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Chemistry · Practice book · Senior 5 · Unit 5 of 15

Alcohols and ethers

Senior 5 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 5 Alcohols and ethers

Key unit competence: Relate the physical and chemical properties of alcohols and ethers to their structure and uses.

Key ideas

  • Alcohols contain –OH on an sp3 carbon; general formula CnH2n+1OH. Classified as primary (RCH2OH), secondary (R2CHOH) or tertiary (R3COH). Named with suffix -ol (propan-2-ol); diols (ethane-1,2-diol) and triols (propane-1,2,3-triol, glycerol).
  • Physical properties: hydrogen bonding gives higher boiling points than alkanes or ethers of similar mass; lower alcohols are miscible with water; solubility decreases as the chain lengthens.
  • Preparation: hydration of alkenes (steam, H3PO4, 300 °C, 60 atm); hydrolysis of halogenoalkanes; reduction of aldehydes/ketones (LiAlH4 or NaBH4); fermentation of sugars by yeast (C6H12O6 → 2C2H5OH + 2CO2, 30–40 °C, anaerobic).
  • Reactions: with Na → alkoxide + H2; with carboxylic acids (conc. H2SO4) → esters; with PCl5/HX → halogenoalkanes; dehydration (conc. H2SO4) → alkenes (170 °C) or ethers (140 °C, excess alcohol); oxidation with acidified K2Cr2O7 (orange → green): primary → aldehyde → carboxylic acid; secondary → ketone; tertiary not oxidised.
  • Tests: Lucas test (ZnCl2/HCl): tertiary cloudy immediately, secondary in 5–10 min, primary no change at room temperature; iodoform test (I2/NaOH, yellow ppt of CHI3) for CH3CH(OH)– (ethanol and methyl secondary alcohols).
  • Ethers (R–O–R'): named as alkoxyalkanes (methoxyethane). Prepared by Williamson synthesis (RO−Na+ + R'X → ROR' + NaX) or dehydration of alcohols. Volatile, low boiling points (no O–H hydrogen bonding), fairly unreactive, good solvents; diethyl ether was used as an anaesthetic. Uses of alcohols: fuels, solvents, drinks, antiseptics, antifreeze; methanol is poisonous.

Common misconceptions

  • "Tertiary alcohols are oxidised easily like primary ones." No: Tertiary alcohols have no H on the C–OH carbon and are not oxidised by dichromate.
  • "Ethers have high boiling points because they contain oxygen." No: Ethers cannot hydrogen bond with each other (no O–H), so they are volatile.
  • "All alcohols give the iodoform test." No: Only alcohols with the CH3CH(OH)– group (e.g. ethanol, propan-2-ol) give yellow iodoform.

Worked examples

Example 1. Describe the oxidation of primary, secondary and tertiary alcohols by acidified potassium dichromate(VI).

Solution

Primary (ethanol): distil to get the aldehyde CH3CHO; reflux with excess oxidant to get ethanoic acid CH3COOH; orange → green.

Secondary (propan-2-ol): oxidised to the ketone propanone CH3COCH3; orange → green.

Tertiary (2-methylpropan-2-ol): no H on the C–OH carbon, not oxidised; stays orange.

Example 2. Describe the production of ethanol by fermentation and how it is concentrated.

Solution

Mix sugar solution (e.g. from sugarcane or bananas) with yeast in the absence of air at 30–40 °C.

Yeast enzymes convert glucose: C6H12O6 → 2C2H5OH + 2CO2.

Fermentation stops at about 12–15% ethanol (yeast is killed).

Filter, then fractionally distil to obtain about 96% ethanol.

Practice questions

  1. Name CH3CH(OH)CH2CH3.

    Show answer

    Butan-2-ol.

    Common mistake: Writing butan-3-ol.

  2. Write the equation for ethanol burning in air.

    Show answer

    C2H5OH + 3O2 → 2CO2 + 3H2O.

    Common mistake: Unbalanced O.

  3. State the conditions for fermentation.

    Show answer

    Yeast, sugar solution, 30–40 °C, absence of air.

    Common mistake: Saying high temperature and nickel.

  4. What is seen when propan-2-ol is warmed with acidified dichromate?

    Show answer

    Orange turns green.

    Common mistake: Saying green turns orange.

  5. Give the product of propan-1-ol distilled with acidified dichromate.

    Show answer

    Propanal.

    Common mistake: Saying propanone.

  6. How does the Lucas test distinguish primary, secondary and tertiary alcohols?

    Show answer

    Tertiary cloudy at once; secondary in minutes; primary no change at room temperature.

    Common mistake: Reversing the order.

  7. Why is methanol dangerous to drink?

    Show answer

    It is metabolised to methanal and methanoic acid, which cause blindness and death.

    Common mistake: Saying it is weaker than ethanol.

  8. Write the Williamson synthesis of methoxyethane.

    Show answer

    C2H5O−Na+ + CH3I → C2H5OCH3 + NaI.

    Common mistake: Using ethanol directly with methane.

  9. Explain why ethanol is completely miscible with water.

    Show answer

    It forms hydrogen bonds with water molecules.

    Common mistake: Saying it is ionic.

  10. Give two uses of ethanol.

    Show answer

    Fuel, solvent, beverages, antiseptic/sanitiser (any two).

    Common mistake: Saying fertiliser.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Heating excess ethanol with conc. H2SO4 at 140 °C gives:a. ethoxyethane
    2. Sodium reacts with ethanol to give sodium ethoxide and:b. hydrogen
    3. Reaction of ethanol with ethanoic acid (conc. H2SO4) gives:c. tertiary alcohols
    4. The Lucas test gives immediate cloudiness with:d. ethyl ethanoate
    Show answer

    1 → a (ethoxyethane) · 2 → b (hydrogen) · 3 → d (ethyl ethanoate) · 4 → c (tertiary alcohols)

    • Intermolecular dehydration.
    • 2C2H5OH + 2Na → 2C2H5ONa + H2.
    • Esterification.
    • Tertiary chlorides form fastest.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    nickel and hydrogen  ·  oxidises a primary or secondary alcohol  ·  steam with phosphoric acid catalyst  ·  an anaesthetic
    1. Ethanol is hydrated from ethene industrially using .
    2. Diethyl ether was once used as .
    3. Acidified potassium dichromate changes from orange to green when it .
    Show answer

    1: steam with phosphoric acid catalyst · 2: an anaesthetic · 3: oxidises a primary or secondary alcohol

    • 300 °C, 60 atm.
    • It is volatile and causes unconsciousness.
    • Cr2O72− → Cr3+.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which alcohol is poisonous and causes blindness?a. yeast
    2. Oxidation of a secondary alcohol gives a:b. Methanol
    3. Ethanol is produced by fermentation of glucose using:c. ketone
    4. Glycerol is a:d. triol
    Show answer

    1 → b (Methanol) · 2 → c (ketone) · 3 → a (yeast) · 4 → d (triol)

    • Oxidised to methanal and methanoic acid.
    • Two C groups stay on the carbonyl carbon.
    • Enzymes in yeast.
    • Propane-1,2,3-triol.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    butan-2-ol  ·  secondary  ·  Williamson synthesis  ·  methanol
    1. Propan-2-ol is classified as .
    2. In the Lucas test, compound A turns cloudy after 5 minutes. A could be .
    3. Ethers are prepared in the lab by the .
    Show answer

    1: secondary · 2: butan-2-ol · 3: Williamson synthesis

    • C–OH carbon bonded to two carbons.
    • Step 1. Cloudy after a few minutes: secondary.
      Step 2. Butan-2-ol.
    • Alkoxide + halogenoalkane.

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