Primer Flashcards
What are the units for acid dissociation constant?
pKa
What is pKa the unit for?
Acid dissociation constant
What is the acid dissociation constant?
An experimentally determined value indicating how tightly protons (H+) are bound to molecules
What does a low or negative pKa indicate?
Molecules donate their protons readily.
What can molecules that have a low or negative pKa be described as?
Strong Bronstead-Lowry acids
What can molecules that gain protons easily be described as being?
Bronstead-Lowry bases
What is an aqueous solvent?
A water-based liquid
What is a water-based liquid called?
An aqueous solvent
What is chromatography?
A physical separation method where components are distributed between a stationary phase and mobile phase.
What is LogD?
The distribution constant
What does Log D (distribution constant) describe?
The likelihood of molecule partitioning (moving) into non-polar (organic) solvent (as Log P) but also considers ionisable groups (multiple in solution).
What is Log P?
Lipophilicity constant
What does Log P (lipophilicity constant) describe?
The likelihood of molecule partitioning (moving) into non-polar (organic) solvent.
What is a mobile phase? Describe:
A fluid that percolates through/along stationary bed in definite direction.
o Liquid in liquid chromatography
o Gas in gas chromatography
o Supercritical fluid in supercritical-fluid chromatography
Define ‘sample’
Mixture of components to be separated on the chromatographic sorbent as they are carried/eluted by the mobile phase
What is a solute?
A solid dissolved in a liquid.
What is a solid dissolved in a liquid known as?
A solute
What is a stationary phase? Describe:
Phase in a chromatographic system
Can be a solid, gel or liquid
o If liquid, it may be distributed on a solid that may/may not contribute to the separation process
o Liquid may be chemically bonded to the solid (bonded phase) or immobilised onto it (immobilised phase)
Prefix Symbol Decimal Base ten
for the values you need to know
milli m 0.001 10-3
micro µ 0.000 001 10-6
nano n 0.000 000 001 10-9
pico p 0.000 000 000 001 10-12
femto f 0.000 000 000 000 001 10-15
What are the units for the Octanol-water partition coefficient
(Kow or log P)
or Log K
What is the octanol-water partition coefficient?
How does it work?
Measure of the hydrophilic (water-soluble) or lipophilic (fat-soluble) properties of a molecule.
Water and octanol are immiscible which means the solvents won’t mix and the densest solvent will be at the bottom.
By measuring the concentration (in [ ]) of the molecule in both the water and octanol, the Log P value can be calculated.
What does a Log P < 1 indicate?
Molecule is more hydrophilic and soluble in water
What does a Log P = 1 indicate?
Molecule is likely to be equally distributed (at the same concentration) in both the water and octanol.
What are some factors affecting whether a molecule is hydrophilic or hydrophobic?
- Number of double bonds
- Type and number of functional groups (alcohols (-OH), amines (-NH3), carboxylic acids (-COO2H), etc.)
- Whether molecule is a chain or ring
What is significant about the size of the value of the acid dissociation constant and what is done about this?
The value of Ka tend to be very small so they are converted to the negative log (base 10) scale to make it easier :
e.g. acetic acid Ka = 1.8 x 10-5 = pKa = 4.76.
What is significant about different functional groups and pKa value?
Each polar functional group will have an associated pKa and are ionisable (we can control the group to gain or lose a proton (H+))
By adjusting the pH (amount of H+) around the pKa, we can control these functional groups as to whether they are charged or neutral
Are acids proton donators or acceptors?
acids are proton donators
are bases proton donators or acceptors?
bases are proton acceptors
How does solubility change for neutral and charged functional groups?
neutral = soluble in organic solvents
charged = soluble in water
When are neutral molecules soluble?
In organic solvents
When are charged molecules soluble?
In water
What type of molecules are soluble in organic solvents?
Neutral (non-polar) molecules
What type of molecules are soluble in water?
Charged (polar) molecules
Explain the 2pH rule:
2pH rule states that to protonate an acid, the mobile phase pH must be 2 pH units below the compound’s pKa. Likewise, to force organic amines into the free-base form the mobile phase pH should be 2 pH units above the compound’s pKa.
