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2.11Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

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

5 min read 12 parts Lesson 10 of 12 2 quizzes

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2.11 Organic Chemistry: Halogenoalkanes — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Name and classify halogenoalkanes, describe their reactions with OH⁻, CN⁻, NH₃ and ethanolic hydroxide, explain the nucleophilic substitution mechanism and compare hydrolysis rates.Prefixes — fluoro-…

  • Repeated halogens
  • Polar bond
  • Nucleophile
  • With aqueous hydroxide
  • With water
  • With cyanide

Section 1 of 12Naming halogenoalkanes

Prefixes
  • Prefixes fluoro-, chloro-, bromo- and iodo- are added to the alkane name
Position
  • Position a number shows which carbon carries the halogen, and substituents are listed alphabetically
  • CH3CH2CH2Br is 1-bromopropane and CH3CH(Br)CH3 is 2-bromopropane.
  • CH3CH(Cl)CH2Br is 1-bromo-2-chloropropane.
Repeated halogens
  • Repeated halogens di- and tri- are used for identical halogens

Section 2 of 12Classifying halogenoalkanes

Classifying halogenoalkanes
Primary
  • Primary the carbon bearing the halogen is attached to one other alkyl group
Secondary
  • Secondary the carbon bearing the halogen is attached to two other alkyl groups
Tertiary
  • Tertiary the carbon bearing the halogen is attached to three other alkyl groups
Polar bond
  • Polar bond the C–X bond is polar because halogens are more electronegative, so the carbon is δ+ and open to nucleophilic attack

Section 3 of 12Nucleophiles and substitution

1Nucleophile
Nucleophile an electron-pair donor that is attracted to positively charged or δ+ species
  • Examples are OH-, CN- and NH3.
2Nucleophilic substitution
  • Nucleophilic substitution a nucleophile attacks the δ+ carbon and replaces the halogen
  • The C–X bond breaks heterolytically, releasing X-.

Section 4 of 12Reactions of halogenoalkanes

With aqueous hydroxide
  • With aqueous hydroxide forms an alcohol, in a hydrolysis reaction
  • CH3CH2Br + OH- → CH3CH2OH + Br-.
  • Warm aqueous NaOH or KOH is used, with ethanol as a co-solvent, and water must be the main solvent.
  • Hydroxide is a better nucleophile than water because it has a full negative charge.
With water
  • With water forms an alcohol much more slowly: RX + H2O → ROH + H+ + X-
  • Silver nitrate in ethanol shows the halide ions as they form: Ag+ + X- → AgX.
With cyanide
  • With cyanide forms a nitrile when heated under reflux with ethanolic KCN
  • Bromoethane forms propanenitrile.
  • The carbon chain is lengthened by one carbon atom.
With ammonia
  • With ammonia forms a primary amine when heated under pressure with excess ethanolic ammonia
  • Bromoethane forms ethylamine.
  • Excess ammonia prevents further substitution because the amine product is more reactive than ammonia.
With ethanolic hydroxide
  • With ethanolic hydroxide heated with ethanolic NaOH, hydrogen halide is eliminated to form an alkene
  • CH3CH2Br + OH- → CH2=CH2 + H2O + Br-.
  • Water as the main solvent gives substitution, but ethanolic conditions give elimination.

Section 5 of 12The nucleophilic substitution mechanism

1
The carbon of the polar C–X bond is δ+, so the lone pair of the nucleophile is attracted to it.
2
A curly arrow goes from the lone pair to the carbon, forming a new bond.
3
A second curly arrow goes from the C–X bond to the halogen.
4
The C–X bond breaks heterolytically and X- leaves, giving the substituted product.
  • Ammonia
    • reacts in two steps, with a second NH3 removing H+ from the intermediate
    • CH3Cl + NH3 → [CH3NH3]+Cl-.
    • [CH3NH3]+Cl- + NH3 → CH3NH2 + NH4+Cl-.

Section 6 of 12SN1 and SN2 (A-level extension)

SN2
  • SN2 typical of primary halogenoalkanes: one step, with the nucleophile attacking from the side opposite the halogen
  • It passes through a transition state with partial bonds.
  • The rate is k[RX][OH−].
SN1
  • SN1 typical of tertiary halogenoalkanes: the C–X bond breaks first to form a carbocation
  • The carbocation is stabilised by electron-releasing alkyl groups, and bulky groups hinder backside attack.
  • The rate is k[RX].

Section 7 of 12Measuring the rate of hydrolysis

Measuring the rate of hydrolysis
Set up test tubes in a 50 °C water bath, each with ethanol and acidified silver nitrate.
Add a few drops of a chloroalkane, a bromoalkane and an iodoalkane, and start a stopwatch.
Time how long each precipitate takes to appear.
1Results
  • Results iodoalkane gives a yellow precipitate fastest, then bromoalkane (cream), with chloroalkane (white) slowest
  • Fluoroalkanes do not react, because silver fluoride is soluble.
2Primary, secondary and tertiary
  • Primary, secondary and tertiary with the same halogen and formula, tertiary reacts quickest and primary slowest
  • 1-chlorobutane, 2-chlorobutane and 2-chloro-2-methylpropane can be compared.

Section 8 of 12Why bond enthalpy decides the trend

Bond enthalpy order
  • C–F > C–Cl > C–Br > C–I
  • The C–I bond is the weakest and is easiest to break, so iodoalkanes react fastest.
Polarity
  • Polarity decreases down the group but is a less important factor than bond strength
  • Polarity alone would predict fluoroalkanes to be the quickest to react.
Bond enthalpy, not polarity, decides the rate of hydrolysis

Section 9 of 12More on this topic

1Reactions with nucleophiles
  • Reactions with nucleophiles the C–X bond is polar, so halogenoalkanes undergo nucleophilic substitution
  • Aqueous hydroxide substitutes to give the alcohol.
2
Using polarity to explain the halogenoalkane rate order — bond enthalpy is the deciding factor

Section 10 of 12More from the specification

  • Nucleophilic substitution in alcoholic solution
    • heating halogenoalkanes with ethanolic potassium hydroxide solution produces alkenes via base-induced beta elimination.

Section 11 of 12Key terms

TermMeaning
HalogenoalkaneAn alkane in which one or more hydrogens are replaced by halogen atoms.
NucleophileAn electron-pair donor attracted to δ+ carbon atoms.
Nucleophilic substitutionReplacement of an atom or group by a nucleophile.
HydrolysisBreakdown of a compound by water or aqueous hydroxide.
EliminationRemoval of a small molecule such as HBr from a molecule to form a C=C bond.
Primary halogenoalkaneOne where the C–X carbon is attached to one other alkyl group.
CarbocationA positive ion with a positively charged carbon atom.

Section 12 of 12Exam technique

1
Explaining the rate order by polarity — C–I is least polar but reacts fastest because its bond is weakest
2
Using ethanolic hydroxide and expecting substitution — ethanolic conditions give elimination to an alkene
3
Starting a curly arrow at the nucleophile's charge rather than at the lone pair
4
Using a limited amount of ammonia and expecting only a primary amine — excess ammonia is needed
5
Forgetting acidified silver nitrate in the hydrolysis test — halide ions must be detected as they form

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