Preparation of Alcohols - UNSOLVED PRACTICE SET

Q7. Write the reaction for the preparation of ethanol from ethene by hydration. What catalyst is used?

Q8. What is hydroboration-oxidation? Why does it give anti-Markovnikov product?

Q9. Write the reaction of CH₃MgBr with acetone followed by hydrolysis. What type of alcohol is formed?

Q10. Explain why the reduction of carboxylic acids with LiAlH₄ gives primary alcohols.

Q11. Write the reaction showing the fermentation of glucose to ethanol. What enzyme catalyzes this reaction?

Q12. Your school lab has a bottle of propanal. How would you convert it to propan-1-ol? Write the reaction and name the reagent.

Q13. (a) Describe the preparation of alcohols from alkenes by:

(i) Acid-catalyzed hydration (Markovnikov addition)

(ii) Hydroboration-oxidation (anti-Markovnikov addition)

Write the reactions and mention the regioselectivity of each method.

Q14. (a) Describe the preparation of alcohols using Grignard reagents.

(b) Write the reactions of Grignard reagents with:

(i) Formaldehyde

(ii) Aldehydes (other than formaldehyde)

(iii) Ketones

(iv) Esters

(c) What type of alcohol is formed in each case?

Q15. (a) Describe the industrial preparation of ethanol by fermentation.

(b) What are the conditions required for fermentation?

(c) Why is the ethanol obtained by fermentation not 100% pure? How is it purified?

(d) Write the reaction for the preparation of methanol from synthesis gas (water gas).

Q16. A student wants to prepare butan-2-ol from but-1-ene.

(a) Write two different methods for this conversion, naming the reagents used in each.

(b) Which method gives better regioselectivity? Explain why.

(c) If the student starts with 28 g of but-1-ene (molar mass = 56 g/mol) and obtains 22.2 g of butan-2-ol (molar mass = 74 g/mol) by acid-catalyzed hydration, calculate the percentage yield.

(d) What are the likely by-products in each method? How can they be minimized?

(e) Why is hydroboration-oxidation preferred over acid-catalyzed hydration for preparing primary alcohols from terminal alkenes?

Q17. The following reactions are used to prepare alcohols:

Reaction 1: CH₃CHO + NaBH₄ → CH₃CH₂OH

Reaction 2: CH₃COCH₃ + LiAlH₄ → (CH₃)₂CHOH

Reaction 3: CH₃MgBr + HCHO → CH₃CH₂OH

Reaction 4: C₂H₄ + H₂O → C₂H₅OH (dilute H₂SO₄)

(a) Classify each reaction by type (reduction, Grignard addition, hydration).

(b) For Reaction 1, explain why NaBH₄ is preferred over LiAlH₄ for reducing aldehydes in aqueous medium.

(c) For Reaction 2, calculate the mass of propan-2-ol obtained from 29 g of propanone (molar mass = 58 g/mol) if the yield is 80%.

(d) For Reaction 3, if 0.5 mol of CH₃MgBr is used with excess formaldehyde, calculate the volume of ethanol obtained at STP. (Molar mass of ethanol = 46 g/mol, density = 0.789 g/mL)

(e) For Reaction 4, why is dilute H₂SO₄ used instead of concentrated H₂SO₄?

Q18. In a school laboratory, students prepare ethanol by fermentation.

(a) Write the reaction for the fermentation of glucose to ethanol. What enzyme catalyzes this reaction?

(b) The students use 50 g of glucose (C₆H₁₂O₆, molar mass = 180 g/mol). Calculate the theoretical yield of ethanol.

(c) In practice, the students obtain 11.5 g of ethanol. Calculate the percentage yield.

(d) Why is the yield less than 100%? (Consider at least two reasons.)

(e) The teacher asks: "Why can't we prepare methanol by fermentation?" What is your answer?