6.12.1 Structure and physical properties of nitriles
The nitrile functional group
consists of a carbon to nitrogen triple bond
C≡N
The aliphatic name is based on the longest
carbon chain, including the C of the nitrile group.
Nitriles are colourless and liquid at room temperature and all poisonous
compounds.
Name |
Formula |
Mpt/oC |
Bpt/oC |
Comments including solubility in water |
Methanenitrile |
HC≡N |
-14 |
26 |
Very soluble in water, also called hydrogen cyanide |
Ethanenitrile |
CH3CN |
-46 |
82 |
Very soluble in water. |
Propanenitrile |
CH3CH2CN |
-92 |
97 |
Very soluble in water. |
Butanenitrile |
CH3CH2CH2CN |
|
|
|
|
|
|
|
|
Benzonitrile |
C6H5CN |
13 |
191 |
An aromatic nitrile, -C≡N directly
attached to the benzene ring. |
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and sub-index
6.12.2
The preparation of nitriles from halogenoalkanes
The reaction between potassium cyanide and
halogenoalkanes
You must know the structures of
primary, secondary and tertiary halogenoalkanes (haloalkanes)
This
is an important reaction for extending a carbon chain and a method of
synthesising carboxylic acids.
A nitrile
functional group replaces the halogen atom in the halogenoalkane:
The halogenoalkane is refluxed with an ethanolic
solution of potassium cyanide.
The cold water cooled Liebig vertical condenser
prevents the loss of volatile molecules e.g. solvent or product.
It is better to use ethanol as the solvent rather than
water to avoid hydrolysis to an alcohol i.e. -X replaced with -OH.
R-X +
KCN ===> R-CN + KX (R = alkyl, X =
halogen)
R-X +
CN- ===> R-CN + X-
(ionic equation)
The cyanide ion is a nucleophile (electron pair donor,
N≡C:-)
(a)
The reaction between potassium cyanide and
bromoethane
Strictly speaking all the reactants and products
should be suffixed by (aq)
bromoethane + potassium cyanide
===> propanenitrile + potassium bromide
+ KCN
+ KBr
(displayed formula
equation)
Since the cyanide and bromide are free ions,
the equations are better written as ...
bromoethane + cyanide ion ===>
propanenitrile + bromide ion
+ CN–
+ Br–
(displayed formula equation)
+ CN–
+ Br–
(structured formula equation)
+ CN–
+ Br–
(abbreviated structured formula equation)
+ CN–
+ Br–
(skeletal formula equation)
This is an important synthesis reaction
because it is one of the few methods of increasing the length of the carbon
chain
(b)
The reaction between 2-bromopropane
(bpt 59oC) and potassium cyanide
2-bromopropane + cyanide ion
===> 2-methylpropanenitrile + bromide ion
TOP OF PAGE
and sub-index
6.12.3 Preparation of nitriles from carbonyl compounds -
aldehydes and ketones
The reaction between
hydrogen cyanide and aldehydes or ketones
The
product of the nucleophilic addition of hydrogen cyanide is a hydroxynitrile (a
cyanohydrin).
The reaction is equivalent to adding H-CN
across the C=O bond, to give a N-C-O-H bonding situation.
The addition begins with the initial addition of a
cyanide ion (see details of mechanism in
section
6.4.3).
So, the reagent and reaction conditions must be just
right.
Hydrogen cyanide is a very weak acid (Ka = 5 x 10-10
mol dm-3), and, on its own at equilibrium, it produces very few
cyanide ions:
(i) HCN + H2O
H3O+ + :CN-
Therefore a base (alkali) must be present to raise the pH
>7.
The presence of a strong base (e.g. hydroxide ion)
generates a sufficiently high concentration of cyanide ions to allow the
addition of HCN to proceed efficiently.
(ii) HCN + OH-
H2O + :CN-
If the pH is too low, there are insufficient cyanide ions
for the reaction to proceed quickly.
In practice, a solution of potassium cyanide (KCN) is
used, buffered to about pH ~8.
KCN is the salt of a strong base and
a very weak acid, and is naturally alkaline by hydrolysis (the reverse
of reaction (ii) above), so providing a higher concentration of cyanide
ions than hydrogen cyanide.
To
get the right pH, a little dilute sulfuric acid is added to a solution
of sodium/potassium cyanide.
Using pure HCN solution, the reaction takes weeks, add a drop of NaOH
and it goes in hours, so the base (alkali) has quite a catalytic effect.
Examples of
nucleophilic addition of hydrogen cyanide to aldehydes and ketones
to give hydroxynitriles
(a)
+ HCN ===>
ethanal + hydrogen
cyanide ===> 2-hydroxypropanenitrile
(b)
+ HCN ===>
propanone + hydrogen
cyanide ===> 2-hydroxy-2-methylpropanenitrile
(c)
+ HCN ===>

butanone + hydrogen
cyanide ===> 2-hydroxy-2-methylbutanenitrile
Need some bigger skeletal formulae equations?
TOP OF PAGE
and sub-index
6.10.4 Use of nitriles in synthesis:
Reduction of nitriles to primary
aliphatic amines
Sodium tetrahydridoborate(III), NaBH4 (sodium borohydride),
is not a powerful enough reducing agent to reduce nitriles to primary
amines.
LiAlH4
is a more powerful reducing agent than NaBH4 and reacts
violently with water (and reacts with ethanol too), so the reaction must be carried out in an inert
non-aqueous solvent
like dry ethoxyethane ('ether').
(a) The reduction reaction of nitriles can be summarised
as:
RC
N + 4[H]
===> RCH2NH2 (R = H, alkyl or aryl)
e.g. propanenitrile to
propylamine: CH3CH2C≡N + 4[H] ===> CH3CH2CH2NH2
The initial product is hydrolysed by
dilute sulphuric acid and the amine then freed by adding strong alkali like
sodium hydroxide.
(b) The reduction of hydroxy-nitriles
e.g.
RCH(OH)C≡N + 4[H]
==> RCH(OH)CH2NH2 (R = H, alkyl or aryl)
The product is an amino alcohol (or hydroxy-amine).
This is an example of the reduction of a polar C-N pi
bond via the hydride ion (H-) acting as a nucleophile
generated by the tetrahydridoaluminate(III) ion, which does not reduce
the non-polar pi bond C=C you find in alkenes.
Tin and hydrochloric acid, Sn(s)/HCl(aq), is not a
powerful enough reducing agent to reduce nitriles to amines.
H2(g)/150oC/Ni(s)
catalyst conditions will allow hydrogen gas to reduce nitriles to primary aliphatic amines
in the chemical industry.
RC
N
+ 2H2 ==> RCH2NH2 (R = H,
alkyl or aryl)
TOP OF PAGE
and sub-index
6.12.5
Use of nitriles in
synthesis:
Hydrolysis to prepare carboxylic acids
The hydrolysis of nitriles to give carboxylic acids
The
hydrolysis of the resulting nitriles e.g. propane nitrile
If the nitrile is refluxed with dilute
hydrochloric/sulfuric acid or sodium hydroxide (strong base - alkali) the
corresponding carboxylic acid or its sodium salt is formed.
The hydrolysis with pure water is to
slow, but the reaction is speeded up by a strong acid or strong alkali.
Strictly speaking all the reactants and products
should be suffixed by (aq)
(i) Equations for the dilute mineral acid hydrolysis of a nitrile to give the
free (weaker) acid
In this case converting
propanenitrile to propanoic acid or its salt, sodium propanoate
+ 2H2O + H+
+ NH4+
Here the free acid and an ammonium
ion are formed.
(more detailed structured formula hydrolysis equation)
+ 2H2O + H+
+ NH4+
(less detailed structured formula hydrolysis equation)
+ 2H2O + H+
+ NH4+
(skeletal formula hydrolysis equation)
(ii) Equations for the alkaline hydrolysis of a nitrile to give the sodium salt
(if aqueous sodium hydroxide is used), in the equations you write out the
product as the carboxylate anion.
In this case converting
propanenitrile to its salt, e.g. sodium propanoate
+ H2O + OH-
+ NH3
Here the carboxylate anion
(propanoate ion) and free ammonia are formed.
(structured formula hydrolysis equation)
+ H2O + OH-
+ NH3
(abbreviated structured formula hydrolysis equation)
+ H2O + OH-
+ NH3
(skeletal formula hydrolysis equation)
The
hydrolysis of 2-methylpropanenitrile
2-methylpropanenitrile
=== hydrolysis ===> free 2-methylpropanoic acid or its salt
or

(CH3)2CHC≡N
+ 2H2O + H+
(CH3)2CHCOOH + NH4+
(acid hydrolysis, free acid)
(CH3)2CHC≡N
+ H2O + OH-
(CH3)2CHCOO- + NH3
(alkaline hydrolysis, salt of acid)
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