Download Lecture 27 – Chemistry of the Alkaline Metals (Group 1A).

2P32 – Inorganic Chemistry
Lecture 27 – Chemistry of the Alkaline Metals
(Group 1A).
1. Alkaline metals
2. Crown ethers and Valinomycin – host guest chemistry
3. The alkaline metals - melting points and lattice
energies.
4. The alkaline metals – general properties
5. Fireworks
1. Alkaline Metals
„
Ion
Cell (mM)
Blood(mM)
K+
Na+
ClX-
139
12
4
138
4
145
116
9
Communication between cells is mediated by nerve impulses: signals electrically
transmitted as traveling waves of ionic currents – made possible by
concentration gradients of ions.
„
How do biochemical processes recognize the difference between Na+ and K+
when their chemistry is much alike?
„
Na+ - 116 pm
K+
- 152 pm
The sizes are different and this does not affect their chemistry, but think of
them traveling through a channel for a specific ion.
1
The Nobel Prize was awarded to Bill Mckennan in 2003 for determining what the
bacterial K+ channel looks like.
From the cytosolic side, the pore opens up into a vestibule; the vestibule
facilitates transport by allowing the K+ ions to remain hydrated even though they are
Only 2 of the identical subunits are shown
in the middle of the membrane.
Bacterial K+ Channel
„
Structure of the selectivity filter explains the ion selectivity.
„
For K+ to enter the filter, it must lose most of its bound water molecules
(hydration sphere).
„
It then interacts with the carbonyl oxygen's lining the selectivity pore.
„
These are rigidly spaced at the exact distance to accommodate a K+ ion.
„
Na+ unable to enter the filter.
„
Carbonyl oxygen's are too far away from the smaller Na+ ion to compensate for
the energy expense associated with the loss of water molecules required for
entry.
2
K+ loses its bound water and interacts with the carbonyl oxygen atoms
Mutual repulsion of the ions as they move through the channel in
single file helps them move into the extracellular fluid
Ions are hydrated
in the vestibule
3
2. Crown Ethers, Cryptands and Valinomycin
„
Charles Pederson –1960‘s at DuPont (during his last 9 years)
„
flexible
„
bind alkali metal ions
O
O
O
O
O
-M
N
A
JE
O
O
A
H
L
IE
R
C
N
P
S
E
L
R
A
H
N
E
S
R
D
K+
K+
O
O
O
18-Crown-6-K+ complex
(cavity organised by K+)
18-Crown-6
ƒ
O
O
+
Oxygen atoms are capable of acting as Lewis bases due to the presence of
the lone pairs (e.g. in hydrogen bonding between water molecules or hydration of a
metal ion in aqueous solution, see below).
ƒ
„
„
The interaction with metal ions with ethers is also important:
In regular ethers, only weak complexes are formed.
However, in certain polyethers (where multiple interactions are possible) the
complexes are much stronger.
ion
„
„
„
18-Crown-6
Complex
Hydration of a metal
These ethers are called "crown ethers" due to their shape.
They are based on repeating -OCH2CH2- units, derived from ethylene glycol
HOCH2CH2OH
The name 18-crown-6 indicates that there are 18 atoms in the ring, 6 of which
are oxygen.
4
Dionaea muscipula (Venus Flytrap)
An example of a highly preorganized host
„
These compounds are important co-solvents.
„
The interior of the cavity is water like, whereas the exterior is hydrocarbon
like.
„
So a metal ion inside the cavity can be "carried" into an organic solvent.
„
This allows ionic systems such as KF to be dissolved in organic solvents and used
as reagents where the metal ion is in a complex, but the anion is unsolvated or
naked and therefore quite reactive.
„
Varying the size of the crown ether varies the cavity size and some metal ions
fit better than others.
„
For example, 18-crown-6 is an good fit for K+
12-Crown-4 is selective for Li+ ions
5
Host-Guest Chemistry
„
The characteristic chemistry of crown ethers involves complexation of the
ether oxygens with various ionic species.
„
This is termed "host-guest" chemistry, with the ether as host and the ionic
species as guest.
„
Crown ethers may be used as phase-transfer catalysts and as agents to
promote solubility of inorganic salts in organic solutions.
„
For example, "purple benzene" is a solution of benzene, 18-crown-6, and
potassium permanganate that finds utility as an oxidizing agent.
„
The crown ether dissolves in benzene, the potassium ion complexes with the
crown ether, and the permanganate is forced to dissolve in the benzene in
order to ion-pair with the potassium ion.
„
This type of chemistry (host-guest) is found in nature with cyclodextrins and
macrocyclic polyether antibiotics.
Specific Applications
1.
Ion recognition approach to volume reduction of alkaline tank waste by
separation and recycle of sodium hydroxide and sodium nitrate (U. S.
Department of Energy, Environmental Science Management Program)
2.
Novel caged crown ligands for waste remediation: Toxic heavy metals in
the Rio Grande river valley (maquilladora region).
6
Valinomycin – a Natural Macrocycle
Valinomycin (VM) is a dodecadepsipeptide, made up of twelve alternating amino
acids and hydroxy acids to form a macrocyclic molecule.
It is a member of the group of natural neutral ionophores because it doesn't have
a residual charge.
O
O
N
O
H
O
O
H
O
VM is highly selective towards potassium
ions over sodium ions. This selectivity is
important in biological systems because VM
transports potassium ions across cell
membranes (this is the antibiotic action).
N
H
N
O
O
O
O
O
O
H
H
O
N
O
O
N
O
H
O
N
O
Valinomycin bound to a Potassium Ion
7
Mechanism of Ionophore Action
This black circle depicts the circular valinomycin molecule. Note that K+ ions bind
to valinomycin and are shuttled back and forth across the membrane. In the
absence of a potassium specific ionophore like valinomycin, K+ only rarely crosses a
lipid bilayer. In the presence of valinomycin, K+ is freely permeable.
3. Melting points and lattice energy of the alkaline metals (Group 1A).
NaF
„
highest lattice energy
9960C
CsF
7030C
CsI
6210C
CsClO3
2500C
Remember the lattice energy is the energy required to change 1 mol of solid to 1
mole of gaseous atoms.
„
Two factors affect the magnitude of the lattice energy:
„
Ionic Charge, the more charged the ions, the greater is the lattice energy.
„
„
„
Interionic distance – the smaller this distance, the greater is the lattice energy.
For a given cation or anion, we can combine these factors into what is known as a
charge-to-radius ratio or the charge density, As Z+/r or Z-/r increases, so does
the lattice energy.
Compare NaF to CsF can see as the ionic radius of the cation increases, the
lattice energy decreases.
Compare, CsF, CsI, CsClO3, as the anion radius increases the lattice energy
decreases.
8
4. The Alkali Metals – General Properties
All are extremely reactive with water – remember the U-tube video!
Li
Na
K
Rb – Rubidium – brilliant red flame test it is used in fireworks to give the purple
color.
Cs – Cesium – latin word for sky – flame test is sky blue color
Fr – Francium- this is a highly radioactive alkali metal that is found in very small
amounts in uranium and thorium ores.
223Fr
is the longest lived isotope and it
has a half life of 22 minutes.
5. Fireworks
„
Fireworks generate three very noticeable forms of energy:
„
a tremendous release of sound, bright light, and heat.
„
The tremendous booms heard at ground level are the result of the rapid release
of energy into the air, causing the air to expand faster than the speed of sound.
This produces a shock wave, a sonic boom.
„
The colors are produced by heating metal salts, such as calcium chloride or
sodium nitrate, that emit characteristic colors. The atoms of each element
absorb energy and release it as light of specific colors. The energy absorbed by
an atom rearranges its electrons from their lowest-energy state, called the
ground state, up to a higher-energy state, called an excited state.
9
„
The excess energy of the excited state is emitted as light, as the electrons
descend to lower-energy states, and ultimately, the ground state.
„
The amount of energy emitted is characteristic of the element, and the amount
of energy determines the color of the light emitted.
„
For example, when sodium nitrate is heated, the electrons of the sodium atoms
absorb heat energy and become excited.
„
This high-energy excited state does not last for long, and the excited electrons
of the sodium atom quickly release their energy, about 200 kJ/mol, which is the
energy of yellow light.
„
The amount of energy released, which varies from element to element, is
characterized by a particular wavelength of light. Higher energies correspond to
shorter wavelength light, whose characteristic colors are located in the
violet/blue region of the visible spectrum. Lower energies correspond to longer
wavelength light, at the orange/red end of the spectrum.
„
The colors you see exploding in the sky are produced by the elements with the
characteristic emissions listed in the table.
10
Colour
Compound
Wavelength
(nm)
strontium salts, lithium salts
lithium carbonate, Li2CO3 = red
strontium carbonate, SrCO3 = bright red
652
calcium salts
calcium chloride, CaCl2
668
yellow
sodium salts
sodium chloride, NaCl
610-621
green
barium compounds + chlorine producer
barium chloride, BaCl2
589
blue
copper compounds + chlorine producer
copper(I) chloride, CuCl
505-535
purple
mixture of strontium (red) and
copper (blue) compounds or Rubidium
420-460
silver
burning aluminum, titanium, or magnesium
red
orange
11