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Chemistry · Practice book · Senior 5 · Unit 4 of 15

Halogenoalkanes

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

Unit 4 Halogenoalkanes

Key unit competence: Relate the physical and chemical properties of halogenoalkanes to their structure and uses, and assess their significance and dangers.

Key ideas

  • Halogenoalkanes (R–X, X = F, Cl, Br, I) are named as halo-substituted alkanes (2-chlorobutane). They are classified as primary (X on a C attached to one other C), secondary (two) or tertiary (three).
  • Physical properties: polar C–X bond, insoluble in water, denser than water (bromo and iodo), boiling points higher than alkanes of similar size and increase from F to I and with chain length.
  • Preparation: from alcohols with HX, PCl5, PCl3, SOCl2 (thionyl chloride gives SO2 and HCl gases) or PBr3/PI3; from alkenes with HX; free-radical halogenation of alkanes.
  • Nucleophilic substitution: OH−(aq) gives alcohols; CN− (ethanolic KCN) gives nitriles (chain lengthening); NH3 (excess, ethanolic, heated in sealed tube) gives amines; RO− gives ethers (Williamson). Reactivity: R–I > R–Br > R–Cl > R–F (C–X bond strength decreases down the group).
  • Mechanisms: SN2 (primary; one step, rate = k[RX][OH−], inversion) and SN1 (tertiary; carbocation intermediate, rate = k[RX]). With hot ethanolic KOH, elimination gives alkenes.
  • Test: warm with aqueous NaOH, acidify with dilute HNO3, add AgNO3: white (Cl), cream (Br), yellow (I) precipitate. Uses: solvents, anaesthetics (halothane), refrigerants and aerosols (CFCs), PVC, pesticides. CFCs destroy the ozone layer (Cl radicals); they are replaced by HFCs under the Montreal Protocol.

Common misconceptions

  • "Halogenoalkanes give a precipitate directly with silver nitrate." No: The halogen is covalently bonded; it must first be released as X− by hydrolysis with NaOH.
  • "Fluoroalkanes are the most reactive because F is most electronegative." No: C–F is the strongest bond, so fluoroalkanes are least reactive; bond strength matters more than polarity.
  • "Aqueous and ethanolic KOH give the same product." No: Aqueous KOH mainly substitutes (alcohol); hot ethanolic KOH mainly eliminates (alkene).

Worked examples

Example 1. Describe the SN1 and SN2 mechanisms for the hydrolysis of halogenoalkanes.

Solution

SN2 (primary, e.g. bromoethane): OH− attacks the carbon from the side opposite to Br; a transition state forms with partial bonds; Br leaves as Br−; one step, rate = k[RX][OH−], inversion of configuration.

SN1 (tertiary, e.g. 2-bromo-2-methylpropane): the C–Br bond breaks first (slow) to form a tertiary carbocation; OH− then attacks quickly; rate = k[RX]; a racemic product if the carbon is chiral.

Example 2. Describe how to identify the halogen in an unknown halogenoalkane.

Solution

Warm a few drops of the halogenoalkane with aqueous NaOH (hydrolysis releases X−).

Acidify with dilute nitric acid (removes excess OH−, which would give a brown Ag2O precipitate).

Add silver nitrate solution: white = chloride, cream = bromide, yellow = iodide.

Confirm with ammonia: AgCl dissolves in dilute NH3, AgBr in concentrated, AgI not at all.

Practice questions

  1. Classify 1-chlorobutane, 2-chlorobutane and 2-chloro-2-methylpropane.

    Show answer

    Primary, secondary, tertiary.

    Common mistake: Classifying by number of Cl atoms.

  2. Write the equation for 1-bromobutane with aqueous NaOH.

    Show answer

    CH3CH2CH2CH2Br + NaOH → CH3CH2CH2CH2OH + NaBr.

    Common mistake: Writing but-1-ene.

  3. Why is dilute nitric acid added before silver nitrate in the halide test?

    Show answer

    To neutralise excess OH−, which would precipitate silver oxide.

    Common mistake: Saying it dissolves the halogenoalkane.

  4. State the conditions for elimination.

    Show answer

    Hot, concentrated ethanolic KOH (or NaOH).

    Common mistake: Saying aqueous NaOH at room temperature.

  5. Why is the reaction with KCN useful in synthesis?

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    It adds one carbon atom to the chain.

    Common mistake: Saying it removes carbon.

  6. Give two uses of halogenoalkanes.

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    Solvents, anaesthetics, refrigerants, PVC manufacture, pesticides (any two).

    Common mistake: Saying food additives.

  7. Explain why iodoalkanes react faster than chloroalkanes.

    Show answer

    C–I bond is weaker and breaks more easily.

    Common mistake: Saying iodine is more electronegative.

  8. Write the equation for ethanol with PCl5.

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    C2H5OH + PCl5 → C2H5Cl + POCl3 + HCl.

    Common mistake: Missing POCl3.

  9. How do CFCs destroy ozone? (equation)

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    Cl· + O3 → ClO· + O2; ClO· + O → Cl· + O2 (Cl regenerated).

    Common mistake: Saying CFCs react with ozone directly without radicals.

  10. Name the product of 2-bromopropane with excess ammonia.

    Show answer

    Propan-2-amine (2-aminopropane).

    Common mistake: Saying propanamide.

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. Which is used as an anaesthetic?a. Halothane
    2. Halogenoalkanes react with aqueous NaOH to give:b. iodine
    3. Halogenoalkanes have higher boiling points than alkanes of similar size because:c. alcohols
    4. A yellow precipitate in this test shows:d. the C–X bond is polar and molecules are larger
    Show answer

    1 → a (Halothane) · 2 → c (alcohols) · 3 → d (the C–X bond is polar and molecules are larger) · 4 → b (iodine)

    • CF3CHBrCl.
    • Nucleophilic substitution by OH−.
    • Dipole–dipole and London forces.
    • AgI is yellow.

  2. Drag and drop

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

    radical steps  ·  a carbocation intermediate  ·  one step  ·  rate = k[RX][OH−]
    1. SN1 reactions involve .
    2. The rate equation for SN2 hydrolysis is .
    3. The SN2 mechanism occurs in .
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    1: a carbocation intermediate · 2: rate = k[RX][OH−] · 3: one step

    • Tertiary halides ionise first.
    • Both reactants in the slow step.
    • Bond breaking and forming together.

  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. (CH3)3CCl is a:a. chlorine radicals
    2. Thionyl chloride, SOCl2, converts alcohols into:b. tertiary halogenoalkane
    3. CFCs damage the ozone layer by producing:c. chloroalkanes
    4. The test for the halogen in a halogenoalkane uses:d. NaOH, then dilute HNO3, then AgNO3
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    1 → b (tertiary halogenoalkane) · 2 → c (chloroalkanes) · 3 → a (chlorine radicals) · 4 → d (NaOH, then dilute HNO3, then AgNO3)

    • Three C groups on the C–Cl carbon.
    • By-products are gases.
    • UV splits C–Cl bonds.
    • Hydrolysis releases X−.

  4. Drag and drop

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

    donates a pair of electrons  ·  secondary halogenoalkane  ·  nitriles  ·  amides
    1. A nucleophile is a species that .
    2. Halogenoalkanes react with KCN in ethanol to give .
    3. CH3CHBrCH3 is classified as a .
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    1: donates a pair of electrons · 2: nitriles · 3: secondary halogenoalkane

    • Electron-pair donor.
    • RX + CN− → RCN + X−.
    • The C–Br carbon is bonded to two carbons.

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