Part 3. The chemistry of HALOGENOALKANES

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

email doc brown - comments - query?

3.5 The reaction between halogenoalkanes and potassium cyanide - another nucleophilic substitution reaction

Halogenoalkanes were once known as 'haloalkanes' or 'alkyl halides', but the correct IUPAC nomenclature is based on calling halogenated alkanes halogenoalkanes.

However, it seems ok to refer to chloroalkanes, bromoalkanes and iodoalkanes.

I've written the equations for the reactions showing the formation of nitriles from the halogenoalkane in multiple styles and added the nucleophilic substitution mechanisms where appropriate.

Sub-index for this page

3.5.1 The reaction between potassium cyanide and halogenoalkanes

3.5.2 The hydrolysis of nitriles to give carboxylic acids

3.5.3 The S_N1 nucleophilic substitution mechanism of halogenoalkanes and cyanide ion

3.5.4 The S_N2 nucleophilic substitution mechanism of halogenoalkanes and cyanide ion

See also Nucleophilic substitution by cyanide ion to give a nitrile [S_N1 or S_N2] (written before the above pages)

I've often added the boiling point (bpt) so can see what is a liquid and could be hydrolysed in a school/college laboratory.

Strictly speaking all the reactants and products should be suffixed by (aq) apart fro water (l).

3.5.1 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^–    (CH₃)₂CHC≡N  +  Br^-

TOP OF PAGE and sub-index

3.5.2 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 (shown on right) or its salt

(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)

TOP OF PAGE and sub-index

3.5.3 The S_N1 nucleophilic substitution mechanism of halogenoalkanes and cyanide ion

Remember: A neutral or negative nucleophile, Nuc: or Nuc:^-, is an electron pair donor that can attack an electron deficient partially/wholly positive carbon atom to form a new C-Nuc bond.

Mechanism diagram 36 shows the general pathway for the S_N1 unimolecular nucleophilic substitution reaction of halogenoalkanes where a cyanide ion (the nucleophile, electron pair donor) displaces a halide ion via a carbocation.

chlorine and iodine

The S_N1 unimolecular mechanism is favoured by tertiary halogenoalkanes.

S_N1 unimolecular, a two step ionic mechanism via carbocation formation [mechanism 1 above]

In step (1) the C^δ+-X^δ- polar bond of the halogenoalkane splits heterolytically to form a carbocation and a free halide ion (X^-, e.g. chloride or bromide) and this is a reversible reaction.

The C-Br bond is most likely to break because it is a weaker bond than the C-C or C-H bond AND breaks heterolytically because of the difference in electronegativity between carbon (2.5) and bromine (2.8) giving a polar C^δ+-Br^δ- bond.

So the C-Br bonding pair of electrons leaves with the Br atom as the Br^- ion.

In step (2) the cyanide ion is a negative electron pair donor and rapidly combines with the carbocation, forming the C-O bond in the alcohol product. The cyanide ion is the nucleophile.

 Step (1) is the rate determining step with the much larger activation energy (see reaction profile diagram 45 below)

This mechanism is most likely with tertiary halogenoalkanes.

i.e. the mechanism is the same for tertiary chloroalkanes or iodoalkanes.

Primary halogenoalkanes tend to react by the S_N2 mechanism NOT involving a carbocation.

 

Mechanism diagram 72b shows the S_N1 unimolecular nucleophilic substitution reaction of 2-bromo-2-methylpropane where a cyanide ion displaces a bromide ion via a carbocation.

2-bromo-2-methylpropane  + cyanide ion  ===>  2,2-dimethylpropanenitrile  +  bromide ion

(CH₃)₃C-Br  +  :C≡N^-  ===>  (CH₃)₃C-C≡N  +  :Br^-

The unimolecular S_N1 mechanism is favoured by tertiary halogenoalkanes like 2-bromo-2-methylpropane.

Secondary halogenoalkanes react with cyanide ions by both S_N1 and SN₂ mechanisms.

Secondary halogenoalkanes react with cyanide ions by both S_N1 and SN₂ mechanisms.

See also Nucleophilic substitution by cyanide ion to give a nitrile [S_N1 or S_N2] (written before the above pages)

TOP OF PAGE and sub-index

3.5.4 The S_N2 nucleophilic substitution mechanism of halogenoalkanes and cyanide ion

Remember: A neutral or negative nucleophile, Nuc: or Nuc:^-, is an electron pair donor that can attack an electron deficient partially/wholly positive carbon atom to form a new C-Nuc bond.

S_N2 'bimolecular', a one step bimolecular collision mechanism [mechanisms 34]

The C^δ+-Br^δ- bond is polar because of the difference in electronegativity between carbon (2.5) and bromine (3.0), so the electron rich nucleophile, the cyanide ion, attacks the slightly positive carbon.

The nucleophile acts as an electron pair donor (Lewis base) to bond with the C^δ+ carbon to make the C-O bond in the newly formed C-CN nitrile group.

Simultaneously the bromine atom is ejected, taking with it the C-Br bond pair, so forming the bromide ion on expulsion.

This mechanism is most likely with primary halogenoalkanes.

i.e. the mechanism is the same for primary chloroalkanes or iodoalkanes.

Tertiary halogenoalkanes tend to react by the S_N1 mechanism involving a carbocation, secondary halogenoalkanes react via both mechanisms.

 

Mechanism diagram 72a shows the bimolecular S_N2 nucleophilic substitution of bromoethane by the cyanide ion.

Note the 'transition state' or 'activated complex' carries a negative sign because of the negative charge of the cyanide ion.

The S_N2 bimolecular mechanism is favoured by primary halogenoalkanes such as bromoethane.

Secondary halogenoalkanes react with cyanide ions by both S_N1 and SN₂ mechanisms.

See also Nucleophilic substitution by cyanide ion to give a nitrile [S_N1 or S_N2] (written before this page)