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Brown's GCE Chemistry
Revising
Advanced Level Organic Chemistry - GCE A Level Revision Notes PART 10
Summary of organic reaction mechanisms
A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
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10.5 ALCOHOLS -
10.5.1 Introduction
1.5.3 Dehydration of alcohols
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Elimination of water from
an alcohol to give an alkene [acid catalysed, E1 and E2] elimination
reaction mechanisms
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The reaction
mechanism described involve acid catalysis and the initial step in
each case involves the protonation of the alcohol, this enables a
subsequent elimination reaction to take place.
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IMPORTANT NOTE on
structure classification
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In the mechanism
diagrams you will see part of the molecular structure shown as R3C
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PLEASE do not
assume this means a tertiary (tert) alcohol.
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R3C–
is used repeatedly to minimise the number of graphic images
needed.
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In general an
aliphatic alcohol has the structure R3C–OH where R =
H, alkyl or aryl.
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A primary
aliphatic alcohol can be shown as RCH2–OH where R =
H, alkyl or aryl.
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A secondary (sec)
alcohol can shown as R2CH–OH where R = alkyl or aryl.
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A tertiary (tert)
alcohol can be shown as R3C–OH where R = alkyl or
aryl.
10.5.3 The
acid catalysed elimination of water from an alcohol
The organic synthesis of alkenes by 'dehydration' of alcohols
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Examples of
elimination of water from an alcohol, its conversion to an alkene
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(i)
==>
+ H2O
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(ii)(a)
==>
+ H2O
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(ii)(b)
==>
+ H2O
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butan-2-ol ==>
but-1-ene/but-2-ene + water
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Note in (ii) the
possibility of isomeric products, two possible structural isomers
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What is the mechanism
for the acid catalysis of dehydrating an alcohol?
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e.g.
R2CH–C(OH)R2
==> R2C=CR2
+ H2O
[see mechanism 30 below]
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The alcohol can be
heated with mod. conc. sulphuric acid or conc. phosphoric(V) acid to
produce an alkene.
mechanism 30
– acid catalysed elimination of water from an alcohol
keywords phrases: reaction conditions formula
intermediates organic chemistry reaction mechanisms elimination reaction
R2CH–C(OH)R2 ==> R2C=CR2 + H2O CH3CH2CH(OH)CH3 ==> {CH3CH2CH=CH2 or CH3CH=CHCH3}
+ H2O C4H10O isomers: (CH3)3COH > CH3CH2CHOHCH3 > CH3CH2CH2CH2OH
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APPENDIX
COMPLETE MECHANISM
and Organic Synthesis INDEX
(so far!)
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