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.
2 Synthesis and Manufacture of
Alcohols
Sub-index for this page
4.2.1
Hydrolysis of
halogenoalkanes - a laboratory synthesis
4.2.2
Industrial
manufacture of alcohols
INDEX of notes on ALCOHOLS
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4.2.1
The hydrolysis of
halogenoalkanes
You must know the structures of
primary, secondary and tertiary halogenoalkanes (haloalkanes)
You must know the structures of
primary, secondary and tertiary alcohols
(a) The reaction between a halogenoalkane
and the hydroxide ion and reagents and method
Both these soluble strong bases
(alkalis) are source of a powerful nucleophile, the hydroxide ion (OH–).
This is a nucleophilic substitution reaction
because the attacking reagent is a nucleophile and the halogen
functional group/atom is replaced by the hydroxy functional group
(alcohol):
RX +
OH- ===> ROH + X-
(R = alkyl, X = Cl, Br or I)
The hydrolysis usually takes faster with an
alkali than pure water because water is a weaker nucleophile than
the hydroxide ion.
Strictly speaking all the reactants and products
should be suffixed by (aq) apart from water (l).
The reaction
is usually carried out by refluxing the halogenoalkane with aqueous sodium
hydroxide (NaOH) and potassium hydroxide (KOH) (left diagram above)
The cold water cooled Liebig vertical condenser
prevents the loss of volatile molecules e.g. solvent or product.
The alcohol can be distilled from the mixture using
fractional distillation (right diagram above).
When an organic compounds like
halogenoalkanes are not very soluble in water, the reaction with an aqueous reagent is very
slow because of a much reduced chance of reactant particle
interactions e.g. compared to a homogenous mixture where the
haloalkane dissolves in the reagent solvent.
So, even under reflux, the hydrolysis reaction can
be quite slow because halogenoalkanes are insoluble in water, but
using aqueous ethanol solvent increases their solubility and speed
of the reaction, BUT ... ... NaOH, and in particular KOH, are very strong bases, and if pure ethanol is used as the solvent, you can get
significant quantities of an alkene via an elimination reaction. See
Part 3.7
The elimination reactions of
halogenoalkanes (haloalkanes) with potassium hydroxide to give alkenes
(b) bromoethane +
sodium hydroxide ===> ethanol + sodium
bromide
+ NaOH
+ NaBr since hydroxide
and bromide are free ions the equations are better written as ...
bromoethane + hydroxide ion ===>
ethanol + bromide ion
+ OH–
+ Br–
(displayed formula equation)
+ OH–
+ Br–
(structured formula ionic equation)
+ OH–
+ Br–
(skeletal formula equation) An
example of primary alcohol formation.
(c) 1-bromopropane + sodium hydroxide
===> propan-1-ol + sodium bromide
+ NaOH
+ NaBr since hydroxide
and bromide are free ions the equations are better written as ...
1-bromopropane + hydroxide ion ===>
propan-1-ol + bromide ion
+ OH–
+ Br–
+ OH–
+ Br–
Another example of primary alcohol formation.
(d) bromocyclohexane + hydroxide ion
===> cyclohexanol + bromide ion
+ OH–
+ Br–
An example of secondary alcohol formation.
(e) 2-bromopropane + sodium
hydroxide ===> propan-2-ol + sodium bromide
CH3CHBrCH3
+ OH- ===> CH3CH(OH)CH3
+ Br-
Another example of secondary alcohol
formation.
(f)
2-chloro-2-methylpropane + sodium hydroxide ===>
2-methylpropan-2-ol + sodium chloride
(CH3)3CCl
+ OH- ===> (CH3)3COH
+ Cl-
Another example of tertiary alcohol formation.
(g) 1,3-dichlorobutane + sodium
hydroxide ===> butane-1,3-diol + sodium bromide
+ 2OH- ===>
+ 2Cl-
The diol product is both a primary and secondary
alcohol (reverse one of the images to match up!)
See also Parts 3.3
Reactivity trends of halogenoalkanes - introduction to their nucleophilic
substitution reactions
3.4
The
substitution reaction of halogenoalkanes (haloalkanes) with sodium/potassium hydroxide to
give alcohols
and
Nucleophilic substitution by water/hydroxide ion
[SN1 or SN2
mechanisms, hydrolysis to
give alcohols]
4.2.2
Industrial manufacture of alcohols
Examples of how alcohols are
manufactured and synthesised on a large scale.
(a)
The industrial
manufacture of methanol
Methanol can be manufactured by
mixing hydrogen and carbon monoxide gases and passing them over a zinc
oxide and chromium oxide catalyst.
2H2(g) +
CO(g) ===> CH3OH(g)
The methanol can be condensed and purified by
fractional distillation.
(b)
Methods of manufacturing ethanol ('alcohol')
The production of ethanol (i) from sugar
fermentation or (ii) synthesis from ethene:
(i)
===>
(acid catalysed synthesis in the chemical industry)
Electrophilic addition mechanism of
adding water to alkenes
[acid catalyst] to form alcohols
(ii)
C6H12O6(aq)
====> 2C2H5OH(aq) + 2CO2(g)
(in the brewing and wine industry)
All
the manufacturing processes are fully described on the
Ethanol manufacture page
(c)
Other
alcohols and other methods
Most of the other alcohols are
manufacture by far more complex processes, often involving several
steps. e.g. the synthesis
of propan-1-ol can
be made in two stages using transition metal based catalysts.
(i) Ethene (from oil cracking) is
reacted with carbon monoxide and hydrogen using a rhodium or cobalt
compound catalyst to propanal.
H2C=CH2
+ CO + H2 ===> CH3CH2CHO
(ii) The propanal is hydrogenated
to make propan-1-ol (1-propanol) using an Ni/Pt catalyst.
CH3CH2CHO
+ H2 ===> CH3CH2CH2OH
Propanol
alcohols, as with many other compounds, can be manufactured from basic
carbohydrates like sugars in a fermenter. The microbe enzymes do the chemistry
- examples of biosynthetic pathways - considered more 'green' chemistry.
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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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