Structures of interhalogen compounds

INTERHALOGEN COMPOUNDS OR INTERHALOGENS

Each halogen combines with another halogen to form several compounds known as interhalogen compounds. The less electronegative element is written first. In naming also, the less electronegative element is mentioned first.

They are divided into four types.

AX  : CIF , BrF ,BrCl, ICl, IBr.

AX3: ClF3 BrF3 ICl3

AX5  : BrF5  IF5

AX7 : IF7

They can all be prepared by direct combination or by the action of a halogen on a lower interhalogen, the product formed depends on the conditions.

473K

Cl2+ F2 (equal volume) ----- --- >      2ClF (AX type)

I2 + Cl2 liquid (equi molar) ® 2ICl (AX type)

573K

Cl2 + 3F2 (excess) ---- --- >        2 ClF3 (AX3 type)

Br2 + 3F2 (diluted with nitrogen)® 2Br F3 Br2 + 5F2 (excess) ® 2Br F5 (AX5  Type)

I2 + 5F2 (Excess) ® 2IF5 (AX5 Type)

IF5 + F2 (Excess) ------- 573K  ----- --- > IF7 (AX7 Type)

The bonds are essentially covalent because of the small electronegativity difference, and the melting and boiling points increase as the difference in electronegativity increases.

The interhalogens are generally more reactive than the halogens (except F) because the A-X bond is weaker than the X-X bond in the halogens. The reactions are similar to those of the halogens. Hydrolysis gives halide and oxyhalide ions, the oxyhalide ion being formed from the larger halogen present.

BrF5 + 3 OH-  --- --- >          5F¯ + BrO3¯ + 3 H+

Bromate

ICl + OH-   --- - ----------- ---         >         Cl ¯ + OI ¯ + H+

hypoiodite

Structures of interhalogen compounds

Interhalogen compounds are generally covalent compounds in which the larger halogen forms the central atom.

1. Type AX. As excepted, the compounds of the type AX are linear. Thus CIF, BrF, BrCl, IBr and ICI are all linear in structure.

Electronic structure of Chlorine atom, in the ground state and hybridised state is represented as in Fig.

Although the spatial arrangement of the four electron pairs (bp = 1 and lps = 3) round the central chlorine atom is tetrahedral, due to the presence of three lone pairs of electrons in three hybrid orbitals, the shape of AX molecule gets distorted and become linear.

2. Type AX3 Compounds of the type AX3 have trigonal bipyramidal structure, Fig.  for the ClF3 molecule.

Bipyramidal structure arises out of sp3d hybridisation involved in the formation of this compound, as illustrated in the Fig.. The three dotted arrows indicate electrons contributed by the three fluorine atoms (without lone pair it is T-shaped).

3. Type AX5 (IF5, BrF5, etc.) These compounds are formed by sp3d2  hybridisation and hence have an octahedral structure, as shown in Fig. for  the formation of IF5 molecule (without lone pair it is square pyramidal).

4. Type AX7 (IF7). This compound has a pentagonalbipyramidal structure since this is formed by sp3d3hybridisation.

Problem

An element A occupies group number 17 and period number 2, shows anomalous behaviour. A reacts with water forms a mixture of B, C and acid D. B and C are allotropes. A also reacts with hydrogen violently even in dark togive an acid D. Identify A,B,C and D. write the reactions.

Solution

i) The element A that occupies group number 17 and period number 2 is fluorine.

ii) Fluorine reacts with water and forms a mixture of B and C

2F2 + 2H2O ® 4HF + O2

3F2 + 3H2O ® 6HF + O3

Therefore, B is Oxygen and C is Ozone.

iii)    Fluorine reacts with hydrogen to give D.

F2 + H2 ® 2HF

D is Hydrofluoric acid.