2.10.2 Reactions of the Halogenoalkanes
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 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
Practical 2.15 reactions of the halogenoalkanes
Bond strengths
Bond
C-F
C-Cl
C-Br
C-I
Bond length
(nm)
0.138
0.177
0.194
0.214
Bond enthalpy
(kJmol-1)
467
346
290
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 + OHCH3CH2CH2OH + 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?
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.
In the presence of excess alcoholic ammonia* further reactions
occur to produce a mixture of all possible products:
CH3CH2CH2Br + CH3CH2CH2NH2
CH3CH2CH2Br + (CH3CH2CH2)2NH
CH3CH2CH2Br + (CH3CH2CH2)3N
(CH3CH2CH2)2NH + HBr
di-(1-aminopropane)
(CH3CH2CH2)3N + HBr
tri-(1-aminopropane)
(CH3CH2CH2)4N+Brtetrapropylammonium bromide
* 1-bromopropane is heated with a concentrated solution of ammonia
in ethanol, in a sealed tube to prevent the ammonia escaping.
Reactions of halogenoalkanes
Elimination reaction using alcoholic KOH
HH
l l
H-C-C-H + OHl l
H Br
bromoethane
H H
ethanol
l l
H-C=C-H + H2O + Br ethene
•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
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.