2.10.2 Halogenoalkanes (haloalkanes)
a. demonstrate an understanding of the nomenclature and corresponding
structural, displayed and skeletal formulae for halogenoalkanes, including
the distinction between primary, secondary and tertiary structures
b. interpret given data and observations comparing the reactions and reactivity
of primary, secondary and tertiary compounds
c. carry out the preparation of an halogenoalkane from an alcohol and explain
why a metal halide and concentrated sulfuric acid should not be used when
making a bromoalkane or an iodoalkane
Connector:
1. Write the general formula for the halogenoalkanes.
2. Draw the displayed and skeletal formulae of the four isomers of
bromobutane, name them and then identify, them as 1o, 2o, or 3o
structures.
3. Draw the displayed formulae for
a) 1,1,1-trichloroethane
b) 1-bromo-2,3-dichloropropane
c) 1,2-dibromo-3,6-difluorocyclohexane (skeletal formula)
Crowe2009
d) Z-1-bromo-2-chloro-1-fluoroethene
Halogenoalkanes (haloalkanes)
General formula - CnH2n+1X
where X = F, Cl, Br, I.
Order of reactivity:
RI > RBr > RCl > RF
Primary, secondary or tertiary?
2o
1o
Br
Br
butan-1-ol
1- bromobutane
butan-2-ol
2- bromobutane
1o
Br
Br 3o
1- bromo2- methylpropane
2-methylpropan-1-ol
2- bromo- 2- methylpropane
2-methylpropan-2-ol
1,1,1-trichloroethane
Cl
Cl
H
C
C
Cl
H
1-bromo-2,3-dichloropropane
H
H
Br Cl
Cl
C
C
C
H
H
H
H
F
1,2-dibromo-3,6-difluorocyclohexane
Br
Br
F
c) Z-1-bromo-2-chloro-1-fluoroethene
Making halogenoalkanes from alcohols
using hydrogen halides
e.g. Making impure bromoethane in the lab
The alcohol is treated with a mixture of sodium or potassium
bromide and concentrated sulphuric acid.
This produces hydrogen bromide which reacts with the alcohol.
The mixture is warmed to distil off the bromoalkane.
Practical 2.14 – 1. Preparation of Bromoethane
Bromoethane has a low boiling point but is denser than water and
almost insoluble in it. To prevent it from evaporating, it is often
collected under water in a flask surrounded by ice.
The big problem with this method is that the
bromoethane contains unwanted by-products,
and needs to be purified (See next slide.).
It is therefore recommended that –
A metal halide and concentrated sulphuric
acid should not be used when making a
bromoalkane or an iodoalkane, because conc.
sulphuric acid is an oxidising reagent and
unwanted side reactions occur.
Practical 2.14 – 1. Preparation of Bromoethane.2
Impurities in the bromoethane include:
•hydrogen bromide (although most of that will dissolve in the
water if you are collecting the bromoethane under water);
•bromine - from the oxidation of bromide ions by the concentrated
sulphuric acid*;
•sulphur dioxide - formed when concentrated sulphuric acid
oxidises the bromide ions*;
•unreacted ethanol;
•ethoxyethane (diethyl ether) - formed by a side reaction between
the ethanol and the concentrated sulphuric acid.
What reagents should be used to obtain a good yield?
Red phosphorus and bromine or iodine, heat under reflux
Preparation of halogenoalkanes
The alcohol is heated under reflux with a mixture of red phosphorus
and either bromine or iodine.
The pure halogenoalkane can be distilled off.