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8/14/2019 (II) Group 1 Elements
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(ii) Group 1 Elements
Li
Na
K
Rb
Cs
Fr
Allaki Metals(Alkali Arabic word al kalya for soda Na2CO3)
electron configuration:ns1
Conduct electricity and heat, soft, have low m.p. that decreases
down the group
Electropositive, reducing.
Common Oxidation states: 0, +1 (Li+, Na+, K+)
Na-1
is observed
Metals are prepard from electrolysis of molten salts
They form a wide range of salts which exhibit typical ionic
properties high mp and are water soluble
The thermal stability of the carbonates, nitrates and suflates,
peroxides and superoxides increases down the group.
The solubilities of the hydroxides in water increases down the
groupLiOH < NAOH < KOH < RbOH < CsOH
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The reactivity of the metals towards O2 and water
increases down the group
4 Li + O2 2Li2O (oxide)
2Na + O2 Na2O2 (peroxide)
K+ O2 KO2 (superoxide)
React vigorously with water: reactivity increases
down the group
2 M(s) + 2 H2O (l) 2 MOH + H2
Li Na K Rb Cs140
160
180
200
220
240
260
280
Atomicradius/pm
Li Na K Rb Cs360
380
400
420
440
460
480
500
520
Firstionizationen
ergy/kJmol-1
Na metal + water
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(a) Lithium
Preparation of Metallic Lithium : electrolysis of molten LiCl
2 LiCl (l) 2Li(s) + Cl2(g)
The melting point of LiCl (610 oC) is reduced by adding KCl
LiCl is obtained mainly from spodumene LiAlSi2O6
spodumene
LiCl
Lithium metal
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Lithium in compounds :
present in all its compounds in +1 oxidation state
Most important coordination number + 4
(e.g. Li(H2O)4+, Li(NH3)4
+ and six (octahedral in LiH, LiCl)
The lithium cation is the most polarizing of the alkali metal cations significant
covalent character in bonding.
Unique in reacting with N2 to form purple Li3N (lithium nitride) and only Li and Na
can react with carbon to form Li2C2 and Na2C2 (carbides)
Applications of Li compounds:
Li2CO3, Li citrate are drugs for manic depression and bipolar disorder
Organolithium compounds are commonly used in organic synthesis, e.g.
methyllithium
Lithium citrate
O
+ MeLi
O-Li+
CH3
+ H+OH
CH3
+ Li+
2 Li + MeBr LiMe + LiBr
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CH3Li methyllithium
Structure of CH3Li : a distorted cube
LiH3C
Li
CH3
CH3Li
C
H3
Li
231 pm
268 pm
68o
4 Li+ ions occupying the corners of a tetrahedron whose 4 faces
are capped by CH3-groups
As two lone-pair electrons are shared by three Li+ ions, the bonding
between CH3- and Li+ is two-electron-four-center bond
Li-Li distance (263 pm) is very close to the Li-Li distance (267.3
pm) in gaseous Li2 molecule : strong Li-Li bond
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The bonding between Li+ and CH3- ions is highly covalentThe covalent character is due to the high polarizing power
associated with high charge density.
Mg2+ also form highly covalent compounds.
Similarity between Li+ and Mg2+ - diagonal relationshipdue to
similar ionic radii (Li+ = 76 pm, Mg2+ = 72 pm)
e.g. Li and Mg form normal oxide, whereas other group 1 metals
form peroxide and superoxide
Li is the only group 1 metal that form Li3N. All gp 2 metals can form
nitrides.
They form similar organometallic compounds: MeLi, Me2Mg
Carbonates of Li and Mg decompose upon heating to metal oxides
whereas carbonates of other gp 1 metals are thermally stable
Li
Na
K
Rb
Cs
Fr
Be
Mg
Ca
Rb
Cs
Fr
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Lithium Nitride as a Potential H2 storage Material
BMW Hydrogen 7 Luxury Automobile
6 Li (s) + N2 (g) 2 Li3N (s)
synthesis
Li3N (s) + 2 H2 (g) LiNH2
(s) + 2 LiH (s)
elevated temp,
high pressure
1700 oC
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(b) Crown Ethers
Charles J. Pedersen
Du Point
1909 1989Nobel prize in chemistry 1987
Cyclic Polyethers
O O
O
O
O
15-crown-5
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O
O O
O
O
OO
O
What are the names of these crown ethers?
O O
O
O
O
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O O
O
O
O Na
OH2+
O
O
O
OLi
OH2+
O
O
O
OO
O
K
+
Some Complexes
O
OO
OO
O
OO
O
O
K+
[K(dibenzo-30-crown-10]+
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Selective Binding of Alkali Ions
The stability of the complex of crown ethers and alkali ions
depends critically on the fit between the hole in the crownether and the cation
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e.g. [K(dibenzo-
30-crown-10]+
Different Structures: relative sizes of ions and
the interior space of the donor
encircled Partially embedded
Encircled with the crown
ether shaped like seam on
a tenis ball
outside
e.g. [K(18-crown-6]+ e.g. [Na(H2O)18-crown-6]+
e.g. [Rb(SCN)(18-crown-6)]+
Sandwiched
e.g. [K(benzo-15-crown-5)2]+
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Ionic Nature of the Interactions
12-Crown-4 Electric Potential
Surface: The electric potential surfaceis a measure of charge
distribution. Red indicates regions of
negative charge, green corresponds to
neutral areas, and blue indicates
regions of positive charge.
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K+ Ion Transporters in Nature: Ionophore
It is a circular molecule, made up of 3 repeats
of the sequence shown above
Very selective for binding K+: bind K+ more strongly
than Na+ by a factor of 104 at room temp
Valinomycin : isolated from cultures ofStreptomyces fulvissimus
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O
O
O
O
O
O
K+
Puckering of the ring, stabilized by H-bonds, allows
valinomycin to closely surround a single unhydrated K+ ion.
Six oxygen atoms of the ionophore interact with the bound
K+, replacing O atoms
of waters of hydration.
Whereas the interior of the valinomycin-K+ complex is polar, the
surface of the complex is hydrophobic. This allows valinomycin to
enter the lipid core of the bilayer, to solubilize K+ within this
hydrophobic environment
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Valinomycin is a passive carrier for K+. It can bind or
release K+ when it encounters the membrane surface.
Valinomycin can catalyze net K
+
transport because itcan translocate either in the complexed or uncomplexed
state. The direction of net flux depends on the
electrochemical K+ gradient
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An Interesting Application of Crown Ether
Isolation of Allkalides
When the alkali metals (except Li) are dissolved in aliphatic
amines or polyethers, they disproportionate to a small
extent into solvated cation M+ and solvated anion (alkalide)with a helium shell. H2C CH2
NH2H2N
MeO
OO
Me
2 M M+(solv) + M-(solv)
Solvent
Addition of crown ether can drastically increase the
solubility of the alkali cations in the solvent and thusincreases the concentration of the alkalide M-.
e.g. [Na(18-crown-6)]+Na-
