Consider a Reaction in Which Both Lewis Acids Are Soft

8/14/2019 Consider a Reaction in Which Both Lewis Acids Are Soft

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Softness

Group 17 I > Br > Cl > F

Group 16 Te Se > S >> O

Group 15 As P > N

Relative Softness

Evaluating the relative softness of the metal ions

has proved somewhat harder.

It can be said that the further the metal is from gold

(which all seem to agree is the softest acid), the

harder it is. (Gold is the metal with the highest

electronegativity)


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Application of HSAB Principle

Solubility of Halides and Chalcogenides

Ag+ + Cl-

Consider the following reaction:

AgCl

At first glance, the reaction cannot be treated by the HSAB principle

[Ag(H2O)n]+ + [Cl(H2O)m]

- AgCl + H2O:H2O

SA HB Borderline

base/BB

HA

SA/BB HA/HB


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Predictions:

Chlorides, bromides, iodides of the soft acids areinsoluble

e.g. CuCl (Not CuCl2

), AgCl, AuCl (but not AuCl3

), TlCl,

OSCl2, IrCl2, PdCl2, PtCl2

Pseudohalides: cyanide (using its C donor atom) andthiocyanide (using its S donor atom) are soft bases, and

azide is borderline base: cyanides, thiocyanides andazides of soft acids are insoluble.

Sulfides, selenides and tellurides of soft and borderlineacids are insoluble (S2-, Se2-, Te2- are stronger bases than

the halides. The combination of softness and strength

enables them to combine with borderline acids)


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-

Hard base Soft base

ShanzerNature1995


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6 Chemistry of Selected Main

Group Elements(i) Hydrogen

3 isotopes : protium (1H), deuterium (2H/D) and tritium(3H/T)

Natural abundance: 1H: 99.9855%, 2H: 0.0145%, 3H:10-5%

Although H has 1 electron in 1s orbital, its chemistry

is very different from that of alkali metals

Three oxidation states: H+ (proton), H0,& H-(hydride)


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(a) Dihydrogen or molecular hydrogen H2Dihydrogen H2 (b.p. 20.28K) is a colorless, odorless gas virtually insoluble

in water.It is produced industrially by the steam reforming of hydrocarbons, notablymethane, or form coke, followed by water shift gas reaction.

CH4 + H2O = CO + 3 H2 (a)

Or

C + H2O = CO + H2 (b)

CO + H2O = CO2 + H2 (water-gas shift reaction)

Reaction (a) is highly endothermic H= 206.2 kJmol-1 and theprocess is therefore run at a high temp (900 110 oC) in the

presence of a catalyst (Ni or Ni/Al2O3)

Most of H2 produced is consumed in large-scaled processes suchas production of methanol, using Cu/ZnO catalyst.

CO + 2 H2 = CH3OH

Synthesis gas/syngas


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H2 is not an exceptionally reactive element at low temp because

the bond dissociation of the molecule is very endothermic H2 = 2HH= 434.1 kJmol-1

Major reactivity: reduction

2 H2 + O2 2 H2O

Hindenburg disaster 1937

MO2 + H2 M2O3 + H2O

MO + H2 M + H2O


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H forms at least 1 compound with the empirical formula EHn with every

element E except noble gases

The hydrogen atom(s) in the compounds can have either positive ornegative partial charge, depending on the relative electronegativity of E

Except Be, all groups 1 and 2 metals form ionic metal hydrides

LiH

NaH

KH

RbH

CsH

MgH2

CaH2

SrH2

BaH2

Covlent character

of M-H bond


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Covalent Hydrogen Compounds

H E H

H

E

H

H

H

E

H

HH

H

EHH

EHH

Gp 2 or 12 linear

Gp 13 trigonal planar

Gp 14 tetrahedral

Gp 15 pyramidal Gp 16 angular


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Boiling Points of H Compounds of Gps 14 17 Elements


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Hydrogen Bonding

a hydrogen bond is formed between an H atom

attached to an electronegative atom, and anelectronegative atom that possesses a lone pair of

electrons

X H Y

+- -

Hydrogen bond


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565F-H29F-HFH

464O-H22HO-HOH2

386N-H17H2N-HNH3

363S-H7HS-HSH2

H-bond enthalpy and the correspondingcovalent bonds (kJmol-1)


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Solid State and Gas Phase Structures of HF

HF

HF

H

F

HF

H

F

HF

F

H

FH

F

HF

H

F

H

HF

hexameric

polymer 157 pm


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Typical H bonds are due largely to electrostatic attraction of

H and Y

In the case of very strong H bonds, the X---Y distancebecomes quite short and the X-H and X-Y distances come

close to being equal. In these cases there are presumably

covalent and electrostatic compounds in X-H and H-Y bonds.

e.g. [F-H-F]-

F H F113 pm

Bond enthalpy = 163 kJmol-1

c.f. H-F : bond enthalpy = 570 kJmol-1

c.f. F-F : bond enthalpy = 158 kJmol

-1


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(b) Hydrides

Covalent metal hydrides

Covalent non-metal hydrides

ionic metal hydrides


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(i) Covalent hydrides

Neutral binary XH4 compounds of Group 14 e.g. CH4

Basic binary XH3 compounds of Group 15 e.g. NH3, PH3

Weakly acidic or amphorteric binary XH2 of Group 16 e.g. H2O andH2S

Strongly acidic binary HX compounds of Group 17 e.g. HCl

Numerous covalent hydrides of boron

Hydridic, complex compounds of hydrogen e.g. LiAlH4 and NaBH4

which serve as reducing agents despite the fact that the Al-H andB-H bonds are essentially covalent(compounds with strong hydridic character react vigorously with

Brnsted acid by transfer of an H- ion with the evolution of H2)


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(ii) Ionic Metal Hydrides (H-)

Effective radius for the free H- ion is 2.08 (c.f. He radius of 0.98 , and 0.5 for H)

The tendency of the H atom to form H- is much lower than for theelectronegative halogen elements

H2(g) H(g) H= 218 kJmol-1

Br2(g)

2 Br(g)

H= 113 kJmol-1

H (g) + 1e- H- (g) H= -67 kJmol-1+

H2(g) + 1e- H-(g) H= 151 kJmol-1

Br (g) + 1e- Br- (g) H= -345 kJmol-1+

Br2(g) + 1e- Br-(g) H= -232 kJmol-1

exothermic

endothermic


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Because of the endothermic character fof H- ion, only the most

electropositive elements, the alkali metals and the larger alkalineearth metals, react directly with H2 to form hydrides with high ioniccharacter such as CaH2 and NaH

2 M(l) + H2 MH

M(l) + H2 MH2

The metal hydrides are truly hydridic substances as they can beconsidered to contain metal cations and H- ions. Their ionic

character is shown by their high conductivities at their m.p.

Since the metal hydrides are hydridic, they react vigorously withwater

NaH + H2O NaOH + H2

They are strong hydride transfering reagents:

8 LiH + Al2Cl6 2 LiAlH4 + 6 LiCl

2 NAH + B2H6

2 NaBH4NaH + B(OCH3)3 Na[HB(OCH3)3]

Consider a Reaction in Which Both Lewis Acids Are Soft

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