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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