2.10.2 Reactions of the Halogenoalkanes

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2.10.2 Reactions of the Halogenoalkanes

• interpret given data and observations comparing
  the reactions and reactivity of primary,
  secondary and tertiary compounds.
• d. describe the typical behaviour of
  halogenoalkanes
• i. aqueous alkali eg KOH (aq)
• ii. alcoholic potassium hydroxide
• iii. water containing dissolved silver nitrate
• iv. alcoholic ammonia
• e. carry out the reactions described in 2.10.2d
  i, ii, iii
• f. discuss the uses of halogenoalkanes, eg as
  fire retardants and
• modern refrigerants.

Connector Explain why a metal halide and
concentrated sulphuric acid should not be used
when making a bromoalkane or an iodoalkane.

• Crowe2009

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Practical 2.15 reactions of the halogenoalkanes
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Bond strengths

• Bond Bond length Bond enthalpy
  (nm) (kJmol-1)
• C-F 0.138 467
• C-Cl 0.177 346
• C-Br 0.194 290
• C-I 0.214 228
• What is the link between bond length and bond
  strength?
• In a substitution reaction the C-X bond is
  broken. Which type of halogenoalkane will be the
  most reactive?
• Why are fluorocarbons very unreactive?

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Reactions of halogenoalkanes Nucleophilic
substitution .1

• With aqueous potassium hydroxide solution
• H2O
• CH3CH2CH2Br OH-
  CH3CH2CH2OH Br-
• What is a nucleophile?
• The C-X bond is polar, which C-X bond will be the
  most polar?
• Compare your answer above, to the order of
  reactivity of RX compounds.
• Which is the most important factor in deciding
  the rate of reaction the strength of the C-X
  bond, or the polarity of the bond?

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Reactions of halogenoalkanes Nucleophilic
substitution .2

• With alcoholic ammonia
• CH3CH2CH2Br NH3 CH3CH2CH2NH2
  HBr
• 
  1-aminopropane
• How does ammonia act as a nucleophile?
• Is 1-aminopropane a nucleophile? Explain your
  answer.
• Suggest what could happen once an appreciable
  amount of 1-aminopropane has formed.

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In the presence of excess alcoholic ammonia
further reactions occur to produce a mixture of
all possible products
CH3CH2CH2Br CH3CH2CH2NH2
(CH3CH2CH2)2NH HBr

di-(1-aminopropane) CH3CH2CH2Br
(CH3CH2CH2)2NH (CH3CH2CH2)3N
HBr

tri-(1-aminopropane) CH3CH2CH2Br
(CH3CH2CH2)3N
(CH3CH2CH2)4NBr-

tetrapropylammonium bromide
1-bromopropane is heated with a concentrated
solution of ammonia in ethanol, in a sealed tube
to prevent the ammonia escaping.
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Reactions of halogenoalkanesElimination reaction
using alcoholic KOH

• H H H H
  
• l l l
  l
• H-C-C-H OH- H-CC-H H2O
  Br -
• l l
  ethene
• H Br
• bromoethane

ethanol

• Why is this called an elimination reaction?
• How does this reaction differ from that which
• was used to make an alcohol?

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Identification of halogenoalkanes
To a sample of an halogenoalkane add 1cm3 of
ethanol followed by 1cm3 of aqueous silver
nitrate. Stand the test tube in a beaker of hot
water for a few minutes.
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ion present Observation
Cl- White ppt.
Br- Pale cream ppt.
I- Yellow ppt
Further tests
White precipitate of silver chloride soluble in
dilute ammonia. Cream precipitate of silver
bromide, only soluble in concentrated ammonia.
Yellow precipitate of silver iodide insoluble in
concentrated ammonia.