It sounds like this“Every hydrogen in a molecule can be given away as H+, so any molecule with hydrogen is an acid.”
Thinking any hydrogen can act as an acid
Why it feels right
Acids are defined by giving away H+, and most molecules we meet are full of hydrogens. If an acid is something that can give away a hydrogen ion, it seems anything with a hydrogen to give must count, so sugar, petrol and water all look like weak acids.
Why it’s wrong
Most hydrogens hold on very tightly. Only the ones whose loss leaves a stable ion come off in water. The hydrogens in methane, bonded to carbon, are about 10^34 times less willing to leave than the one in acetic acid.
A worked example
Whether a hydrogen leaves depends on what is left behind. When H+ departs, it leaves its bonding electrons with the rest of the molecule, which becomes a negative ion. If that ion is stable, because the charge sits on an electronegative atom such as oxygen and can be spread around, the hydrogen comes off easily. If it is not, the hydrogen stays.
Acetic acid loses the hydrogen on its oxygen to give acetate, whose charge is shared by two oxygens, so its pKa is 4.76. Ethanol loses the O-H hydrogen to give an ion with the charge stuck on one oxygen: pKa about 16. Methane would have to put the charge on a carbon, which is very poor at holding it: pKa about 50.
The pKa is a logarithm, so each step of 1 is a factor of ten. The difference between acetic acid at 4.76 and methane at about 50 is a factor of 10 to the power 45. The hydrogens on carbon in an ordinary molecule are, for practical purposes, not acidic at all.
How to catch yourself
- You call any hydrogen-containing molecule an acid.
- You think petrol or sugar are acidic in water.
- You ignore what the molecule would look like after losing H+.
What to hold instead
Only hydrogens whose loss leaves a stable negative ion come off easily. A hydrogen on oxygen next to a C=O, as in acetic acid, does; a hydrogen on a carbon, as in methane, essentially never does.
Where you’ll meet it
- Scanning a structure for the acidic hydrogen.
- Understanding why vinegar is acidic and petrol is not.
- Reading pKa tables.
Questions people ask
- Which hydrogens in a molecule are acidic?
- Those whose removal leaves a stable negative ion, usually hydrogens on oxygen, and especially when the leftover charge can be shared with a neighbouring C=O. Hydrogens on carbon are generally not acidic.
- Why is acetic acid acidic but methane is not?
- Losing H+ from acetic acid leaves acetate, whose negative charge is shared by two oxygens. Losing it from methane would leave a charge on carbon, which is very unstable, so the hydrogen does not leave.