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Chapter 17

Group 17 Elements

Physical Properties

The elements

Hydrogen Halides

Interhalogen compounds and polyhalogen ions

Oxoacids and salts

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Bromine resources and commercial demand

2NaClO3 + 2NaCl + 2H2SO4 2ClO2 + Cl2 + 2Na2SO4 + 2H2O

5NaClO2 + 4HCl 4ClO2 + 5NaCl + 2H2O

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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)

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[I(py)2]

Although anions are the most commonly observed ion form a group 17 elements, cations can be stabilized by complexation or solvation.

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.

Fluorine-19 NMR

19F is (spin I =1/2, 100%) is valuable in structure elucidation and investigating reaction mechanisms.

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.)

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Typical F2 source is gas cylinders, but F2 may be produced by decomposition:

K2MnF6 + 2SbF5 ---420 K 2KSbF6 + MnF2 + F2

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 HFNaF + HF NaHF2

SiO2 + 2F2 SiF4 + O2 *Reaction is slow unless SiO2 is powderedSiO2 + 4HF SiF4 + 2H2O2H2O + 2F2 4HF + O2

Difluorine

Solid State Structures

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

van der Waals radius, rvCl 180 pmBr 195 pmI 215 pm

Layered solid

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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), Br2and 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 (230-330 nm) region arising a charge transfer band.

Example charge transfer complexes

charge transfer complexes involving Br2-

chain structure of C6H6•Br2

1,2,4,5-(EtS)4C6H2•(Br2)2

2MeCN•Br

Ph3P•Br2

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. 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

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-ICl

IBr

ClF

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[I2F12]2- dimer[Sb2F11]- dimer

Cation-anion interaction

[BrF4]

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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

[I2]+ dimerizes at 193 K to give [I4]2+

2I2 + 3AsF5 –liq SO2 [I4][AsF6]2 + AsF3

3Br2 + 2[O2]+[AsF6]- 2[Br3]+[AsF6]- + 2O2

3I2 + 3AsF5 –liq SO2 2[I3]+[AsF6]- + AsF3

I

I

I

I

2+

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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

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Br2 Br2O3 Br2O5O3, 195 K O3, 195 K

Brown Orange Colorless

Charge separated species for coordination numbers greater than 4 and still obey the octet rule.

CCl3F, 195 KH2O + 2BrOTeF5 Br2O + 2HOTeF5

2Cl2 + 3HgO Cl2O + Hg3O2Cl2

2Cl2 + 2Na2CO3 + H2O 2NaHCO3+2NaCl+Cl2O

Cl2O + H2O 2HOCl

2KClO3 + 2H2C2O4 K2C2O4 + 2ClO2 + 2CO2 + 2H2O

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ClO2F + HClO4 Cl2O6 + HF

May decompose by:Cl2O6 + H2O - HClO4 + HClO3

Cl2O7 – anhydride of perchloric acid

Oxofluorides of chlorine, bromine, and iodine

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[F5IO2]2-trans-[F2BrO3]-

HIO3

HClO4 ClO4-

H5IO6

HIO4

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Aqueous solution chemistry

Potential diagrams