Part 4.
The chemistry of ALCOHOLS
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK
KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
Part 4.7
Halogenation with hydrogen halides (not substitution)
Conversion of alcohols to
halogenoalkanes (haloalkanes)
INDEX of notes on ALCOHOLS
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Introduction to the reaction between an
alcohol and a hydrogen halide
These reactions involve substituting a halo functional
group for a hydroxy group, either directly or indirectly using the
equivalent hydrogen halide.
i.e.
ROH + HX ===> RX + H2O
(R = alkyl, X = Cl, Br or I)
The reaction is used as a
method of synthesising halogenoalkanes.
Aliphatic alcohols
Reminders:
You need to know the structures of the sub-classes of alcohols - primary,
secondary and tertiary.
(1) The reaction between phosphorus(V) chloride and an alcohol
If a dry alcohol is treated with
phosphorus(V) chloride (solid PCl5) the hydroxy functional
group is replaced by the chloro functional group.
Beware of the nasty
acidic fumes of hydrogen chloride gas evolved (in a fume cupboard) - though
it can also
convert the alcohol to the haloalkane! (see the next method)
The chloroalkane and phosphorus oxychloride are left
behind in the reaction vessel.
e.g. hexan-1-ol is converted to 1-chlorohexane
CH3CH2CH2CH2CH2CH2OH
+ PCl5 ===> CH3CH2CH2CH2CH2CH2Cl
+ POCl3 + HCl
(2) The reaction between thionyl chloride and an alcohol
If an alcohol is treated with thionyl
chloride (liquid SOCl2), again the hydroxy functional group
is also replaced by the chloro functional group.
Again, beware of the
nasty acidic fumes of hydrogen chloride and sulfur dioxide gases evolved
- though this is also an advantages - the waste gases are automatically
separated from the liquid product in a fume cupboard.
e.g. hexan-1-ol is converted to 1-chlorohexane
(bpt 133oC)
CH3CH2CH2CH2CH2CH2OH
+ SOCl2 ===> CH3CH2CH2CH2CH2CH2Cl
+ SO2 + HCl
(3) The reaction between an
alcohol and hydrogen chloride
If conc. sulfuric is added to a
mixture of the alcohol and potassium chloride which form potassium
hydrogenslfate and hydrogen chloride.
The alcohol is protonated
by the generated hydrogen chloride which converts the hydroxy group of the
alcohol to the chloro group of the haloalkane.
KCl + H2SO4
===> KHSO4 + HCl
e.g. hexan-1-ol is converted to 1-chlorohexane
(bpt 133oC)
CH3CH2CH2CH2CH2CH2OH
+ HCl ===> CH3CH2CH2CH2CH2CH2Cl
+ H2O
This a less efficient method if a secondary or
tertiary alcohol is used, because of dehydration converting the alcohol
to an alkene.
The
mechanism of converting an alcohol to a halogenoalkane via a hydrogen
halide
(4)
The reaction between an alcohol and potassium bromide - sulfuric
acid reagent
You cannot use potassium bromide and
potassium iodide to prepare bromoalkanes and iodoalkanes using the method
described in (3) above because conc. sulfuric acid oxidises the bromide
ion/hydrogen bromide molecule to bromine and the iodide ion/hydrogen iodide
molecule to iodine - see method (5). For more details on Halogen chemistry see
Redox reaction between halide salts and conc.
sulfuric acid However, you can use potassium
bromide by using ~50% sulfuric acid (~50% water).
The 50% sulfuric acid is added
slowly to the alcohol, keeping the mixture cool in a beaker.
This mixture is then added to solid potassium
bromide and the mixture gently heated under reflux in a suitable
flask.
The bromoalkane can be distilled from the
mixture. e.g. making
1-bromopropane (bpt ~71oC) from propan-1-ol (bpt ~97oC).
KBr + H2SO4
===> KHSO4 + HBr
CH3CH2CH2OH
+ HBr ===> CH3CH2CH2Br
+ H2O
(5) The reaction between phosphorus, iodine and an alcohol
With reference to method (4),
unfortunately, even 50%
sulfuric acid will still oxidise hydrogen iodide/iodide ion, so
another procedure must be adopted. An iodoalkane can be made
by gently warming a mixture of damp red phosphorus, iodine
and the appropriate alcohol. The water helps facilitate the
reaction between the phosphorus and iodine solids.
The iodine reacts with
the phosphorus to form phosphorus(III) iodide, which in turn
reacts with the alcohol to give iodoalkane e.g. to make
2-iodopropane from propan-2-ol.
2P +
3I2 ===> PI3
3CH3CH(OH)CH3
+ PI3
===> 3CH3CHICH3 +
H3PO3
(6) A summary of alcohol reactions (1) to (5) with
hydrogen halides
|
Chlorination |
Bromination |
Iodination |
Reagent |
PCl5, SOCl2
or H2SO4/KCl |
KBr + 50% sulfuric acid |
Moist P(red) + I2 |
Conditions |
Room temperature |
Heat under reflux |
Gentle warming |
Reaction type -
substitution Product - haloalkane
R = alkyl |
ROH + HCl ==> RCl + H2O
chloroalkane |
ROH + HBr ==> RBr + H2O bromoalkane |
ROH == PI3 ==> RI iodoalkane |
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INDEX of notes on ALCOHOLS
chemistry
All Advanced Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
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