Preparation Properties & Structure of Interhalogen Compounds
Due to different electronegativities, the halogens combine with each other and give rise to their corresponding binary compounds which are called as ‘Interhalogen compounds’.
These compounds are of four types:
(i) AB type s e.g. CIF, BrF, F, BrCl, ICl and IBr.
(ii) AB3 type : e.g CIF3, BrF3,IF3 and ICl3
(iii) AB5 type : e.g. ClF5, BrF5 and IF5 and
(iv) AB7 type : e.g. IF7
Interhalogen compound of the type ABx, ‘A′ is central halogen atom which is larger than B halogen atom and also XA<XB. The halogen atom having less electronegativity shows positive oxidation state. Due to this reason, interhalogen compounds are named as the halides of less electronegative halogen atom. For example : CIF is named as chlorine mono fluoride. In case of halogen fluorides, it is observed that greater the electronegativity difference between F and A(XF⋅XA), greater is the number of fluorides given by atom A. For exampla : lodine forms four fluorides viz IF, ∣F3,IF5 and IF7 while Bromine forms only three fluorides viz BrF, BrF3 and BrF5 because XF−X1(4.0−2.5)>XF−XBc(4.0−2.8).
General Methods of preparation :
(1) By direct combination of halogens :
All interhalogen compounds except IF7 can be prepared by the direct combination of halogens under suitable condition.
(2) From lower Interhalogens :
Higher interhalogens can be prepared by the interaction of lower interhalogen with suitable halogen. This method is generally used for the preparation of halogen fluorides, e.g.
(3) Miscellaneous Methods :
1- ClF,BrF,ClF3 and IF7 are covalent gases. BrF3,BrF5 and IF5 are liquids. ICl, IBr, IF3 and ICl3 are solids.
2- Since all the valence electrons in interhalogen compounds are present either as bonding or non bonding electron pairs, such compounds are ‘diamagnetic’ in nature.
3. Thermal stability of AB type interhalogen compounds decreases with the decrease in electronegativity difference between A and B atoms. Thus, the order of stability of some AB compound is as follows:
IF(1.5) > BrF(1.2) > ClF(1.0) > ICl(0.5) > Br(0.3) > BrCl(0.2)
- For each type of interhalogen compounds there is an increase in the b.p. With the increase In the electronegativity difference between atom A and B.
- AB type of compounds are more reactive than A2 and B2 molecules, since A−B, bond in A−B compounds is weaker than A−A and B−B bonds in A2 and B2 molecules respectively. Due to this reason AB type compounds converts the metals into a mixture of two halides. e.g.
The order of reactively of interhalogen compounds are as follows :
ClF3 > BrF3 > IF7 > BrF5 > IF5 > BrF
Chemical Properties :
1- Hydrolysis :
Interhalogen compounds on hydrolysis gives halogen acid and ox halogen acids, e.g.
BrCl + H2O ⟶ HCl + HOBr
ICl+H2O⟶ HCl+ HIO
ICl3+2H2O⟶3HCl + HIO2
IF5+ 3H2O ⟶ 5HF + HIO3
IF7+ 6H2O ⟶ 7HF + H5IO6
2-Addition reactions :
The diatomic interhalogens add at olefinic double bond to give addition product. e.g.
3- Reaction with alkali-metal halides :
When interhalogen compounds are treated with alkali metal halides yield poly-halides e.g.
IBr + NaBr ⟶ NaIBr2
ICl + KCl ⟶ KICl2
BrCl + CsI ⟶ CsIBrCl
ICl3 + KCl ⟶ KlCl4
4. Self- ionization and acid-base reactions :
When interhalogens compounds salt ionize to give polyamide anions.
ICl + ICl ⟶ I+ + ICl2 –
Base1 Acid2 Acid1 Base2
Auto ionization of ICl shows the compounds which produce it ions in liquid ICl act as salvo acids while those which produce ICl2− ions act as salvo bases. e.g. covalent halides like AlCl3,SbCl5. SiCl4. TiCl4 etc. give I+ ions in liquid ICl and hence act as salvo acids while the ionic halides like KCl,PbCl,CsCl,NH4Cl etc give ICl2−ions in this solvent and hence act as salvo bases thus,
AlCl3 + ICl ⟶ I+ + [AlCl4]– ⟶ I+ [AlCl4]–
SbCl5 + ICl ⟶ I+ + [SbCl5]– ⟶ I+ [SbCl6]–
KCl + ICl ⟶ K+ + ICl2– ⟶ K+[ICl2]–
Similarity, auto-ionization occurs in IBr, BrF3 and IF5
IF5 + IF6 ⟶ IF4+ + IF6–
Acid1 Base2 Acid1 Acid2
BrF3 + BrF3 ⟶ BrF2+ + Br4–Acid1 Base2 Acid1 Acid2
The substances like SnF4,VF5,AuF3.SbF5 and TaF5 produce BrF2+ ions in liquid BrF3, act as acids and KF, AgF, BaF2 etc. which produce BrF4− ions in liquid BrF3 act as bases. Thus,
SnF5 + 2BrF3(I) ⟶ 2[BrF2]+[SnF6]–
BaF2 + 2BrF3(I) Ba+2[BrF4]2-
5- Halogenation reactions.
Inter halogen compounds are good halogenating agents. For examples:
C6H6 + ICl —> C6H5I + HCl
The oxidizing power of fluoride is in the order of
ClF3 > BrF3 > IF3
This tendency increases for a particular halogen fluorides with increase of oxidation number.
(i) A solution of ICl in glacial acetic acid is used for the determination of iodine number which measures the unsaturation of oils and fats.
(ii) Because of the formation of I+ cations, ICl is also used as an electrophilic iodination agent e.g.
(iii) ClF3 it used as n fluorinating agent and also an oxidizer for propellent.
(IV) BrF3 is also used as an oxidizer for propellent and the for propellent.