Question

List the charge of the functional groups at: the pH indicated (right) The pKa is indicated in () Amino (9) Carboxylic Acid (2) Hydroxyl (>14) Sulfhydryl (8)

Answer 1

In general:

H2O <--> H+ and OH-

pH low --> H+ rich

pH high --> OH- rich

therefore:

a)

Amino group

it is negatively charged at this pkA (has 1 lone pair)

pKa = 9

if pH = 13; OH- is rich, then it remains NEGATIVE

if pH = 7; there are H+ and OH- almost equally, the substance will attract H+ ions, expect the lone pair to attach to the H+ proton. There is NEUTRAL charge

if pH = 1; there are lots of H+, the substance will attract H+ ions, expect the lone pair to attach to the H+ proton. There is NEUTRAL charge

b)

Carboxilic Acids

it is normally "neutrally charged"

pKA = 2

if pH = 13; OH- is rich, then it the molecule H+ will dissociate in order to react with OH- ions. The moelcule gets NEGATIVE charge

if pH = 7; there are H+ and OH- almost equally, since pKa is pretty low; Neutral charge in general

if pH = 1; there are lots of H+, will not free its H+ easily, it remains NEUTRAL

c)

Hydroxyl group

pKa > 14

this means, it is not likely to give off its H+ as proton

if pH = 13; OH- is rich, then it the molecule H+ will NOT dissociate (since pH is lower than pKa) expect no chang ein charge. It is NEUTRAL

if pH = 7; there are H+ and OH- almost equally, charge remains as neutral

if pH = 1; there are lots of H+, will not free its H+ easily, charge remains as neutral

d)

Sulfhydryl group

pKA = 8

originally neutral charge:

if pH = 13; OH- is rich, then it the molecule H+ will dissociate in order to react with OH- ions. The moelcule gets NEGATIVE charge

if pH = 7; there are H+ and OH- almost equally, since pKa is similar; there is no dissociation, the group remains NEUTRAL

if pH = 1; there are lots of H+, will not free its H+ easily, its pKA is pretty high, meaning it is not acidic. it remains NEUTRAL