Volitox Ignis
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Lewis Acid and Base questions
Suppose that a compound has a central atom bonded to some substituents. How would different combinations of central atom and substituents, and
properties thereof, influence the compound's strength as either a Lewis Acid or Base? I know that a Lewis Acid is something which accepts an electron
pair, while a Lewis Base is an electron pair donor. I've also read that a higher pKa is correlated with higher Lewis Base strength, but that's about
all I know.
For instance:
Supposing that several different compounds have the same central atom acting as the electron donor, how would one decide, without knowing the pKa
values, which is a stronger Lewis Base?
For example: trimethylamine, monomethylamine, ethylamine, triethylamine, and pyridine. Or water vs ethanol vs diethyl ether. Do more massive
substituents increase or decrease Lewis Base strength? What about if the donor atom is part of a cyclic compound?
Also, assuming that the compounds being compared have the same substituents but different central atoms, how would I know which is the stronger Lewis
Base? Example: Ammonia vs water, or various compounds in which the central atom is a transition metal.
The same questions also apply when talking about Lewis Acids.
Also also, what makes Boron tribromide a strong Lewis Acid?
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DraconicAcid
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Basically, a Lewis base (like a Bronsted-Lowry base) is going to share its electrons with an acid to form a bond. The more willing it is to do this,
the stronger a base it will be.
To a first approximation (I teach first year, not advanced), this will depend on several factors- charge, electronegativity, size, and resonance.
Charge- the more anionic something it, the more willing it's going to be to share electrons. Hydroxide is a stronger base than water, and oxide is
much stronger than hydroxide.
Electronegativity- the less electronegative the atom with the lone pair is, the more willing to give up that lone pair it is. CH3F is a crappy base
(Lewis or BL), CH3OH is a weak base, CH3NH2 is much better.
Electronegativity also depends on the hybridization of the atom (the more s character, the higher the effective electronegativity). Ammonia (with an
sp3 N) is a stronger base than pyridine (sp2 N), which is a stronger base than acetonitrile (CH3CN, sp hybridized N). Acetonitrile is not a BL base
in aqueous solution, but can act as a Lewis base to form complexes with transition metals (although I'm not sure if they're stable in aqueous
solution).
Electronegative substituents on a central atom will pull electron density away from that central atom, making it a weaker base (NF3 would be a feeble
base, if it was a base at all; 4-fluoropyridine would be a weaker base than pyridine, but a stronger base than 2,4,6-trifluoropyridine).
Electron-donating substituents (such as alkyl groups replacing hydrogens) would make something a stronger base (if you check the pKb values for
ammonia, methylamine, dimethylamine and trimethylamine, you will *not*, however, see a steady progression of stronger bases, though, because of
solvent effects, which are beyond the scope of this lecture).
Resonance will spread out the negative charge or lone pair, making it harder for the acid to find it. Thus, phenoxide is a weaker base than ethoxide,
but a stronger base than acetate.
The size of the atom with the lone pair, for simple acid-base chemistry, is similar to resonance. The larger the atom, the more spread out the
negative charge, and the less attractive it is for the H+ ion. So iodide is a weaker base than chloride, which is a weaker base than fluoride (so HF
is a weak acid, HCl is a strong acid in aqueous solutions, and HI is a strong acid in aqueous solutions and a whole raft of nonaqueous systems as
well). This goes against the electronegativity argument- if you're comparing atoms within a column, go with size; if you're comparing different atoms
within a row of the periodic table, go with electronegativity.
For other Lewis basicity cases, size can be tricky. If you're looking at whether something will be a good nucleophile, then the size argument is
often reversed, or depends on solvents. If you're looking at whether something will act as a ligand towards a transition metal, then it depends a lot
on the metal (fluoride tends to be the worst ligand out of all the halides, but whether a metal will preferentially bond to a phosphine or an amine
depends on the metal and its oxidation state).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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