lect 22 Flashcards
what is the outline of this lecture?
partition equilibria
-equilibrium partition constants
-structure and property relationships
-fate and transport of environmental contaminants
what is partition (P) and constant (KD)?
what are some Kow values for various compounds?
-negative reflects a polar molecule
-as molecular weight goes up, the log Kow increases (becomes more nonpolar)
what can we use a separatory funnel to determine?
what is the significant of Log P for this course and beyond?
because we can make predictions on fate in environment (partition in soil, water, atmosphere)
what is the connection between Log Kow and general anesthesia?
what is the movement of compounds in the environment?
what are some example of concentration of some compounds in soil-pore water?
what determines water solubility?
how is activity coefficient related to water solubility?
what is the example of ethane vs ethanol?
Enthalpy: Ethanol forms favorable hydrogen bonds (good), ethane disrupts them (bad).
Entropy: Ethanol minimally disrupts water’s structure (good), ethane forces water into order (bad).
what about the molecule itself (ethane vs ethanol)?
does size matter?
yes, in fact it does!
- Dissolving the smaller molecule releases more favorable interaction energy (or requires less input of energy) than dissolving the large, bulky molecule.
- For larger molecules, dissolving usually requires more energy to break solvent-solvent interactions, and they give back less favorable interactions, making the dissolution process enthalpically worse.
what is relative hydrophobicity in some compounds
what are some differences in water solubility?
- Polarity and Dipole Moment
Fluorine is more electronegative than chlorine, which makes the C–F bond more polar than the C–Cl bond.
A more polar molecule tends to have better interactions with polar solvents like water.
As a result, fluorobenzene has a larger dipole moment (~1.6 D) than chlorobenzene (~1.7 D), but the polarity from the highly electronegative fluorine allows for stronger dipole-dipole interactions with water.
Even though chlorobenzene has a slightly higher dipole moment, the nature of the C–F bond results in more favorable water interactions.
- Size and Hydrophobicity
Chlorine is bulkier than fluorine, adding more hydrophobic character to chlorobenzene.
The larger size and greater polarizability of Cl also means that chlorobenzene is less able to interact effectively with water molecules.
- Hydrogen Bonding Potential
While neither C–F nor C–Cl bonds form hydrogen bonds directly, fluorine can engage in weak hydrogen bonding or dipole-dipole interactions due to its high electronegativity.
Chlorine is less capable of such interactions due to its lower electronegativity and larger atomic radius.
C₆Cl₆ is bigger, more hydrophobic, and disrupts water’s structure more than benzene, both enthalpically and entropically — so it’s way less soluble
Enthalpic factors:
Benzene is nonpolar, but it’s relatively small and can induce weak dipole interactions (temporary dispersion forces) with water.
C₆Cl₆ replaces all the hydrogens with bulky, heavy chlorine atoms. Chlorine is electronegative, but in this structure, their effects cancel out (due to symmetry), so C₆Cl₆ is very nonpolar overall. Plus, it’s more hydrophobic because chlorines are big and block water from getting close.
Also, dissolving a huge, heavy molecule like C₆Cl₆ would seriously disrupt water’s hydrogen bonding, without providing strong compensating interactions — making it very enthalpically unfavorable.
Entropic factors:
Water would have to order itself even more tightly around the giant hydrophobic C₆Cl₆ molecules than around benzene — leading to a bigger entropy loss.
More ordering = lower entropy = bad for solubility.
what are some other effects on water solubility?
what is effective solubility? what is it due to?
what is another model for solubility?
why is Kow important?
what does our lipid bilayer look like?
what is the relationship between Kow and water solubility (Wsol)?
how is this done what is the underlying principle of Kow?
