Advanced Level Organic Chemistry Nitrogen compounds: Acid-base chemistry of amines

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Organic Chemistry Part 8. Chemistry of organo-nitrogen compounds

Part 8.4 The acid-base chemistry of aliphatic amines, their comparative strength as bases and reactions with acids

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Part 8.4 The acid-base chemistry of aliphatic amines, their comparative strength as bases and reactions with acids

Sub-index for this page

8.4.1 The ionisation of aliphatic amine bases in water

8.4.2  The strength of aliphatic bases - database of pKb and Kb values

8.4.3 The reaction of aliphatic bases with acids - neutralisation and salt formation

8.4.4 Conjugate acids - why are salts solutions of amine bases acidic?

See also 7.10 The chemical properties of phenylamine (aromatic amine) including its acid-base chemistry

More on Definition of a weak base, theory and examples of Kb, pKb, Kw weak base CALCULATIONS

8.4.1 The ionisation of aliphatic amine bases in water

Reminders:

The term weak base refers to one that only ionises (dissociates) to a small extent in aqueous solution.

A base is defined as a proton acceptor (See Lewis and Bronsted–Lowry acid–base theories)

 e.g. ammonia is a weak inorganic base in water:

NH3(aq)  +  H2O(l)  == ~2% ==>  NH4+(aq)  +  OH-(aq)

A strong base like sodium hydroxide ionises completely:  NaOH(s)  +  aq  == 100%  ==> Na+(aq)  +  OH-(aq)

The equilibrium involved when a weak base B dissolves in water:

(i) :B(aq)  +  H2O(l)    BH+(aq)  +  OH-(aq)  

(for any soluble base, note the lone pair on N: which makes this molecule a base,  and BH+ is the conjugate acid - proton donor)

The B:→H bond is a dative covalent bond (or co-ordinate bond) - the proton from the acid accepts the pair of electrons donated from the base e.g. the non-bonding lone pair of electrons on the nitrogen atom of ammonia or aliphatic/aryl organic bases.

(ii) :NH3(aq)  +  H2O(l)    NH4+(aq)  +  OH-(aq)  

(for ammonia giving an alkaline solution, the NH4+ ion is the conjugate acids of the base NH3)

ionisation of ammonia in water Kb pKb values diagram balanced equation of ionization ammonium ion hydroxide ion

(iii) CH3CH2NH2(aq)  +  H2O(l)    CH3CH2NH3+(aq)  +  OH-(aq)  

(for ethylamine, primary aliphatic amine base, the cation formed is called the ethylammonium ion)

ionisation of prmary aliphatic amine ethylamine in water Kb pKb values diagram balanced equation of ionization ethylammonium ion hydroxide ion

From equation (iii) for a weak organic base, the ionisation constant, the equilibrium constant (Kb) for equilibrium (iii), is given by the expression (water is a subsumed constant):

Kb =

  [CH3CH2NH3+(aq)] [OH–(aq)]

 ––––––––––––––––––––––––––  mol dm-3

           [CH3CH2NH2(aq)]

Like pH, the range of Kb is so great, it is often quoted on the logarithmic scale where

 pKb = -log10(Kb/mol dm-3)   and  Kb = 10-pKb

The stronger the base, the greater the Kb and the lower the pKb value.

More on Definition of a weak base, theory and examples of Kb, pKb, Kw weak base CALCULATIONS

See also 7.10 The chemical properties of phenylamine including its acid-base chemistry

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8.4.2 The strength of aliphatic bases - database of pKb and Kb values

Aliphatic amines are weak bases, but generally stronger bases than aromatic bases like phenylamine

Name Structure pKb Kb/moldm-3 Solubility in water Comments

Ammonia and a primary aromatic amine are added for comparison purposes
ammonia NH3 4.75 1.78 x 10-5 very high Inorganic base
phenylamine structural formula phenylamine molecular structure C6H5NH2 9.38 4.16 x 10-10 relatively low, 3.6g/100g water Aromatic organic amine base, much weaker than simple aliphatic amine bases listed below.
methylamine (c) doc b CH3NH2 3.38 4.17 x 10-4 ~miscible Primary aliphatic amine base ...
ethylamine (c) doc b CH3CH2NH2 3.27 5.37 x 10-4 ~miscible Primary aliphatic amine base
propylamine CH3CH2CH2NH2 3.36 4.37 x 10-4 ~miscible Primary aliphatic amine base
butylamine CH3CH2CH2CH2NH2 3.39 4.07 x 10-4 ~miscible Primary aliphatic amine base
Three for comparison          
propylamine CH3CH2CH2NH2 3.36 4.37 x 10-4 ~miscible A series of three structurally isomeric molecules (based on C3H9N) in the sequence: primary ==> secondary ==> tertiary aliphatic amines
N-ethylmethylamine CH3CH2NHCH3 3.24 5.75 x 10-4 very high
trimethylamine (CH3)3N 4.20 6.31 x 10-5 very high
           

Comments on the data table

(a) The simpler aliphatic amines are stronger bases than the simplest aromatic amines.

pKb(aliphatic)  <  pKb(aromatic)  and  Kb(aliphatic)  >  Kb(aromatic)

In the case of aromatic amines, the benzene ring has an minus inductive effect (-I electron shift) making the non-bonding electron pair on the nitrogen is less energetically favourable for protonation.

This weakens their basic character, increasing so if there are very electronegative substituents in the benzene ring.

However, in aliphatic amines, the alkyl groups have a plus inductive effect (+I electron shift) so the non-bonding electron pair on the nitrogen is more energetically favourable for protonation, making them stronger bases.

Remember we are talking about the relative strengths of weak bases, but there is a wide range in terms of Kb and pKb values (that is why the latter is on a logarithmic base 10 scale).

See also 7.10 The chemical properties of phenylamine including its acid-base chemistry

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8.4.3 The reaction of aliphatic bases with acids - neutralisation and salt formation

When ammonia or an organic amine base are neutralised, the base is protonated by the acid to give

protonation of inorganic base ammonia in water diagram balanced equation of formation of conjugate acid ammonium ion cation

ammonia   == protonation ==>  ammonium ion

 

protonation of base primary aliphatic amine ethylamine in water diagram balanced equation of formation of conjugate acid ethylammonium ion cation

ethylamine  == protonation ==> ethylammonium ion

 

Therefore, you can write neutralisation equations with strong mineral acids, just as you do with ammonia e.g.

NH3(aq)  +  HCl(aq)  ===>  NH4+(aq)  +  Cl-(aq)  == evaporation ==> NH4+Cl-(s)

(preparation of ammonium chloride)

2NH3(aq)  +  H2SO4(aq)  ==>  2NH4+(aq)  +  SO42-(aq)  = evap. => (NH4+)2SO42-(s)

(preparation of ammonium sulfate)

CH3CH2NH2(aq)  +  HCl(aq)  ==>  CH3CH2NH3+(aq)  +  Cl-(aq)  = evap. => CH3CH2NH3+Cl-(s)

(preparation of ethylammonium chloride, note the names of the alkylammonium ions in these examples)

2CH3CH2NH2(aq)  +  H2SO4(aq)  ==>  2CH3CH2NH3+(aq)  +  SO42-(aq) 

= evap. => (CH3CH2NH3+)2SO42-(s)

(preparation of ethylammonium sulfate)

Don't be put off by coming across a cycloalkyl amine e.g. cyclohexylamine

Cyclohexylamine is neutralised by nitric acid to yield the salt, cyclohexylammonium nitrate preparation structural formula equation molecular structure 

Cyclohexylamine is neutralised by nitric acid to yield the salt, cyclohexylammonium nitrate

 

Cyclohexylamine is neutralised by hydrochloric acid to yield the salt, cyclohexylammonium chloride preparation structural formula equation molecular structure

Cyclohexylamine is neutralised by hydrochloric acid to yield the salt, cyclohexylammonium chloride

 

Cyclohexylamine is neutralised by sulfuric acid to yield the salt, cyclohexylammonium sulfate preparation structural formula equation molecular structure

Cyclohexylamine is neutralised by sulfuric acid to yield the salt, cyclohexylammonium sulfate

Watch the balancing of the equation 2 : 1 mole ratio of base : acid

Note:

(i) Using evaporation, the salts can crystallised out from aqueous solution, as you can with ammonium salts.

(ii) If in an amine preparation, the initial product is a salt, you add strong alkali like aqueous sodium hydroxide to free the base.

CH3CH2NH3+(aq)  +  OH-(aq)  ===> CH3CH2NH2(aq)  +  H2O(l)

Unfortunately, aliphatic amines are very soluble in water, so the separation is difficult, even using fractional distillation.

In the case of aromatic amines, they can be separated by steam distillation - see preparation of phenylamine.

 

See also 7.10 The chemical properties of phenylamine including its preparation and acid-base chemistry

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8.4.4 Conjugate acids - why are salts solutions of amine bases acidic?

The conjugate acids of weal bases are quite strong, so solutions of organic amine bases are acidic, with pH often <4.

You find the same with ammonium salts e.g. if put a piece of magnesium ribbon in ammonium chloride solution, it fizzes as hydrogen is evolved. Simple proof that the hydrated proton ion (H3O+) was formed.

NH4+(aq)  +  H2O(l)  ===>  NH3(aq)  +  H3O+(aq)

Therefore you get exactly the same effect with the ethylammonium ion

CH3CH2NH3+(aq)  +  H2O(l)  ===>  CH3CH2NH2(aq)  +  H3O+(aq)

If the add sodium hydrogencarbonate to an ethylammonium salt solution, carbon dioxide is evolved.

You can write the equation in terms of the ethylammonium ion acting as the acid e.g.

CH3CH2NH3+(aq)  +  HCO3-(aq)  ===>  CH3CH2NH2(aq)  +  H2O(l)  +  CO2(g)

 

See also 7.10 The chemical properties of phenylamine (aromatic amine) including its acid-base chemistry

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