Ch 17. Group 17

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Prepn of elements

F2 colorless gas

Mineral source of F is CaF2 or Na3AlF6 (cryolite)

2 HF H2 + F2 E = -2.87 V

(Moisson, 1886)

Fluorine is very reactive with almost all other elements, but not with materials such as (CF2)n, SiO2 (dry), Cx at RT

Chemical prepn:

K2MnF6 + 2 SbF5 2 KSbF6 + MnF3 + ½ F2

anhyd KF, electrolysis

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Fluorine vacuum line

Ni, SS, or Monel, passivation layer such as NiF2

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Prepn of elementsCl2 pale yellow gas, source = brine

2 Cl Cl2 + 2e E = -1.36V

13M tons in 2004

Since H2O/O2 = 1.23 V (or higher at pH > 0), Cl2 production requires there be a greater overpotential for O2 than for Cl2 (RuO2 electrode works best)

Br2 deep red liquid, source = brine

Cl2 + 2 Br Br2 + 2 Cl E = +0.26V

0.4M tons in 2000

I2 violet solid that sublimes, source = brine, kelp, or NaIO3

Oxidation of iodide with Cl2 21k tons 2003

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Periodic TrendsXp Ea D(X2) D(HX) D(CX4) Hf(LiX) E(X2/X-)**

F 4.0 328* 159 574 456 1037 2.8

Cl 3.0 344 243 428 327 832 1.4

Br 2.8 325 193 363 272 813 1.1

I 2.5 295 151 294 239 750 0.5

* all in kJ/mol

** V vs SHE for

e- + ½ X2(g) X- (aq)

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Periodic Trends

Reactivity of F2 >> Cl2 > Br2 > I2

Reason for reactivity for F2 is the strong bond of F with other elements and weak F-F bond

ex: Hhyd(F) - Hhyd(Cl) = 143 kJ/mol = 1.48 eV

note that difference in E (X2/X-) is similar (1.4 V)

High E to generate F2 means F ligands can often stabilize high ox states

ex: PtF6, PbF4, BiF5, IF7 none of these exist with other halides

High means substitution can lead to incr. acidity

CH3SO3H pKa ~ -2

CF3SO3H pKa ~ -15

inductive effect of F

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Interhalogens:Structures: Larger atom is always central

VSEPR works well

XY XY3 XY5 XY7

ClF ClF3 ClF5

BrF BrF3 BrF5

IF (IF3)n IF5 IF7

BrCl

ICl (ICl3)2

IBr

pentag. bipyr (D5h)

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Interhalogens:Prepn, reactions:

I2 + Cl2 2 ICl

Br2 + 3 F2 2 BrF3

All interhalogens are good oxidizing agents, unstable in air

ClF3 and BrF3 are very good fluorinating agents

BrF3 + asbestos burns brightly liberating O2(g) + Br

LA LB

2 BrF3 BrF2+ + BrF4

KF is a LB, add to incr [BrF4]

SnF4 is a LA, add to incr [BrF2+

]+

(BrF2)2SnF6 SnF4 + 2 BrF3

(Sn + 2 F2 is not practical due to passivation layer)

Sn + 2 Br2 SnBr4 (l)

BrF3

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Polyhalides and cations:

XYn or Xn+1

(n even)

n = 2 I3 , Br3

(linear)

ICl2, IBr2, BrCl2, IBrCl

n = 4 I5

ClF4, ICl4, IBrCl3 sq. planar

n = 6 I7

ClF6

XYn+ or Xn+1

+ (n even)

I3+

ClF2 +, BrF2+ (all C2v)

XF4+ X = Cl, Br, I

XF6+

(IF7 + SbF5 IF6+SbF6

)

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Polyhalides

I3- is linear

(12 valence orbitals, 22 e-, so just maximize E of unfilled orbital)

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Polyiodide structures

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Conduction mechanism in polyiodides

Li / I2 batteries: http://www.greatbatch.com

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Iodine complexes

:B

I2

transition E in uv for F2, but HOMO-LUMO transition decreases from F2 to I2

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Halogen OxidesF: OF2 MP ~ -224 C, BP ~ -145 C easily hydrolyzed to HF + O2

O2F2 very strong oxidant and fluorinator

Pu (s) + 3 O2F2(g) PuF6 (g) + 3 O2(g)

Cl, Br, I: oxoacids, oxoanions

ox.

state pKa conj. base

+1 HXO (hypochlorous acid) C∞v 7.5 XO (hypochlorite)

+3 HXO2 (chlorous acid) C2v 2.0 XO2 (chlorite)

+5 HXO3 (chloric acid) C3v -1.2 XO3 (chlorate)

+7 HXO4 (perchloric acid) Td -10 XO4 (perchlorate)

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Halogen Oxides

Trend in acidity similar for Cl, Br, I

Pauling’s rules pKa = 8 - 5p for XOp(OH)q

Note exception:

IO4 (periodate) has pKa = 3.3

it is actually HIO4 + 2H2O H5IO6 (p = 1)

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Frost diagrams

all ox states above -1 are strong oxidants

disproportionation favorable for many species

oxyhalides are stronger oxidants in acidic solutions

In acid: ClO4 + 2 H+ + 2e- ClO3

+ H2O E = +1.2 V

In base: ClO4 + H2O + 2e- ClO3

+ 2 OH E = +0.37 V

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OxyhalidesReaction rates:

1 XO4 XO3

XO2 XO X2

ex ClO4 is kinetically stable in aqueous soln (but can explosively

decompose in organic soln or anhydrides)

(reaction mechanism often involves nucleophilic attack on “X” which is shielded in XO4

but ClO is a labile oxidant (bleach)

2. Rates with central atom Cl Br I

ex ClO undergoes slow disproportionation

but IO has only been detected as a reaction intermediate

3. Rates increase in acidic solutions

X = O X - OH (weaker interaction)H+

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Oxyhalides

Representative reactions:

Cl2 (aq) + 2 OH- (aq) ClO (aq) + Cl- (aq) + H2O (l) fast

ClO (aq) 2 Cl (aq) + ClO3 (aq) slower

4 ClO3 (aq) 3 ClO4

(aq) + Cl (aq) very slow

Fluorocarbons

CHCl3 + 2 HF → CHClF2 + 2 HCl X exchange

2 CHClF2 → C2F4 + 2 HCl HX elimination

n C2F4 → (C2F4)n polymerization

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