Some Basic Properties of the Halogens – Group 17

The Halogens exhibit some very interesting properties in the periodic table. Elements typically become more metallic or basic on descending a main group. Thus in Groups 14,15 and 16 the first elements C, N and O are non-metals, but the heavier members Sn, Pb, Bi and Po are metals. Metallic properties decrease across a period. Little is known about astatine as a member of the halogens, though it would be expected to show more tendency to metallic cations than the other members of the group. Thus the trend to metallic properties is less obvious in Group 17. The increasing stability of positive ions among the halogens indicates an increasing tendency to basic or metallic character. It must be emphasized that iodine is not a metal.

Fluorine is the most electronegative element and has no basic properties (that is, it has no tendency to form positive ions).

Iodine dissolves in oleum and other strongly oxidizing solvents, forming bright blue solutions which are paramagnetic. For a long time these solutions were thought to contain 1⁺, but they are now known to contain the [1₂]⁺ cation. (The group 16 elements Sulphur, Selenium and Tellurium behave in a similar way. Thus Sulphur dissolves, forming blue coloured paramagnetic solutions containing various [S_”]²⁺ cations such as [S₄]²⁺, [S₈]²⁺ and [S₁₉]²⁺). [Br₂]⁺ is also formed in oleum and is bright red coloured and paramagnetic. [CI₂]⁺ has been observed spectroscopically in discharge tubes. Several crystalline compounds containing [1₂]⁺ or [Br₂]⁺ have been prepared.

Lets take a brief look at some of the reactions of the halogens

The bond length in the [Br₂]⁺ cation is 2.15Å compared with 2.27Å in Br₂. The bond in the cation is shorter and stronger than in the eternal and shows that the electron removed came from an antibonding orbital. In a similar way the bond in [1₂] is stronger than in 1₂. Many other compounds containing cations such as [Cl₃]⁺, [Br₃]⁺ [1₃]⁺, [Br₅]⁺ and [1₅]⁺ have been prepared.

CI₂ + CIF₃ + AsF₅                   [CI₃] ⁺ [AsF₆]^– + F₂        

Br₂ + BrF₃ + AsF₅                  [Br₃] ⁺ [AsF₆]^– + F₂        

  I₂ + ICI + AICI₃                      [I₃] ⁺ [AICI₄]^–         

  2I₂ + ICI + AICI₃                   [I₅] ⁺ [AICI₄]^–

The structures of several compounds containing the cationic halogen ions [Br₃]⁺ and [I₃]⁺ have been established by X-ray crystallography or Raman spectroscopy. These ions are always bent, in contrast to the triiodide ion [I₃]^– which is linear. The structures of the other ions are not known with certainty. A compounds I₇SO₃F has been reported (as a maximum on a phase diagram). It is a black solid but it is no known whether it contains [I₇]⁺.

Positive bromine also exists in other complexes such as Br (pyridine)₂NO₃ and BrF₃. These ionize and give.

Br (pyridine)₂NO₃                   [Br(pyridine)₂]⁺ + NO₃^–

                   2BrF₃                                [BrF₂]⁺ +[BrF₄]^–  

Complexes containing positive iodine are more numerous. ICI and IBr both conduct electricity when molten. Electrolysis of ICI liberates I₂ and CI₂ at both electrodes.

This is consistent with the following ionization:

3ICI                    [I₂CI]⁺ + [ICI₂]^– 

Molten ICN behaves in a similar way to ICI.

3ICN                 [I₂CN]⁺ + [ICN₂]^– in melt

However, electrolysis of ICI dissolved in pyridine gives I₂ only at the cathode. This suggests the more simple type of ionization:

2ICI            [I(pyridine)₂]⁺ + [ICI₂] ^–  

The addition of AICI₃ to a melt of ICI greatly increases its conductivity and is explained by the formation of complex ions:

AICI₃ + 2ICI         [I₂CI]⁺ + [AICI₄] ^–

ICI behaves as an electrophilic iodinating agent. It coverts acetanilide to 4 iodoacetanilide, and salicylic acid to 3.5-diiodosalicylic acid. Because the attacked sites have an electron excess, the iodine must be positive. It a solution of iodine in an inert solvent is passed down a cationic ion exchange column, some iodine is retained in the risen.

H ⁺ Resin^– + 1₂         1⁺ Resin^– + HI

The positive ion retained may be eluted with KI,’ to estimate the amount of I⁺, or it may be allowed to react with various reagents.

       I ⁺ Resin^– + KI        I₂ + K ⁺ Resin^–

       I ⁺ Resin^– + anhydrous H₂SO₄        I₂SO₄ + H ⁺ Resin^–

           I ⁺ Resin^– + alcoholic HNO₃        INO₃ + H ⁺ Resin^–

I⁺ reacts with OH^– in aqueous solutions.

                   I⁺ + OH^–       HOI

             2HOI + OI^–        IO₃^–  + 2I^– + 2H⁺

For this reason I⁺ will only exist in water if it is stabilized by coordination to some other molecule. A large number of compounds are known which contain I⁺ stabilized in a complex ion, many pyridine complexes are known such as [I(pyridine)₂]NO_3, [I(pyridine)₂]CIO₄, [I(pyridine)] acetate and [I(pyridine)]benzoate.

Molten ICI₃ has a high conductivity (8.4 x 10^-3ohm^-1cm^-1). When ICI₃ is electrolysed both I₂ and CI₂ are liberated at both electrodes. This suggests ionization:

2ICI₃       [ICI₂]⁺ + [ICI₄]^–

Treatment of I₂ with fuming HNO₃ and acetic anhydride gives the ionic compound I(acetate)₃. If a saturated solution of I(acetate)₃ in acetic anhydride is electrolysed using silver electrodes, one equivalent of AgI is formed at the cathode for every three Faradays of electricity passed. This would seem to indicate ionization given I³⁺:

I(acetate)₃     I³⁺ + 3(acetate^–)

In the halogens, there is no structural evidence for the presence of I³⁺. Other ionic compounds which may contain I³⁺ are iodine phosphate IPO₄ and iodine fluosulphonate I(SO₃F)₃.