The HalogensChapter 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 pinkish-brown solutions are formed.

colors indicate the formation of weak complexes I2 ---S known as charge-transfer 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 con-

taining 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 Compounds

On evaporation of water, XeO3 is obtained as a white deliquescent solid that is dangerously explosive.