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3.3Organic Chemistry

Halogenoalkanes — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 11 parts Lesson 11 of 55

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Complete PDF with figures and original page layoutPage 1 Oxford AQA International Your notes A Level (IAL) Chemistry Halogenoalkanes Contents Nucleophilic Substitution Elimination Reactions of Halogenoalkanes Page 2 Nucleophilic Substitu…

Part 1 of 9Nucleophilic substitution

Nucleophilic substitution
Nucleophile = an electron-rich species that donates a pair of electrons and is attracted to positive centres, with examples H2O, OH-, NH3 and CN-
Reaction
  • Reaction a nucleophile attacks a carbon with a partial positive charge and replaces an atom with a partial negative charge
Why halogenoalkanes react
  • Why halogenoalkanes react the C–X bond is polar because the halogen is much more electronegative than carbon

Part 2 of 9The mechanism

The mechanism
Draw a curly arrow from the lone pair on the nucleophile to the δ+ carbon of the C–X bond.
Draw a second curly arrow from the C–X bond to the X atom.
The halide leaves as X- and the nucleophile bonds to carbon.
  • Amines
    • the nucleophilic substitution forming an amine needs an extra step to remove the positive charge on nitrogen, with an arrow from the N–H bond to the N atom

Part 3 of 9Formation of nitriles

Nucleophile
  • Nucleophile the CN- ion
Reagent and conditions
  • Reagent and conditions aqueous ethanolic potassium cyanide, warmed with the halogenoalkane
    • (heated under reflux)
Product
  • Product a nitrile, extending the carbon chain by one carbon
Example
  • CH3CH2Br + CN- → CH3CH2CN + Br-, giving propanenitrile

Part 4 of 9Formation of primary amines

ammonia amine protonation
ammonia amine protonation
ammonia primary secondary tertiary amines
ammonia primary secondary tertiary amines
Nucleophile
  • NH3
Reagent and conditions
  • Reagent and conditions excess ethanolic ammonia heated under pressure
Product
  • Product a primary amine, with ammonium halide
Example
  • CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br, giving ethylamine

Part 5 of 9Formation of alcohols by hydrolysis

alcohol to halogenoalkane reagents
alcohol to halogenoalkane reagents
bromoethane aqueous hydroxide hydrolysis
bromoethane aqueous hydroxide hydrolysis
Nucleophile
  • OH-
Reagent
  • Reagent aqueous sodium or potassium hydroxide, warmed, with ethanol as solvent to increase the solubility of the halogenoalkane
Rate
  • Rate the reaction is very slow at room temperature
Example
  • CH3CH2Br + OH- → CH3CH2OH + Br-

Part 6 of 9Substitution against elimination

addition substitution elimination examples
addition substitution elimination examples
Competition
  • Competition hydroxide can cause nucleophilic substitution or elimination, depending on conditions
Favouring substitution
  • A higher proportion of water in the solvent.
  • Aqueous NaOH at room temperature or warmed.
  • Primary halogenoalkanes.
Favouring elimination
  • A higher proportion of ethanol.
  • Higher temperature and higher hydroxide concentration.
  • Tertiary halogenoalkanes.

Part 7 of 9C–X bond enthalpy and reactivity

  • Reason
    • substitution involves breaking the C–X bond, so bond energy explains the rates
BondBond enthalpy / kJ mol-1
C–F467
C–Cl346
C–Br290
C–I228
C–I
  • C–I it is the weakest bond, so iodoalkanes react fastest: R3C–I + OH- → R3C–OH + I-
C–F
  • C–F it is the strongest bond, so fluoroalkanes are least likely to react

Part 8 of 9Elimination reactions

haloalkane ethanolic hydroxide elimination
haloalkane ethanolic hydroxide elimination
bromoethane elimination ethene
bromoethane elimination ethene
Elimination = a molecule loses a small molecule, here a hydrogen halide, forming an alkene
Conditions
  • Conditions heat the halogenoalkane with ethanolic sodium hydroxide, with no water present
Role of OH−
  • Role of OH− it acts as a base, removing an H+ from the carbon next to the C–X carbon
Elimination reactions
Draw an arrow from the lone pair on OH- to the H on the adjacent carbon.
Draw an arrow from that C–H bond to form the C=C bond.
Draw an arrow from the C–X bond to X, which leaves as X-.
Example
  • Example hydrogen bromide is eliminated to form ethene
Aid
  • Aid the mechanism resembles a staircase, so check the first arrow goes to the correct hydrogen

Part 9 of 9Which product forms

PathwayConditionsProduct
Nucleophilic substitutionNaOH in water, room temperature, favoured by primary halogenoalkanesAlcohol
EliminationNaOH in ethanol (no water), high temperature, favoured by tertiary halogenoalkanesAlkene
Isomers
  • Isomers elimination can give a mixture of alkenes, which can show E/Z isomerism
Exam point
  • Exam point state the conditions correctly because different conditions give different reactions

Key terms

TermMeaning
NucleophileAn electron-pair donor that attacks a δ+ carbon.
Nucleophilic substitutionA reaction in which a nucleophile replaces a halogen.
EliminationLoss of a small molecule from a larger one to form a double bond.
NitrileA compound with the –C≡N group.
HydrolysisReaction with water or hydroxide breaking the C–X bond.
Bond enthalpyThe energy to break a bond.

Avoid these mistakes

1
Saying fluoroalkanes react fastest — C–F is the strongest bond.
2
Using aqueous NaOH for elimination — ethanolic NaOH with no water is used.
3
Starting the curly arrow at the carbon — it must start at the nucleophile’s lone pair.
4
Forgetting the extra step for amine formation — the positive nitrogen loses H+.

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