_” NITRILES “_
A. Definision Of Nitriles
A nitrile
is any organic compound that has a -C ≡ N functional
group. The prefix cyano- is used interchangeably with the
term nitrile in industrial literature. Nitriles are found in many useful
compounds, including methyl cyanoacrylate, used in super glue,
and nitrile
butadiene rubber, a nitrile-containing polymer
used in latex-free
laboratory and medical gloves. Organic compounds containing multiple nitrile
groups are known as cyanocarbons.
Inorganic compounds containing the -C≡N
group are not called nitriles, but cyanides instead. Though both nitriles and
cyanides can be derived from cyanide salts, most nitriles are not nearly as
toxic.
B. Making Nitriles
Making nitriles from
halogenoalkanes
The halogenoalkane is heated under reflux
with a solution of sodium or potassium cyanide in ethanol. The halogen is
replaced by a -CN group and a nitrile is produced. Heating under reflux means
heating with a condenser placed vertically in the flask to prevent loss of
volatile substances from the mixture.
The solvent is important. If water is present you tend to get substitution
by -OH instead of -CN.
For example, using 1-bromopropane as a typical halogenoalkane:
You could write the full equation rather
than the ionic one, but it slightly obscures what's going on:
The bromine (or other halogen) in the
halogenoalkane is simply replaced by a -CN group - hence a substitution reaction.
In this example, butanenitrile is formed.
Making a nitrile by this method is a
useful way of increasing the length of a carbon chain. Having made the nitrile,
the -CN group can easily be modified to make other things - as you will find if
you explore the nitriles menu (link a the bottom of the page).
Making nitriles from amides
Nitriles can be made by dehydrating
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, you will get ethanenitrile by dehydrating ethanamide.
Making nitriles from
aldehydes and ketones
Aldehydes and ketones undergo an addition
reaction with hydrogen cyanide. The hydrogen cyanide adds across the
carbon-oxygen double bond in the aldehyde or ketone to produce a
hydroxynitrile. Hydroxynitriles used to be known as cyanohydrins.
For example, with ethanal (an aldehyde) you get 2-hydroxypropanenitrile:
With propanone (a ketone) you get 2-hydroxy-2-methylpropanenitrile:
In every example of this kind, the -OH
group will be on the number 2 carbon atom - the one next to the -CN group.
The reaction isn't normally done using
hydrogen cyanide itself, because this is an extremely poisonous gas. Instead,
the aldehyde or ketone is mixed with a solution of sodium or potassium cyanide
in water to which a little sulphuric acid has been added. The pH of the
solution is adjusted to about 4 - 5, because this gives the fastest reaction.
The reaction happens at room temperature.
The solution will contain hydrogen cyanide
(from the reaction between the sodium or potassium cyanide and the sulphuric
acid), but still contains some free cyanide ions. This is important for the
mechanism.
These are useful reactions because they
not only increase the number of carbon atoms in a chain, but also introduce
another reactive group as well as the -CN group. The -OH group behaves just
like the -OH group in any alcohol with a similar structure.
For example, starting from a hydroxynitrile made from an aldehyde, you can
quite easily produce relatively complicated molecules like 2-amino acids - the
amino acids which are used to construct proteins.
hi puput....
BalasHapuscan you explain to me about hydrolysis of nitrils?
^_~
The hydrolysis of nitriles
HapusWhen nitriles are hydrolysed you can think of them reacting with water in two stages - first to produce an amide, and then the ammonium salt of a carboxylic acid.
For example, ethanenitrile would end up as ammonium ethanoate going via ethanamide.
In practice, the reaction between nitriles and water would be so slow as to be completely negligible. The nitrile is instead heated with either a dilute acid such as dilute hydrochloric acid, or with an alkali such as sodium hydroxide solution.
The end result is similar in all the cases, but the exact nature of the final product varies depending on the conditions you use for the reaction.
1. Acidic Hydrolysis of Nitriles
The nitrile is heated under reflux with dilute hydrochloric acid. Instead of getting an ammonium salt as you would do if the reaction only involved water, you produce the free carboxylic acid.
For example, with ethanenitrile and hydrochloric acid you would get ethanoic acid and ammonium chloride.
Why is the free acid formed rather than the ammonium salt? The ethanoate ions in the ammonium ethanoate react with hydrogen ions from the hydrochloric acid to produce ethanoic acid. Ethanoic acid is only a weak acid and so once it has got the hydrogen ion, it tends to hang on to it.
2. Alkaline Aydrolysis of Nitriles
The nitrile is heated under reflux with sodium hydroxide solution. This time, instead of getting an ammonium salt as you would do if the reaction only involved water, you get the sodium salt. Ammonia gas is given off as well.
For example, with ethanenitrile and sodium hydroxide solution you would get sodium ethanoate and ammonia.
The ammonia is formed from reaction between ammonium ions and hydroxide ions.
If you wanted the free carboxylic acid in this case, you would have to acidify the final solution with a strong acid such as dilute hydrochloric acid or dilute sulphuric acid. The ethanoate ion in the sodium ethanoate will react with hydrogen ions as mentioned above.