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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

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2.10.2 Reactions of the Halogenoalkanes

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  1. 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. 2.10.2 Reactions of the Halogenoalkanes Crowe2009 Connector: Explain why a metal halide and concentrated sulphuric acid should not be used when making a bromoalkane or an iodoalkane.

  2. Practical 2.15 reactions of the halogenoalkanes

  3. 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?

  4. 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?

  5. 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.

  6. 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)4N+Br- tetrapropylammonium bromide * 1-bromopropane is heated with a concentrated solution of ammonia in ethanol, in a sealed tube to prevent the ammonia escaping.

  7. Reactions of halogenoalkanesElimination reaction using alcoholic KOH H H H H l l l l H-C-C-H + OH- H-C=C-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?

  8. 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.

  9. 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.

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