HYDROLYSING AMIDES
This page describes the hydrolysis of
amides under both acidic and alkaline conditions. It also describes the use of
alkaline hydrolysis in testing for amides.
The hydrolysis of amides
What is hydrolysis?
Technically, hydrolysis is a reaction with
water. That is exactly what happens when amides are hydrolysed in the presence
of dilute acids such as dilute hydrochloric acid. The acid acts as a catalyst
for the reaction between the amide and water.
The alkaline hydrolysis of amides actually
involves reaction with hydroxide ions, but the result is similar enough that it
is still classed as hydrolysis.
Hydrolysis under acidic conditions
Taking ethanamide as a typical amide:
If ethanamide is heated with a dilute acid (such as dilute hydrochloric
acid), ethanoic acid is formed together with ammonium ions. So, if you were
using hydrochloric acid, the final solution would contain ammonium chloride and
ethanoic acid.
Note: You might argue that because the hydrochloric acid is changed during the
reaction, it isn't acting as a catalyst. In fact, it is doing two things. It is
acting as a catalyst in a reaction between the amide and water which would
produce ammonium ethanoate (containing ammonium ions and ethanoate ions). It is
secondly reacting with those ethanoate ions to make ethanoic acid.
Hydrolysis under alkaline conditions
Again, taking ethanamide as a typical amide:
If ethanamide is heated with sodium hydroxide solution, ammonia gas is
given off and you are left with a solution containing sodium ethanoate.
Using alkaline hydrolysis to test for an
amide
If you add sodium hydroxide solution to an
unknown organic compound, and it gives off ammonia on heating (but not
immediately in the cold), then it is an amide.
You can recognise the ammonia by smell and
because it turns red litmus paper blue.
The possible confusion using this test is
with ammonium salts. Ammonium salts also produce ammonia with sodium hydroxide
solution, but in this case there is always enough ammonia produced in the cold
for the smell to be immediately obvious.
Note: This test is OK for UK A level purposes, but there are other things which
also give off ammonia on heating with sodium hydroxide solution - for example,
nitriles (but you won't come across them in a practical situation at this
level) and imides (but they are beyond the scope of courses at this level).
OTHER REACTIONS OF
AMIDES
This page explains the reason for the lack
of basic character in amides, and describes their dehydration to give nitriles,
and their reaction with bromine and sodium hydroxide solution to form primary
amines with one less carbon atom (the Hofmann degradation).
Note: The hydrolysis of amides is
described on a separate page.
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the BACK button on your browser to return to this page.
The lack of base
character in amides
Unusually for compounds containing the -NH2
group, amides are neutral. This section explains why -NH2 groups are
usually basic and why amides are different.
The usual basic character
of the -NH2 group
Simple compounds containing an -NH2
group such as ammonia, NH3, or a primary amine like methylamine, CH3NH2,
are weak bases. A primary amine is a compound where the -NH2 group
is attached to a hydrocarbon group.
The active lone pair of electrons on the
nitrogen atom in ammonia can combine with a hydrogen ion (a proton) from some
other source - in other words it acts as a base.
With a compound like methylamine, all that
has happened is that one of the hydrogen atoms attached to the nitrogen has
been replaced by a methyl group. It doesn't make a huge amount of difference to
the lone pair and so ammonia and methylamine behave similarly.
Note: The reasons that these are bases and the differences between them (because
there are slight differences) are explored in some detail on a page
about organic bases. It would
be useful to read this page before you go on because it is relevant to what is
coming next.
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button on your browser to return to this page.
For example, if you dissolve these
compounds in water, the nitrogen lone pair takes a hydrogen ion from a water
molecule - and equilibria like these are set up:
Notice that the reactions are reversible.
In both cases the positions of equilibrium lie well to the left. These
compounds are weak bases because they don't hang on to the incoming
hydrogen ion very well.
Both ammonia and the amines are alkaline
in solution because of the presence of the hydroxide ions, and both of them
turn red litmus blue.
Why doesn't something
similar happen with amides?
Amides are neutral to litmus and have
virtually no basic character at all - despite having the -NH2 group.
Their tendency to attract hydrogen ions is so slight that it can be ignored for
most purposes.
Note: If you haven't already done so, follow the link mentioned above to the page
about organic bases, and read
the bit about phenylamine. It is directly relevant to what's next.
Use the BACK button on your browser to
return to this page.
We need to look at the bonding in the
-CONH2 group.
Like any other double bond, a
carbon-oxygen double bond is made up of two different parts. One electron pair
is found on the line between the two nuclei - this is known as a sigma bond.
The other electron pair is found above and below the plane of the molecule in a
pi bond.
A pi bond is made by sideways overlap
between p orbitals on the carbon and the oxygen.
In an amide, the lone pair on the nitrogen
atom ends up almost parallel to these p orbitals, and overlaps with them as
they form the pi bond.
The result of this is that the nitrogen
lone pair becomes delocalised - in other words it is no longer
found located on the nitrogen atom, but the electrons from it are spread out
over the whole of that part of the molecule.
This has two effects which prevent the
lone pair accepting hydrogen ions and acting as a base:
- Because the lone pair is no longer located on a single atom as an intensely negative region of space, it isn't anything like as attractive for a nearby hydrogen ion.
- Delocalisation makes molecules more stable. For the nitrogen to reclaim its lone pair and join to a hydrogen ion, the delocalisation would have to be broken, and that will cost energy.
Note: If you want to look in more detail at the bonding in the carbon-oxygen
double bond, you could follow this link.
If you do choose to follow this link, it
will probably take you to several other pages before you are ready to come back
here again. Use the BACK button (or HISTORY file or GO menu) on your browser to
return to this page later.
The dehydration of
amides
Amides are dehydrated by heating a solid
mixture of the amide and phosphorus(V) oxide, P4O10.
Water is removed from the amide group to
leave a nitrile group, -CN. The liquid nitrile is collected by simple
distillation.
For example, with ethanamide, you will get
ethanenitrile.
Note: This is a just a flow scheme rather than a proper equation. I haven't been
able to find a single example of the use of the full equation for this
reaction. In fact the phosphorus(V) oxide reacts with the water to produce
mixtures of phosphorus-containing acids.
The Hofmann Degradation
The Hofmann degradation is a reaction
between an amide and a mixture of bromine and sodium hydroxide solution. Heat
is needed.
The net effect of the reaction is a loss
of the -CO- part of the amide group. You get a primary amine with one less
carbon atom than the original amide had.
The general case would be (as a flow
scheme):
