1. 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 d 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

2. Practical 2.15 reactions of the halogenoalkanes

3. Bond strengths Bond Bond length Bond enthalpy (nm) (kJmol-1)
C-F
C-Cl
C-Br
C-I
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 + NH 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
In the presence of excess alcoholic ammonia* further reactions occur to produce a mixture of all possible products:
CH3CH2CH2Br + CH3CH2CH2NH (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 halogenoalkanes Elimination 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. 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.