Thread: Acids Bases pH; why molecules donate or accept?

Hi,

1. Why does H+ leave COOH, but adds to NH2 ? Yes, one is an acid/proton donor, the other a base /proton acceptor (and ok various definitions of acids/bases…Arrhenius/Lewis/Bronsted-Lowry), but what is the reason a given molecule should donate or accept (leaving aside ‘it depends on its environment’). Is it to do with dipoles? Hydrophobicity? Why exactly does COOH tend to kick out the H, yet NH2 tends to take it on?

2. Why does the proton/H+ concentration matter so much (i.e. the pH) is it simply that protons are highly reactive and so run around changing things in a biological system that shouldn’t be changed (or at least shouldn’t be changed at such a high rate). (also I vaguely understand that there are not actually protons on their own but rather hydronium ions and more complicated H5O2, H7O3, etc)

Thanks.
Sean

1. Why does H+ leave COOH, but adds to NH2 ? Yes, one is an acid/proton donor, the other a base /proton acceptor (and ok various definitions of acids/bases…Arrhenius/Lewis/Bronsted-Lowry), but what is the reason a given molecule should donate or accept (leaving aside ‘it depends on its environment’). Is it to do with dipoles? Hydrophobicity? Why exactly does COOH tend to kick out the H, yet NH2 tends to take it on?

The double bonded O resonates with the C, and the H will have a weaker bond with it's O, because it will resonate it's electron between the 2 O's. The elektron is what is holding the H to it's COO group. So if the solution lacks H's, it'll easily pull it off, the more H's there are, the less chance for a COOH group to lose it's proton.

2. Why does the proton/H+ concentration matter so much (i.e. the pH) is it simply that protons are highly reactive and so run around changing things in a biological system that shouldn’t be changed (or at least shouldn’t be changed at such a high rate). (also I vaguely understand that there are not actually protons on their own but rather hydronium ions and more complicated H5O2, H7O3, etc)

H+ is EXTREMELY reactive. So yes, your right. But it still changes this in a high rate.. I can't tell it differently, maybe someone else can.

1. The pull on the NH2 is strong enough to overcome the weak bond holding the H to COOH, so it steals the H. Don't think of it as COOH kicking out the H. There is no force within COOH that actually expels the H.

2. H+ is just very reactive. It's got that single e- and it's desperately seeking a mate.