Role of Ammonia in chemical tests
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Ammonia solution can be used in a test for halide ions (Cl-, Br- and I-). Along with silver nitrate, ammonia solution can be used to identify the halide ions. This is done by precipitation of silver chloride, silver bromide and silver iodide when an aqueous solution containing the appropriate halide ion is treated with an aqueous solution of silver nitrate. The colours of the three silver salts formed with chloride, bromide and iodide ions, and their different solubilities in aqueous ammonia, can be used as a test for the presence of the halide. AgCl gives a white colour and is soluble in dilute NH3(aq) identifying the Cl- ion. AgBr gives a cream colour and is sparingly soluble in dilute NH3(aq) but soluble in concentrated NH3(aq) identifying the Br- ion. AgI gives a yellow colour and is soluble in concentrated NH3(aq) identifying the I- ion.
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Ammonia is used in Tollens’ Reagent, which is used to distinguish between aldehydes and ketones. Tollens’ Reagent contains the complex ion [Ag(NH3)2]+ and is prepared by adding an excess of aqueous ammonia to silver nitrate solution. When gently warmed, aldehydes reduce this complex ion and produce a silver mirror on the walls of the test tube; ketones do not form a silver mirror.
Reactions of Ammonia:
- Ammonia plus haloalkanes: nucleophilic substitution reaction: primary amines are formed.
When haloakanes are warmed with an excess of ammonia in a sealed container, primary amines are formed. For example, bromoethane forms ethylamine:
CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br
Mechanism:
H
CH3CH2—Br → CH3CH2—N+—H
H3N: H + :Br-
H H
CH3CH2— N+— H → CH3CH2—N+—H
H
H3N: + NH4+
The excess of ammonia minimises the chance of further reaction of the primary amines to form secondary or tertiary amines, or quaternary ammonium salts.
- Ammonia plus acyl chlorides: acylation reaction: amides are formed.
H3C CH3 CH3
H3N: C ═ O → H2N+ — C — O- → (- Cl-) H2N+ — C ═ O → CH3CONH2
+ NH4+
Cl H Cl H
:NH3
Unlike the reaction between ammonia and haloalkanes, further acylation is difficult because
the lone pair on the amide nitrogen atom is withdrawn by the carbonyl group so that the
amide is less likely to act as a nucelophile. Consequently amides are much less basic then
amines.
Ammonia as a weak base:
Weak bases are only partially ionised in aqueous solution. The strength of the base is indicated by the position of the equilibrium established when the base is dissolved in water. If the equilibrium lies to the right, the base is strong: if it lies to the left the base is weak. Ammonia is the most commonly encountered weak base:
NH3(aq) + H20(l) ═ NH4+(aq) + OH-(aq)
The equilibrium here is well over to the left; the disassociation into ammonium ions and hydroxide ions is incomplete. Hence, ammonia is a weak base.
Ammonia as a buffer
Ammonia and ammonium chloride in solution can be used as a basic buffer. Such a solution would be able to maintain its pH above 7 when small amounts of acid or base are added, and also on dilution with water. This is because there is an equilibrium between the components in the solution:
NH4+(aq) = NH3(aq) + H+(aq)
Also
Aqueous ammonia molecules and ammonium ions act as a source for protons and will, within limits, maintain an almost constant pH.
Dilution doesn’t affect the pH as the ratio of concentrations is constant under dilution.
Ammonia and Amines:
Replacement of the hydrogen atoms in ammonia by alkyl or aryl groups gives rise to three types of amine:
RNH2 (Primary), R2NH (secondary), R3N (tertiary)
Ammonia and amines act as Bronsted – Lowry bases (proton acceptors) by virtue of the lone pair of electrons on the nitrogen atom. The basicity is related to the availability of the lone pair (electron density) for protonation:
RNH2 H+ ═ RNH3
The introduction of one alkyl group into ammonia strengthens the base due to the inductive effect on the alkyl group which pushes electrons towards the nitrogen atom.
Ammonia and amines also act as nucloephiles (electron-pair donors) and take part in nucleophilic substitution reactions. The reaction between a haloalkane and ammonia is an alkylation producing an alkylammonium salt. Proton exchange with another ammonia molecule produces the primary amine:
NH3 + RBr → [RNH3]+ Br-
[RNH3]+ Br + NH3 ═ RNH2 + [NH4] ]+ Br
primary
Further substitution is possible because the primary amine can compete effectively with ammonia for the haloalkane to generate a dialkylammonium salt. Further proton exchange with either ammonia or with RNH2 liberates the secondary amine:
RNH2 + RBr → [R2NH2]+ Br-
[R2NH2]+ Br + NH3 ═ R2NH + [NH4] ]+ Br
secondary
A third alkylation can then take place to give a trialkylammonium salt which, in turn, will donate a proton to ammonia or to another amine:
R2NH + RBr → [R3NH]+ Br-
[R3NH]+ Br + NH3 ═ R3N + [NH4] ]+ Br
tertiary
The resulting tertiary amine then reacts with the haloalkane in a fourth alkylation step to form a quaternary ammonium salt:
R3N + RBr → [R4N]+ Br
quaternary ammonium salt
A mixture of products is usually obtained. Clearly, this outcome limits the usefulness of direct alkylation in synthesis, although separation of the various components is possible. A high yield of the quaternary ammonium salt is obtained by using a large excess of haloalkane. On the other hand, a large excess of ammonia reduces the possibility of further substitution and gives a better yield of primary amine.
Mechanism:
H H
H3N: C — Br → H3N+ — C :Br -
H H H
Reactions of aqua ions with aqueous ammonia
An aqueous solution of ammonia is alkaline because of the presence of OH- ions. The addition of ammonia solution to a solution of a transition metal ion results initially in the formation of a precipitate of the metal hydroxide. If ammonia solution is added to a solution of copper(II) sulphate, a blue copper(II) hydroxide precipitate is formed.
The equation for this reaction is:
[Cu(H2O)6]2+ + 2OH- = [Cu(OH)2(H2O)4] + 2H2O
The colours of common hydroxides formed in these reactions are:
[Fe(H2O)6]2+ → [Fe(H2O)4(OH)2]
green solution green precipitate (turns brown in air)
[Co(H2O)6]2+ → [Co(H2O)4(OH)2]
pink solution blue-green ppt (turns pink on standing then turns brown in air)
[Fe(H2O)6]3+ → [Fe(H2O)3(OH)3]
pale violet solution brown precipitate
[Cr(H2O)6]3+ → [Cr(H2O)3(OH)3]
red-violet solution green precipitate
[Al(H2O)6]3+ → [Al(H2O)3(OH)3]
colourless solution white precipitate
Further (substitution) reactions can occur in the presence of excess ammonia (see below), but initially the metal hydroxide precipitate is formed.
Ligand Substitution
Ammonia is a neutral ligand and can replace water molecules in an aqua ion. This can be written as:
[M(H2O)6]2+ + 6NH3 = [M(NH3)6]2+ + 6H2O
The equation above disguises the fact that what is written in as a one step equilibrium can be broken down into six steps, with only one water molecule being replaced in each step:
[M(H2O)6]2+ + NH3 = [M(NH3)(H2O)5]2+ + H2O
[M(NH3)(H2O)5]2+ + NH3 = [M(NH3)2(H2O)4]2+ + H2O
[M(NH3)2(H2O)4]2+ + NH3 = [M(NH3)3(H2O)3]2+ + H2O
[M(NH3)3(H2O)3]2+ + NH3 = [M(NH3)4(H2O)2]2+ + H2O
[M(NH3)4(H2O)2]2+ + NH3 = [M(NH3)5(H2O)]2+ + H2O
[M(NH3)5(H2O)]2+ + NH3 = [M(NH3)6]2+ + H2O
Ammonia is uncharged and has a similar size to water. Consequently, no change of shape is expected to occur during these substitution reactions. Thus, all the complexes in the equilibria above are octahedral.
One of the most famous historical examples of such a substitution sequence occurs in the addition of ammonia to cobalt(II) ions. In this sequence, the first change is the formation of a green-blue precipitate. This precipitate is cobalt(II) hydroxide, formed by the acidity reaction of the cobalt(II) ions. The overall reaction for this process can be written as:
[Co(H2O)6]2+ + 2NH3 = [Co(H2O)4(OH)2] + 2NH4+
pink solution green-blue precipitate
When an excess of concentrated aqueous ammonia is added to the reaction mixture, the green-blue precipitate dissolves and a pale straw-coloured solution results. It is important to keep air away from this solution, which otherwise darkens rapidly to form a dark-brown mixture containing cobalt(III) ammines.
The species in the straw-coloured solution is the hexaamminecobalt(II) ion. The green-blue hydroxide dissolves in ammonia according to the equation:
[Co(H2O)4(OH)2] + 6NH3 = [Co(NH3)6]2+ + 4H2O + 2OH-
green-blue precipitate straw-coloured solution
The overall equation starting from the aqua ion is:
[Co(H2O)6]2+ + 6NH3 = [Co(NH3)6]2+ + 6H2O
pink solution straw-coloured solution (turns brown on standing on air)
Thus with cobalt(II), complete substitution of water molecules by ammonia molecules occurs.
With copper(II) ions, however, only four of the six water molecules on copper are replaced. As with cobalt, when aqueous ammonia is added to a solution of a copper(II) salt, the first change observed is the formation of a blue precipitate of the hydroxide. This then dissolves when an excess of ammonia is added and a deep-blue solution of the tetraamminebisaquacopper(II) ion is formed.
[Cu(H2O)6]2+ + 4NH3 = [Cu(NH3)4(H2O)2]2+ + 4H2O
blue solution deep-blue solution
Further substitution can be achieved when the concentration of ammonia is increased (e.g. by cooling the solution in ice and saturating it with ammonia gas, or by using liquid ammonia rather than aqueous ammonia), but in concentrated aqueous ammonia, the equilibrium position reached is that in which the dark-blue [Cu(NH3)4(H2O)2]2+ ion is formed. This ion has four ammonia molecules in a square-planar arrangement around copper with water molecules occupying the other two octahedral positions above and below the plane. The bonds to water are longer and weaker than the bonds to ammonia.
Substitution involving Cr(III) complexes are very slow. [Cr(NH3)3] takes several minutes to form whereas [Cr(NH3)3]2+ forms immediately.
[Cr(H2O)6]3+ → [Cr(NH3)3]2+
red-violet solution purple solution
Ammine complexes are not formed with [Fe(H2O)6]2+, [Fe(H2O)6]3+ and [Al(H2O)6]3+.
Cisplatin is an important anti-cancer drug containing NH3 molecules and Cl- ions are ligands in the four-co-ordinate square-planar complex of platinum(II). It is used in chemotherapy and has a remarkable success rate in curing certain types of cancer.
Cl NH3
Pt
Cl NH3