Transcript Main group VII

The Halogens
Chapter 20
 F,Cl,Br,I,At
 All
elements through the periodic table
of elements do form halides
 Exception: He,Ne,Ar
 They are easiest to prepare
 Use: precursors in synthesis reactions
 Halides with elements with more than
one valence are best known
 In organic compounds the F have special
properties
 At
is a greek name and means unstable.
 At
has no stable isotope.
 At
behaves like I, but is less
electronegative.
 F: occurs
widely as Fluorspar (CaF2)
 Na3AlF6 Cryolite,
 Ca3(PO4)2Ca(F,Cl)2 Fluorapatite.
 It is more abundant than Chlorinne
 F is obtained by electrolysis of molten
fluorides.
 Most common used electrolyte KF.2-3HF
 Under electrolysis the melting point
increases but the electrolyte is
regenerated by HF
 Fluorine
cells are made out of Steel, Cu,
Ni-Cu alloys.
 They become coated with a layer of
fluoride.
 Cathodes are steel or Cu.
 Anodes ungraphitized carbon.
 F2 is handled in metal apparatus
 But it can be handled in glas, but HF
needs to be removed by passing through
anhydrous NaF,KF forming MHF2
 Fluorine
is the most chemically reactive
of all the elements
 combines directly (often with extreme
vigor), at ordinary or elevated
temperatures, with all the elements other
than 02, He, Ne, and Kr.
 Attacks many compounds transforming
them to fluorides.
 Organic material burn in F2.
 The
great reactivity ofF2 is in part
attributable to the low dissociation
energy of the F-F bond, and because
reactions of atomic fluorine are strongly
exothermic.
 The low F-F bond energy is probably due
to repulsion between nonbonding
electrons
 Chlorine
 occurs
as NaCl, KCI, MgCl2 sea water,
salt lakes deposits originating from the
prehistoric evaporation of salt lakes.
 Cl2 is obtained by electrolysis of brine.
 Old technology:
 Mercury cathode
 New technology:
 Membrane cells
 Chlorine
is a greenish gas
 It is moderately soluble in water
 Bromine
occurs in much smaller amounts,
as bromides, along with chlorides
 Bromine
is a dense, mobile, dark red
liquid at room temperature
 It is moderately soluble in water and
miscible with nonpolar solvents such as
CS2 and CCI4
 Iodine occurs as iodide in brines and as
iodate in Chile saltpeter NaNO3
 Various forms of marine life concentrate
iodine
 Production
of 12 involves either oxidizing
1-or reducing iodates to 1-followed by
oxidation
 An acid solution of Mn02 is commonly
used as the oxidant.
 Iodine is a black solid with a slight
metallic luster
 I2 sublimes at 1atm without melting
 Soluble
in nonpolar solvents such as CS2
and CCI4
 Colour: Purple
 In polar solvents, unsaturated hydro
carbons, and liquid S02' brown or pinkishbrown solutions are formed.
 colors indicate the formation of weak
complexes I2 ---S known as chargetransfer complexes.
 The
bonding energy results from partial
transfer of charge in the sense I2-S+
 I2,Br2,Cl2 and Icl can sometimes be
isolated as crystalline solids at low
temperatures
 Iodine forms a blue complex with starch,
in which the iodine forms linear I5- ions in
channels in the polysaccharide amylose
 At
has been found as a product of the U
and Th decay series.
 About 20 isotopes of At are known
 The longest lived has a half-life of 8.3 h
 At seems to follow the trend of the other
halogens.
 It is volatile, somewhat soluble in water
 There
are many types of halides.
 Binary Haliddes
 Form simple molecules, complex infinite
arrays.
 Metal halides in +1,+2,+3 oxidation state
are ionic.
 Many metals show their highest oxidation
state in fluorides
 Preparation
of Anhydrous Halides
 1. Direct interaction with the elements
• Direct fluorination normally gives fluorides in
the higher oxidation states
• Most metals and nonmetals react very
vigorously with F2
• nonmetals such as P4 the reaction may be
explosive
• Metal and halide react faster in THF. The
Halide is a solvate.
 2. Dehydration
of hydrated halides
• The dissolution of metals, oxides, or carbonates
in aqueous halogen acids followed by
evaporation or crystallization gives hydrated
halides.
• Dehydration of chlorides can be effected by
thionyl chloride
 3. Treatment
of oxides with other halogen
compounds
• Oxides may often be treated with halogen-
containing compounds to replace oxygen with
halogen
 4. Halogen exchange
 Many halides react to
exchange halogen
with
 (a) elemental halogens
 (b) acid halides
 (c) halide salts
 (d) an excess of another halogen containing substance
 Chlorides
can often be converted to either
bromides (by KBr) or especially to iodides
(by KI)
 Halogen
exchange is especially
important for the synthesis of fluorides
from chlorides, using various metal
fluorides such as CoF3or AsF5.
 Molecular
Halides
 Molecular Halides are also called
covalent halides.
 Between 2 metal atoms, most common 2
halogen atoms.
A
fairly general property of molecular
halides is their easy hydrolysis, for
example
 Reaction
of Halogens with H2O and OH The halogens are all soluble in water to
some extent.
 In such solutions there are species other
than solvated halogen molecules
 disproportionation reaction occurs
rapidly.
 Occurrence:
 Minor
constituents of the atmosphere
 He : radioactive minerals, natural gas
 Origin fromt he decay of U, Th that emit
alpha particles.
 The alpha particles are He nuclei athat
acquire electrons from surounding medium.
 He stays trapped in the rocks.
 Rn is radioactive, comes formt he decay
series of U, Th
 Properties
of the Noble Gases:
 Ne,Ar,Kr,Xe
prepared by fractionation of
liquid air.
 Known as inert gases
 Thought not to have any reactivity
 Key to the problem of valency, the
interpretation od the periodic table, and
the concept of the closed electron shell
configuration.
 Point of reference
 He:
 Main
use cryoscopy.
 Ar:
 Used
to provide an inert atmosphere
 Ne:
 Used
in discharge lamps
 Rn:
 Health
hazard in houses, Cancerogenicc
 Chemistry
of Xe
 Fluorides:
 Thermodynamic
studies show that only
these 3 fluorides exist
 Chemistry
of Xe:
 Xenon difluoride (XeF2)
 preparation by interaction of Xe with a
deficiency of F2 at high pressures
 Soluble in water
 Hydrolysis is slow in acid solution, but
rapid in the presence of bases
 Xenon
tetrafluoride (XeF4 ) is the easiest
of the three fluorides to prepare. On
heating a 1:5 mixture of Xe and F2 at
400°C and about 6-atm pressure for a few
hours, XeF4 is formed quantitatively.
 Fluorination agent in organic chemistry
 Xenon
hexafluoride (XeF6 ) is obtained
by the interaction ofXeF4 and F2 under
pressure or directly from Xe and F2 at
temperatures above 250°C and pressures
greater than 50 atm. Xenon hexafluoride
is extremely reactive, attacking even
quartz
 Xenon
hexafluoride is a strong acid
according to the Lux-Flood definition
 It accepts oxide ion from other
compounds and inserts fluoride ion in its
place.
 The
xenon fluorides will react with strong
Lewis acids such as SbFs or 1rFs
 3 types of adducts formed by XeF2 are:
XeF2.MF5,
 2XeF2.MF5
 XeF2.2MF5
 where M = Ru, Ir, Pt, and so on.
 molecular rather than ionic structure, in
most cases adduct formation involves
fluoride ion transfer to give structures that
contain ions, such as XeF+
 Crystal
structure of XeF
 Xenon
hexafluoride can act as a Lewis
acid toward F-and can be converted to
heptafluoro or octafluoro xenates
 Most
stable compounds, decompose
above 400 degC.
 Xenon-Oxygen
 On
Compounds
evaporation of water, XeO3 is
obtained as a white deliquescent solid
that is dangerously explosive.