4.2.1 (d,e) Elimination and Substitution reactions of Alcohols
Syllabus
(d) Elimination of H2O from Alcohols in the presence of an acid catalyst (e.g. H3PO4 or H2SO4) and heat
to form Alkenes
{Mechanism not required.}
(e) Substitution with Halide ions in the presence of acid (e.g. NaBr/H2SO4) to form Haloalkanes.
{Mechanism not required.}
What does this mean?
What is the difference between Elimination and Substitution?
A Substitution Reaction involves removing an atom (or group) and replacing it with another - like substituting a player for another in football match.
Elimination involves removing an atom (or group) but not replacing it with anything - like taking off an injured player after using all your substitutes.
Eliminating Water from Alcohols
To eliminate water from an Alcohol requires the removal of the -OH from one Carbon atom and the removal of an H atom from a neighbouring Carbon atom.
There alcohols where this may not happen.
Dimethylpropan-1-ol is an example where there are no Hydrogen atoms on Carbon 2 to remove. So it won't dehydrate in a normal way.
This is a minor point but occasionally crops up in exams.
But most Alcohols can be dehydrated this way by using a concentrated strong acid, usually Phosphoric or Sulphuric acid as dehydrating agent.
Since both Carbons in the molecule above have lost an atom/group without gaining anything to replace them a double bond forms.
So elimination reactions of Alcohols always form an Alkene.
Primary Alcohols that can eliminate always form only 1 product
But other Alcohols form two and sometimes three different Alkenes.
How?
In this case, 2-methylbutan-2-ol forms two products because when the -OH is removed from Carbon 2 we can remove a Hydrogen atom from either Carbon 1 or Carbon 3.
This is generally the case for Alcohols that are not Primary.
But how is it possible to make three Alkenes?
Butan-2-ol can obviously make But-1-ene or But-2-ene but looking closely at But-2-ene it should be clear that it can exist as E and Z isomers.
So there would be three different Alkenes formed (but not in equal amounts).
Sometimes examiners try to throw you off the scent by including a strange looking molecule.
A favourite is cyclo-compounds that are alcohols, such as Cyclohexanol.
It is still an Alcohol, the Carbon atoms next to Carbon 1 have Hydrogen atoms available so it will dehydrate.
And asymmetrical Cyclo-alcohols may form more than one product.
Substitution reactions of Alcohols
It is possible to remove the -OH group and replace it with a Halide ion (generally Cl- or Br-).
The Alcohol is mixed with NaBr and a Sulphuric Acid catalyst.
This makes HBr which then reacts with the Alcohol to make a Haloalkane.
So for the reaction to make 2-bromopropane from propan-2-ol the first part is:
NaBr + H2SO4 → HBr + NaHSO4
And the second is:
HBr + CH3CH(OH)CH3 → CH3CH(Br)CH3 + H2O
Hence, overall it is
NaBr + H2SO4 + CH3CH(OH)CH3 → CH3CH(Br)CH3 + H2O + NaHSO4
Although examiners may for an equation for the reaction between HBr & an Alcohol, in which case it would be:
HBr + CH3CH(OH)CH3 → CH3CH(Br)CH3 + H2O
Video
This video starts with esterification which is covered in a later part of the syllabus.
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