Fundamentals 5: Acidity, basicity and pKa Flashcards
what does pH tell us?
the ability or likelihood of a species to act as a proton donor.
what is Ka and what is its relationship to pKa?
the dissociation constant.
pKa = -logKa.
for a strong acid, pKa is low.
for a strong base, pKa is high.
lower pKa = more dissociated.
what 3 factors discuss the extent of dissociation?
strength of the H-A bond.
intrinsic stability of A-
(electronegativity, hybridisation, delocalisation and substituent effects).
environmental factors.
what is the relationship between the relative acidity and pKa values of the halo-acids and their bond strengths?
bond strength decreases as you go down the halo-acids.
pKa values decrease as you go down.
what determines the stability of a conjugate base?
the electronegativity of the atom to which H is bonded.
the hybridisation of the centre where H is to be lost
as electronegativity increases, pKa values decrease.
why are electrons more stable in an orbital with greater āsā character?
the charge sits closer to the nucleus, so receives greater stabilisation.
what effect does delocalisation have on the intrinsic stability of a conjugate base?
makes the anion more stable.
lowers pKa associated with the conjugate base.
why does delocalisation make the anion more stable in a conjugate base?
delocalisation allows negative charge to be distributed over multiple atoms.
reduces electrostatic repulsion between negatively charged particles.
increases electron density around negative charge.
what effect do substituent effects have on the intrinsic stability of a conjugate base?
electron withdrawing substituents on the conjugate base stabilise the anion and lower pKa.
electron donating substituents reduce acidity by destabilising conjugate base anion.
how does an electron withdrawing species stabilise the anion and lower pKa?
withdrawing electron density from negatively charged anion.
reduces tendency of anion to react with other species.
makes anion less basic and more acidic.
this results in a lower pKa, hence more likely to accept a proton.
how do inductive stabilising effects target the stability of a conjugate base?
the further away the inductive substituent is, the electron withdrawing effects are felt less.
what role do environmental factors play in the intrinsic stabilisation of a conjugate base/acid?
(polar protic and aprotic)
solvents that can establish non-covalent interactions with the charged species will stabilise it.
protic:
can stabilise cations via dipoles/lone pairs.
can stabilise anions via hydrogen.
aprotic:
can stabilise cations via dipole/lone pairs.
enolates being stabilised conjugate bases:
enolates are very stable due to the ability to stabilise the conjugate base by resonating the anionic charge onto an electronegative oxygen atom.
