Chapter 17
Group 17 Elements
Physical Properties
The elements
Hydrogen Halides
Interhalogen compounds and polyhalogen ions
Oxoacids and salts
1
Bromine resources
and commercial
demand
2NaClO3 + 2NaCl + 2H2SO4  2ClO2 + Cl2 + 2Na2SO4 + 2H2O
5NaClO2 + 4HCl  4ClO2 + 5NaCl + 2H2O
2
Fluorine has several differences in properties
from later halogens:
• Only exists in -1 formal oxidation state (unlike
HClO4, HClO3 etc.)
• Small size
• Low dissociation energy for F2, bond breaks
easily.
• High oxidation power (+2.87 V)
• High electronegativity (highest element)
The A-F bond is stronger than A-Cl bond
• Small size and good overlap of atomic orbitals
strengthens bonds
Fluorocarbons are volatile, have weak London
forces.
• Nonpolarizable F
Small size also leads to large lattice energies in
solids
• Born-Lande equation. (melting point: NaF 993
°C; NaCl 801 °C)
3
[I(py)2]
Although anions are the most commonly observed ion form a group 17
elements, cations can be stabilized by complexation or solvation.
Fluorine-19 NMR
19F
is (spin I =1/2, 100%) is valuable in structure elucidation and investigating
reaction mechanisms.
Neutron Activation Analysis
Naturally occurring 19F is converted to 20F by neutron bombardment, the
radioactive decay is monitored, allowing the original amount of 19F in the sample
to be determined.
Fluoride ion-selective electrode
An electrode that is sensitive to the concentration of a specific ion is called an
ion sensitive electrode. (a pH meter electrode is sensitive to H+ ions.)
4
Difluorine
Difluorine is a pale yellow gas, but is difficult to store since it is highly reactive
and extremely corrosive.
Glass can be used* if the gas is freed of HF
NaF + HF  NaHF2
SiO2 + 2F2  SiF4 + O2
*Reaction is slow unless SiO2 is powdered
SiO2 + 4HF  SiF4 + 2H2O
2H2O + 2F2  4HF + O2
Typical F2 source is gas cylinders, but F2 may be
produced by decomposition:
K2MnF6 + 2SbF5 ---420 K
2KSbF6 + MnF2 + F2
Diiodine, dibromine, and dichlorine
Cl2 may be purchased for laboratory use, but also
may be prepared in the lab scale by:
MnO2 + 4HCl  MnCl2 + Cl2 + 2H2O
Layered solid
Solid State Structures
van der Waals radius, rv
Cl
180 pm
Br
195 pm
I
215 pm
5
Charge transfer complexes
A charge transfer complex is one in which a donor and acceptor interact
weakly together with some transfer of electronic charge, usually facilitated by
the acceptor.
•HOMO-LUMO gap decreases in
the order F2 > Cl2 > Br2 > I2
•Shift in the absorption maximum
from the near UV to the red region
of the visible spectrum.
•When combined with donor atoms
(e.g. ethers, ketones, pyridine), Br2
and I2 form charge transfer
complexes with the halogen s* MO
acting as the acceptor orbital.
•Solutions of I2 in donor solvents
are typically brown or yellow.
•Electronic spectrum contains an
intense absorption in the UV (230330 nm) region arising a charge
transfer band.
Example charge transfer complexes
charge transfer complexes involving Br22MeCN•Br
chain structure of C6H6•Br2
1,2,4,5-(EtS)4C6H2•(Br2)2
Ph3P•Br2
6
. Proposed bonding scheme for [(AgI2)n]n illustrating the ability of I2 to act
as both a charge donor and a charge acceptor.
Physical Properties of hydrogen halides
7
IBr
-ICl
ClF
8
Cation-anion interaction
[I2F12]2- dimer
[BrF4] [Sb2F11]- dimer
9
Bonding in [XY2]- ions
Review Chapter 5 – MO theory
Polyhalogen Cations
Homonuclear cations are known:
[Br2]+, [I2]+, [Cl3]+, [Br3]+, [I3]+, [Br5]+, [I5]+, [I4]2+
Br2 + SbF5 –--BrF5 [Br2]+[Sb3F16](unbalanced)
2I2 + S2O6F2 ---HSO3F 2[I2]+[SO3F]Using MO theory, predict the change in X-X bond length on going from X2 to [X2]+
[Br2]+[Sb3F16]-
Br-Br stretching is 368 cm-1
Br2
Br-Br stretching is 320 cm-1
2+
I
[I2]+ dimerizes at 193 K to give [I4]2+
2I2 + 3AsF5 –liq SO2 [I4][AsF6]2 + AsF3
I
I
I
3Br2 + 2[O2]+[AsF6]-  2[Br3]+[AsF6]- + 2O2
3I2 + 3AsF5 –liq SO2 2[I3]+[AsF6]- + AsF3
10
Polyhalide anions
Homonuclear polyhalide anions are known:
[I3]-, [I4]2-, [I5]-, [I7]-, [I8]2-, [I9]-, [I10]4-, [I12]2-, [I16]2-, [I16]4-,
[I22]4- , [I26]3-, [I29]3[I5]-
[I7]-
[I8]-
Oxides of chlorine, bromine, and iodine
Iodine is the only halogen to form an oxide which is thermodynamically
stable with respect to decomposition into its elements.
I2 + 5/2 O2  I2O5
DfHo(298 K) = -151.1 kJ mol-1
11
O3, 195 K
Br2
Br2O3
Brown
O3, 195 K
Orange
Br2O5
Colorless
Charge separated species
for coordination numbers
greater than 4 and still
obey the octet rule.
H2O + 2BrOTeF5
CCl3F, 195 K
2Cl2 + 3HgO
Cl2O + Hg3O2Cl2
2Cl2 + 2Na2CO3 + H2O
Cl2O + H2O
2KClO3 + 2H2C2O4
Br2O + 2HOTeF5
2NaHCO3+2NaCl+Cl2O
2HOCl
K2C2O4 + 2ClO2
+ 2CO2 + 2H2O
12
ClO2F + HClO4  Cl2O6 + HF
May decompose by:
Cl2O6 + H2O - HClO4 + HClO3
Cl2O7 – anhydride of perchloric acid
Oxofluorides of chlorine, bromine, and iodine
13
trans-[F2BrO3]-
[F5IO2]2-
H5IO6
HIO3
HIO4
HClO4
ClO4-
14
Aqueous solution chemistry
Potential diagrams
15