acids, bases, pKa, buffers Flashcards
lewis base
electron donor (nucleophile)
Lewis acid
electron acceptor (electrophile)
amphiprotic
- substance can both accept and donate a proton (H+)
- all amphoteric substances can amphiprotic
( bronstead lowry)
amphoteric
- refers to the ability to act as both an acid and a base
- all amphiprotic substances are not amphoteric
(lewis?)
examples amphiprotic
water, amino acids, phosphate progression, sulfate progression.
HSO4-, H2PO4- , HPO42-
solving for [H+] using Ka expression gives….
[H+] = Ka ([HA] / [A-])
using the relationship pH = -log [H+] gives… (Henderson hasselbalch equation)
the henderson hasselbalch (H-H) equation: pH = -log Ka + log ([A-] / [HA])
pKa
pKa = -log Ka, pH at which acid and conjugate base are equal in concentration
pH values to the left of the pKa
the acidic form is predominant
pH values to the right of the pKa
the basic form in predominant
why to be familiar with pKa numbers?
they predict what the acid/base is and the Nuc/E+, and the buffering ability
Buffer
want pKa at or near mid-point of the buffer range (capacity) for greatest ability to neutralize small added amounts of acid or base
What makes a buffer?
- made up of weak acid and salt of conjugate base or weak base and salt of conjugate acid.
zwitterion
dipolar ion with no charge
why must buffer use weak acid/ base?
in order to establish equilibrium. Strong acids and base have equilibrium constant far to the right.
why is zwitterion pH dependent
must be at pI to have the highest concentration.
Why is the pI where the substance has the lowest solubility?
- no net charge, wont dissolve in water
Why are NH2 and COOH protonated and COOH-H+ deprotonated at physiological pH?
- blood is around 7 pKa
- amino groups around 9 pKa
- carboxylic acid pKa around 2.
The pKa of blood is larger than the pKa of carboxylic acid, which means it will exist in its conjugate base form.
The pKa of blood is smaller than the pKa of the amino groups, which means amino groups will exist in their acidic form.
most amino acids pI
5-5.6
If the amino acid is acidic…
the pI is less than 5 or if basic, pI is greater than 6.5.
why are the main chain (alpha) carboxylic acids deprotonated before side chain COOH in Asp/Glu?
- NOT RESONANCE
- inductive effect… through bond effect not involving resonance…
- side chain carboxylic acid is further away from amino group than main chain
- amino group is acting inductively to withdraw electron density away from the conjugate base of the alpha carboxylic acid… but too far away for side chain
Why is the main chain deprotonated for a-N first rather before side chain in Arg/Lys?
- NOT RESONANCE
- inductive effect once again
