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2.7Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Organic Chemistry: Halogenoalkanes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 13 parts Lesson 7 of 53 1 quiz

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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
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Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

2.7 Organic Chemistry: Halogenoalkanes — naming, reactions, mechanism and reactivity — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Name and classify halogenoalkanes, describe their reactions with nucleophiles and with ethanolic hydroxide, explain nucleophilic substitution and compare h…

  • A nucleophilic substitution reaction
  • What the symbols mean
  • Hydrolysis
  • Elimination
  • Bond enthalpy
  • Primary halogenoalkane

Part 1 of 11Naming halogenoalkanes

1
Use the prefixes fluoro-, chloro-, bromo- and iodo- on the parent alkane name.
2
Give the position of the halogen with a number: CH3CH2CH2Br is 1-bromopropane and CH3CH(Br)CH3 is 2-bromopropane.
3
List different substituents alphabetically: CH3CH(Cl)CH2Br is 1-bromo-2-chloropropane.
4
Use di-, tri- for repeated identical halogens.
5
Structural, displayed and skeletal formulae may all be required.

Part 2 of 11Classifying halogenoalkanes

Classifying halogenoalkanes
Classification depends on the number of carbon atoms attached to the carbon bearing the halogen.
Primary
  • Primary the C-X carbon is attached to one other alkyl group.
Secondary
  • Secondary the C-X carbon is attached to two other alkyl groups.
Tertiary
  • Tertiary the C-X carbon is attached to three other alkyl groups.

Part 3 of 11Polar C-X bond and nucleophiles

1
The halogen is more electronegative than carbon, so the C-X bond is polar with Cδ+.
2Nucleophile
  • Nucleophile an electron-rich species that donates a lone pair; it is attracted to positively charged carbon.
3A nucleophilic substitution reaction
  • A nucleophilic substitution reaction a nucleophile attacks the δ+ carbon and replaces the halogen.
4
Halogenoalkanes are insoluble in water and denser than water.

Part 4 of 11Reactions with hydroxide, water and cyanide

Reactions with hydroxide, water and cyanide
Aqueous hydroxide gives an alcohol (hydrolysis).
  • Use warm aqueous NaOH or KOH with a little ethanol as co-solvent;
    • the reaction is very slow at room temperature.
  • CH3CH2Br + OH− → CH3CH2OH + Br−.
  • Water must be the main solvent, otherwise elimination occurs.
Water also acts as a nucleophile but much more slowly than OH−.
  • OH− has a full negative charge, whereas oxygen in water carries only a partial negative charge.
  • RX + H2O → ROH + H+ + X−.
Cyanide gives a nitrile.
  • Heat under reflux with KCN in ethanol.
  • The carbon chain is extended by one carbon, e.g. bromoethane gives propanenitrile.

Part 5 of 11Reactions with ammonia and ethanolic hydroxide

Reactions with ammonia and ethanolic hydroxide
Ammonia gives a primary amine.
  • Heat a primary halogenoalkane with excess ammonia in ethanol under pressure (sealed tube).
  • Excess ammonia prevents further substitution, because the amine product is more reactive than ammonia.
  • Bromoethane gives ethylamine.
  • Two stepsCH3Cl + NH3 → [CH3NH3]+Cl−, then [CH3NH3]+Cl− + NH3 → CH3NH2 + NH4Cl.
Ethanolic hydroxide gives an alkene (elimination).
  • Heat under reflux with KOH or NaOH in ethanol.
  • The C-X bond breaks heterolytically and HX is removed, e.g. bromoethane gives ethene.

Part 6 of 11Nucleophilic substitution mechanism

Mechanism with aqueous hydroxide
  • The lone pair on OH− is attracted to the δ+ carbon.
  • A curly arrow from the lone pair forms a new bond to carbon.
  • A curly arrow from the C-X bonding pair moves onto X.
  • The C-X bond breaks heterolytically, releasing X−, and the alcohol forms.
What the symbols mean:
SN2 (primary halogenoalkanes)
  • The nucleophile attacks from the side opposite the halogen.
  • One step with a transition state; rate = k[RX][OH−].
    What the symbols mean: rate (mol dm−3 s−1) · k = rate constant (units depend on the overall order) · [ ] = concentration (mol dm−3) · each power is the order with respect to that reactant
SN1 (tertiary halogenoalkanes)
  • C-X breaks first to give a carbocation stabilised by electron-releasing alkyl groups.
  • The nucleophile then attacks; rate = k[RX].
  • Bulky groups also hinder back-side attack.
Ammonia as the nucleophile
  • The lone pair on N attacks the δ+ carbon, then a second NH3 removes H+.

Part 7 of 11Measuring the rate of hydrolysis

Measuring the rate of hydrolysis
Put three test tubes of ethanol and acidified aqueous silver nitrate in a water bath at about 50 °C.
Add a few drops of a chloroalkane, bromoalkane and iodoalkane and start the stopwatch.
Time how long the silver halide precipitate takes to appear.
  • Water is the nucleophile; the released halide ion forms the precipitate Ag+(aq) + X−(aq) → AgX(s).
HalogenoalkaneC-X bond enthalpy / kJ mol−1PrecipitateRate
C-Cl346White AgClSlowest
C-Br290Cream AgBrMedium
C-I228Yellow AgIFastest
Measuring the rate of hydrolysis
AgF is soluble, so no precipitate forms and fluoroalkanes are the least reactive.
For the same halogen and molecular formula: tertiary > secondary > primary.
1-chlorobutane is slowest, 2-chlorobutane is intermediate and 2-chloro-2-methylpropane is fastest.

Part 8 of 11Trend in reactivity and bond enthalpy

1
Polarity of C-X falls down the group as electronegativity falls, which alone would suggest fluoroalkanes react fastest.
2
Bond enthalpy is the greater factor: breaking C-X is the rate-limiting step.
3
C-I is the weakest bond and C-F the strongest, so iodoalkanes react fastest and fluoroalkanes least.
  • CH3CH2I + OH− → CH3CH2OH + I−.
4
Astatoalkanes would be predicted to react even faster than iodoalkanes.

Part 9 of 11Uses and environmental issues

Uses and environmental issues
Halogenoalkanes are used as solvents, refrigerants and in synthesis.
CFCs damage the ozone layer because UV light breaks C-Cl bonds, releasing chlorine radicals.

Part 10 of 11More on this topic

  • Explaining the halogenoalkane hydrolysis rate order by polarity — it follows bond enthalpy, since C-F is the most polar but the strongest.

Part 11 of 11More from the specification

  • Aqueous alkaline hydrolysis solution
    • heating esters under reflux with aqueous sodium hydroxide solution yields carboxylate salts and alcohols.

Key terms

TermMeaning
HalogenoalkaneAn alkane in which one or more hydrogen atoms are replaced by halogen atoms.
NucleophileAn electron-pair donor that attacks an electron-deficient carbon.
Nucleophilic substitutionA reaction in which a nucleophile replaces the halogen of a halogenoalkane.
HydrolysisBreaking a bond by reaction with water or hydroxide ions.
EliminationRemoval of HX from a halogenoalkane to form an alkene.
Bond enthalpyThe energy needed to break one mole of a given bond in gaseous molecules.
Primary halogenoalkaneA halogenoalkane in which the C-X carbon is attached to one other carbon.

Avoid these mistakes

1
Using polarity to explain the rate of hydrolysis — the C-X bond enthalpy decides, so C-I reacts fastest although it is the least polar.
2
Using ethanolic hydroxide for substitution — ethanolic KOH gives elimination, aqueous gives substitution.
3
Using a limited supply of ammonia — excess prevents further substitution.
4
Forgetting that the cyanide reaction adds a carbon atom to the chain.

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