Cation receptors Flashcards
(35 cards)
Why is H+ a very strong Lewis acid?
Very small size so has a high charge density
H+ in HSAB sense
Hard Lewis acid so has high affinity for hard ligands (Lewis bases) such as F-, OH- and NH3
But virtually no affinity for soft halide ions e.g. Cl-, Br- and I-
What does H+ (i.e. the proton) form in aqueous solution?
A linear hydronium ion [H5O2]+
i.e. H+ in the middle of 2 water molecules
Why is H able to form the hydride anion, H-?
Because H has a high enough electron affinity
Properties of H-
Very strong Lewis base
Can displace OH- from water (showing that H- is a stronger base than OH-)
H-(aq) + H2O —> OH-(aq) + H2(g)
H- can form saline (salt-like) hydrides, and resembles an F- ion in salts e.g. NaF. H- has approx. the same ionic radius as F-
Can form more covalent hydrides with more electronegative metals e.g. [Al2H6]
What is one of the most important properties of the proton?
Its ability to form H-bonds when attached to more electronegative donor atoms e.g. N, O, F
How are H-bonds judged to be present?
When the separation between the 2 atoms forming the H-bond is less than the sum of the vdW radii
i.e. the molecules are closer together than would be expected from vdW forces alone
Typical O-H—O distance
2.76 A
Typical F-H—F distance
2.55 A
Typical N-H—N distance
3.00 A
Group 1
The alkali metals
Very reactive - react violently with water to give the metal hydroxide and H2(g)
e.g. Li(s) + H2O —> Li+(aq) + OH-(aq) + H2(g)
Very negative reduction potentials
Why do group 1 metal ions have a limited ability to form complexes in solution?
Due to their large size and low charge
Metal hydroxides are completely ionised to give the free metal cations and hydroxide ions so are therefore strong bases
What happens to the coordination numbers of the alkali metals going down the group?
Increases due to their increasing size
Li+ CN4-6
Cs+ CN8-9
Why are alkali metals hard in the HSAB sense?
They have low electronegativity
Most important aspect of alkali metal chemistry
Their ability to bind to crown ethers and cryptands
What are crown ethers/cryptands derived from?
Ethylene glycol HOCH2CH2OH
Interior of crown ether cavity
Water-like
Exterior of crown ether
Hydrocarbon-like
18-crown-6
Strong preference for K+ Stability constant (logKa) = 6.08
Important aspect of crown ethers
Can complex alkali metal cations in solution
Provided an explanation for how the selective passage of Na+ and K+ through ion channels in cell membranes may be achieved
Why does 15-crown-5 bind to K+ with a higher stability constant than Na+?
The 15-crown-5 macrocyclic ring is too small for K+ to ‘fit’ inside, so instead 2:1 ‘sandwich’ complex is formed with K+ sandwiched between 2 crown ether rings
Cryptands
3D cavity
Form much more thermodynamically stable complexes with group 1 and 2 metals c.f. crown ethers because it is kinetically difficult for the metal to leave once it is inside the cryptand
Uses of cryptands
Disposable cassettes for measuring blood pH, pCO2, pO2, Na+, K+, Ca2+ etc
OPTI critical care analyses
Group 2
Alkali earth metals
Resemble the alkali metals in most aspects (low electronegativity, hard), but the main difference is their charge (+2), therefore alkali earth metal cations are stronger Lewis acids
