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

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1 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 a Cl X Communication between cells is mediated by nerve impulses: signals electrically transmitted as traveling waves of ionic currents made possible by concentration gradients of ions. ow do biochemical processes recognize the difference between a + and K + when their chemistry is much alike? a pm K 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

2 The obel 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 nly 2 of the identical subunits are shown in the middle of the membrane. Bacterial K + Structure of the selectivity Channel 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. a + unable to enter the filter. Carbonyl oxygen's are too far away from the smaller a + ion to compensate for the energy expense associated with the loss of water molecules required for entry. 2

3 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

(during his last 9 years) flexible bind alkali metal ions + K + K+ 18-Crown-6

in hydrogen bonding between water molecules or hydration of a metal ion in

The interaction with metal ions with ethers is also important: In regular

owever, in certain polyethers (where multiple interactions are possible) the

4 JEA-M ARIELE C ARLESP EDERSE 2. Crown Ethers, Cryptands and Valinomycin Charles Pederson 1960 s at DuPont (during his last 9 years) flexible bind alkali metal ions + K + K+ 18-Crown-6 18-Crown-6-K + complex (cavity organised by K + ) xygen 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). The interaction with metal ions with ethers is also important: In regular ethers, only weak complexes are formed. owever, in certain polyethers (where multiple interactions are possible) the complexes are much stronger. ion 18-Crown-6 Complex ydration of a metal These ethers are called "crown ethers" due to their shape. They are based on repeating -C 2 C 2 - units, derived from ethylene glycol C 2 C 2 The name 18-crown-6 indicates that there are 18 atoms in the ring, 6 of which are oxygen. 4

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

5 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

6 ost-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. ovel caged crown ligands for waste remediation: Toxic heavy metals in the Rio Grande river valley (maquilladora region). 6

Valinomycin a atural Macrocycle Valinomycin (VM) is a

and hydroxy acids to form a macrocyclic molecule.

because it doesn't have a residual charge.

This selectivity is important in biological systems because VM

7 Valinomycin a atural 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. 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). Valinomycin bound to a Potassium Ion 7

8 Mechanism of Ionophore Action This black circle depicts the circular valinomycin molecule. ote 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). af highest lattice energy C CsF C CsI C CsCl C 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 af to CsF can see as the ionic radius of the cation increases, the lattice energy decreases. Compare, CsF, CsI, CsCl 3, as the anion radius increases the lattice energy decreases. 8

9 4. The Alkali Metals General Properties All are extremely reactive with water remember the U-tube video! Li a 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. 223 Fr 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

10 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. igher 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

11 Colour red orange yellow green blue purple silver sodium salts sodium chloride, acl Compound strontium salts, lithium salts lithium carbonate, Li 2 C 3 = red strontium carbonate, SrC 3 = bright red calcium salts calcium chloride, CaCl 2 barium compounds + chlorine producer barium chloride, BaCl 2 copper compounds + chlorine producer copper(i) chloride, CuCl mixture of strontium (red) and copper (blue) compounds or Rubidium burning aluminum, titanium, or magnesium Wavelength (nm)

Name 2/14 Bonding Page 1

Name 2/14 Bonding Page 1 Bonding Page 1 Name 2/14 1. Which formula represents an ionic compound? A) NaCl B) N2O C) HCl D) H2O 2. Which type of bond results when one or more valence electrons are transferred from one atom to another?