, heat)→ C₆H₁₀ + H₂O.

What intermediate forms during cyclohexanol dehydration?

The cyclohexyl carbocation (secondary carbocation).

What is the product of cyclohexanol dehydration?

Cyclohexene.

Why does cyclohexanol not rearrange?

The ring structure restricts rearrangement, and the carbocation is stable.

What is a possible side reaction for cyclohexene under acidic conditions?

Acid-catalyzed polymerization.

Is the dehydration of alcohols reversible?

Yes, the reverse is acid-catalyzed hydration of alkenes.

How can dehydration reversal be prevented?

Remove the water or alkene product as it forms.

Why is dehydration of alcohols endothermic?

Bond-breaking (C-O, C-H) requires energy.

Why do tertiary alcohols dehydrate faster than primary ones?

Because tertiary carbocations form more readily and are more stable.

What determines if dehydration occurs by E1 or E2?

E1 happens with stable carbocations and weak bases; E2 with strong bases or primary alcohols.

What is the rate-determining step in E1 dehydration?

Loss of the leaving group (water) to form a carbocation.

What kind of intermediate forms when the hydroxyl group is protonated?

An oxonium ion (R-OH₂⁺).

What does Le Châtelier's principle explain in this reaction?

Removing products (water or alkene) shifts equilibrium toward completion.

What is the function of the acid catalyst in dehydration?

It protonates the alcohol and stabilizes intermediates but is regenerated at the end.

What type of alcohol dehydrates most easily?

Tertiary > secondary > primary.

What are the main steps of an E1 dehydration mechanism?

Protonation → Loss of water → Carbocation formation → β-hydrogen elimination → Alkene formation.

What is the mechanistic difference between E1 and E2 eliminations?

E1 involves carbocation formation (two steps); E2 is a single concerted step.

What determines the orientation of elimination?

Zaitsev's rule and alkene stability.

What stabilizes alkenes?

Alkyl substitution and conjugation.

What general formula represents dehydration of cyclohexanol?

C₆H₁₂O → C₆H₁₀ + H₂O.

What is the catalyst symbol in dehydration equations?

H⁺ (acid catalyst).

What is the physical property difference that helps isolate alkenes?

Lower boiling point compared to alcohols.

Why does primary alcohol dehydration often require E2 mechanism?

Primary carbocations are unstable, so elimination must occur in one step.

What is the purpose of setting up the fractional distillation apparatus with the receiving flask in an ice-water bath?

To condense and collect the cyclohexene product efficiently while minimizing evaporation losses.

Why do we use a 25 mL round-bottom flask as the distillation flask?

It provides enough space for heating and mixing the reaction mixture safely during distillation.

Why do we add a one-inch stir bar to the 25 mL round-bottom flask?

To ensure even mixing and prevent bumping during heating.

What is the purpose of adding 5 mL of cyclohexanol to the round-bottom flask?

It serves as the starting alcohol that will undergo dehydration to form cyclohexene.

Why is 2.5 mL of 9 M sulfuric acid added to the cyclohexanol?

Sulfuric acid acts as the acid catalyst to protonate the alcohol and promote dehydration (E1 elimination).

Why must sulfuric acid be added carefully?

It is highly corrosive and exothermic when mixed with organic compounds, so slow addition prevents splattering.

Why do we attach the flask to the distillation apparatus before heating?

To allow the dehydration product (cyclohexene) to distill out as it forms, driving the reaction forward.

Why is the hot plate turned up to about 250 °C?

To provide enough heat to overcome the activation energy for dehydration since it's an endothermic process.

Why do we collect distillate up to the 80-85 °C range?

Cyclohexene boils in that temperature range, so this ensures the correct product is collected.

Why do we stop heating when no more product is being collected?

It prevents overheating or decomposition of the reaction mixture once all product has distilled.

Why do we place the receiving flask in an ice bath?

To prevent the volatile cyclohexene product from evaporating during collection.

What is the purpose of adding a few spatulas of potassium carbonate to the receiving flask?

Potassium carbonate is a drying agent that removes any water remaining in the product.

Why must we filter off the potassium carbonate before analysis?

To isolate the pure liquid product and prevent solid contamination during mass measurement or spectroscopy.

Why do we record the mass of the liquid product?

To calculate the percent yield of cyclohexene from the starting amount of cyclohexanol.

Why do we obtain an IR spectrum of the product?

To confirm the presence of the C=C double bond and the absence of the O-H stretch, verifying successful dehydration.

What is the purpose of performing the bromine test on the product?

To qualitatively confirm the presence of an alkene (unsaturation) in the product.

Why is dichloromethane used as the solvent in the bromine test?

It dissolves both bromine and cyclohexene and provides an inert medium for the reaction.

Why do we add only 1-2 drops of cyclohexene to the bromine test?

To ensure the test is sensitive and that excess bromine color change can be clearly observed.

What does it mean if the red color of bromine disappears?

It's a positive test for unsaturation, confirming that cyclohexene (an alkene) is present.

Why is the bromine test done last?

To verify the final product after purification and ensure no side reactions or impurities interfere with the test.