2.10.2 Reactions of the Halogenoalkanes

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b. 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 hydroxideiii. water containing dissolved silver nitrateiv. alcoholic ammonia

e. carry out the reactions described in 2.10.2d i, ii, iiif. 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.

Practical 2.15 reactions of the halogenoalkanes

Bond strengthsBond Bond length Bond enthalpy

(nm) (kJmol-1)C-F 0.138 467 C-Cl 0.177 346 C-Br 0.194 290C-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?

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?

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.

Reactions of halogenoalkanes – Nucleophilic substitution .2

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

In the presence of excess alcoholic ammonia* further reactions occur to produce a mixture of all possible products:

* 1-bromopropane is heated with a concentrated solution of ammonia in ethanol, in a sealed tube to prevent the ammonia escaping.

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?

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.

ion present Observation

Cl- White ppt.

Br- Pale cream ppt.

I- Yellow ppt

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.

Further tests: