Halogens.pdf

Group 7, the Halogens

Group 17—The Halogen Group

• All the elements in Group 17 are nonmetals

except for astatine, which is a radioactive

metalloid.

2

• These elements are

called

halogens

, which

means “salt-former.”

• All of the halogens

form salts with

Group 17—The Halogen Group

• The halogen fluorine is the most reactive

of the halogens in combining with other

elements.

• Chlorine is less reactive than fluorine,

and bromine is less reactive than chlorine.

Some facts…

1) Reactivity DECREASES as you go down the group

Decre

as

ing

reactivity

(This is because the electrons are further away from the

nucleus and so any extra electrons aren’t attracted as much).

2) They exist as

diatomic molecules (so that they both have a full outer shell):

Cl Cl

The halogens – some reactions

Na

+

Cl

-Na + Cl

2) Halogen + non-metal:

H + Cl Cl _H

Halogen + metal ionic salt

Interhalogen

The halogens react with each other to

form

interhalogen

compounds.

The general formula of most interhalogen

compounds is XYn, where n = 1, 3, 5 or 7,

and X is the less electronegative of the

two halogens.

Diatomic interhalogens

The interhalogens of form XY have physical properties intermediate between those of the two parent halogens.

The covalent bond between the two atoms has some ionic character, the less electronegative element, X, being oxidised and having a partial positive charge. Most combinations of F, Cl, Br and I are known, but not all are stable.

Chlorine monofluoride (ClF)

The lightest interhalogen compound, ClF is a colorless gas

Bromine monofluoride (BrF)

BrF has not been obtained pure. It dissociates into the trifluoride and free bromine .

Iodine monofluoride (IF)

IF is unstable and decomposes at 0 C, disproportionating into elemental iodine and iodine pentafluoride.

Bromine monochloride (BrCl)

Iodine monochloride (ICl)

Red transparent crystals which melt at 27.2 C to form a choking brownish liquid (similar in appearance and weight to bromine). It reacts with HCl to form the strong acid HICl_2. The crystal structure of iodine monochloride consists

of puckered zig-zag chains, with strong interactions between the chains.

Iodine monobromide (IBr)

Made by direct combination of the elements to form a dark red crystalline solid. It melts at 42 C and boils at 116 C

Tetra-atomic interhalogens

Chlorine trifluoride (ClF₃) is a colourless gas which condenses to a green liquid, and freezes to a white solid.

It is made by reacting chlorine with an excess of fluorine at 250 C in a nickel tube.

It reacts more violently than fluorine, often explosively. The

molecule is planar and T-shaped. It is used in the manufacture of uranium hexafluoride.

Bromine trifluoride (BrF₃) is a yellow green liquid which conducts electricity. It ionises to form [BrF₂+_{] + [BrF}

4-]. It reacts with many metals and metal oxides to form similar ionised

entities; with some others it forms the metal fluoride plus free bromine and oxygen. It is used in organic chemistry as a

Iodine trifluoride (IF₃) is a yellow solid which decomposes above -28 0_{C. It can be synthesised from the elements, but} care must be taken to avoid the formation of IF₅. F₂ attacks I₂ to yield IF₃ at -45 0_{C in CCl}

3F.

Alternatively, at low temperatures, the fluorination reaction I₂ + 3XeF₂ 2IF₃ + 3Xe can be used. Not much is known about iodine trifluoride as it is so unstable.

Iodine trichloride (ICl₃) forms lemon yellow crystals which can be melted under pressure to a brown liquid. It

can be made from the elements at low temperature, or from iodine pentoxide and hydrogen chloride. It reacts with

many metal chlorides to form tetrachloriodides, and hydrolyses in water. The molecule is a planar dimer, with

Hexa-atomic interhalogens

Chlorine pentafluoride (ClF₅) is a colourless gas, made by reacting chlorine trifluoride with fluorine at high

temperatures and high pressures. It reacts violently with water and most metals and nonmetals.

ClF₃ + F₂ --- ClF₅

Bromine pentafluoride (BrF₅) is a colourless fuming liquid, made by reacting bromine trifluoride with fluorine

at 200 C. It is physically stable, but reacts violently with water and most metals and nonmetals.

Iodine pentafluoride (IF₅) is a colourless liquid, made by reacting iodine pentoxide with fluorine, or iodine with

silver fluoride. It is highly reactive, even slowly with glass. It reacts with elements, oxides and carbon halides.

Octa-atomic interhalogens

Iodine heptafluoride (IF₇) is a colourless gas. It is made by reacting the pentafluoride with fluorine.

IF₇ is chemically inert, having no lone pair of electrons in the valency shell; in this it resembles sulfur hexafluoride.

The molecule is a pentagonal bipyramid.

This compound is the only interhalogen compound possible where the larger atom is carrying seven of the smaller

atoms.

Molecular shapes of the main types of interhalogen compounds. ClF linear, XY BrF₅ Square

pyramidal, XY₅ BrF₅

Square

pyramidal, XY₅

IF₇

Pentagonal

bipyramidal, XY₇

IF₇

Pentagonal

bipyramidal, XY₇

ClF₃

T-shaped, XY₃ ClF₃

Chlorine oxides.

dichlorine monoxide Cl₂O

chlorine dioxide ClO₂

lone e

Relative Strength of Halogen Oxoacids

The relative strength of halogen oxoacids depends on both the electronegativity and the oxidation state of the halogen.

For oxoacids with the halogen in the same oxidation state, acid strength decreases as the halogen EN decreases.

HOClO₂ > HOBrO₂ > HOIO₂

For oxoacids of a given halogen, acid strength decreases

as the oxidation state of the halogen decreases.

Pseudohalogens

The pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds.

Pseudohalogens occur in inorganic molecules of the general forms Ps–Ps or Ps–X (where Ps is a pseudohalogen group), such as cyanogen;

as pseudohalide anions, such as cyanide ion; as inorganic acids, such as hydrogen cyanide;

Examples of pseudohalogen

molecules

Examples of symmetrical pseudohalogens include cyanogen (CN)₂, thiocyanogen (SCN)₂, azidodithiocarbonate (N₃CS₂)₂.

Another complex symmetrical pseudohalogen is dicobalt

octacarbonyl, Co₂(CO)₈. This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, Co(CO)_4.

Examples of non-symmetrical pseudohalogens (Ps–X) are cyanogen halides (ICN, ClCN, BrCN), and other compounds.

Cyanogen

Cyanogen is the chemical compound with the

formula (CN)

.

It is a colorless, toxic gas with a pungent odor.

Cyanogen molecules consist of two CN groups –

analogous to diatomic halogen molecules, such as

Cl

, but far less oxidizing.

The two cyano groups are bonded together at their

carbon atoms: N≡C−C≡N.

Certain derivatives of cyanogen are also called

"cyanogen" even though they contain only one CN

group. For example, cyanogen bromide has the

Preparation

Cyanogen is typically generated from cyanide compounds. One laboratory method entails thermal decomposition

of mercuric cyanide:

2 Hg(CN)₂ → (CN)₂ + Hg₂(CN)₂

Alternatively, one can combine solutions of copper(II) salts (such as copper(II) sulfate) with cyanides, an unstable

copper(II) cyanide is formed which rapidly decomposes into copper(I) cyanide and cyanogen.

2 CuSO₄ + 4 KCN → (CN)₂ + 2 CuCN + 2 K₂SO₄

Dicobalt octacarbonyl

Dicobalt octacarbonyl is the inorganic compound

Co

(CO)

. This metal carbonyl is a reagent and

catalyst in organometallic chemistry and organic

synthesis.

It is used as a catalyst for hydroformylation, the

conversion of alkenes into aldehydes.

It is highly reactive towards alkynes, and is

Structure and Synthesis

It is synthesised by the high pressure carbonylation of cobalt(II) salts, often in the presence of cyanide.

The major isomer contains two bridging CO ligands linking the Co atoms and six terminal CO ligands, three on each Co atom. It can be described by the formula (CO)₃Co(μ-CO)₂Co(CO)₃.

This structure resembles Fe₂(CO)₉, but with one fewer bridging CO. The Co---Co distance is 2.52 Å, and the Co---Co–CO_terminal and Co–CO_bridge distances are 1.80 and 1.90 Å, respectively.

Reactions

The most characteristic reaction of Co₂(CO)₈ is its

hydrogenation/reduction to tetracarbonylhydrocobalt, [HCo(CO)₄]: Co₂(CO)₈ + H₂ → 2 HCo(CO)₄

This hydride is the active agent for hydroformylation reactions. It adds to alkenes to give an alkyl-Co(CO)₄ product that then proceeds to insert CO and undergo hydrogenolysis to produce the aldehyde. Reduction of Co₂(CO)₈gives the conjugate base of HCo(CO)₄

Co₂(CO)₈ + 2 Na → 2 NaCo(CO)₄

The CO ligands can be replaced with tertiary phosphine ligands to give Co₂(CO)_8-x(PR₃)_x. These bulky derivatives are more selective catalysts for hydroformylation reactions.

6 C₅H₅N + 1.5 Co₂(CO)₈ → [Co(C₅H₅N)₆][Co(CO)₄]₂ + 4 CO

The Pauson–Khand reaction, in which an alkyne, an alkene, and carbon

Pauson–Khand reaction

,

Co₂(CO)₈ reacts with alkynes to form a stable covalent complex, which is useful as a protective group for the alkyne.