email doc brown - comments - query? * [privacy, cookies and disclaimer]

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.

TOP OF PAGE 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 59^oC) and potassium cyanide

2-bromopropane  +  cyanide ion  ===> 2-methylpropanenitrile  +  bromide ion

CH₃CHBrCH₃  + CN^-  ===>  CH₃CH(CN)CH₃  +  Br^-  

CH₃CH(CH₃)CN = CH₃CH(CN)CH₃

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:

RCN + 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.

H_2(g)/150^oC/Ni_(s) catalyst conditions will allow hydrogen gas to reduce nitriles to primary aliphatic amines in the chemical industry.

RCN + 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)