It sounds like this“If a molecule has an OH in its formula, it is a base.”
Thinking an OH in the formula makes a base
Why it feels right
Sodium hydroxide, NaOH, is a base and has OH in its name. Potassium hydroxide, calcium hydroxide: the pattern is easy to see. So when a formula such as CH3COOH or CH3CH2OH has OH in it, the pattern seems to say base.
Why it’s wrong
The base is the hydroxide ion, OH-, on its own, which takes H+ strongly. An O-H bonded into a molecule is a different thing: in acetic acid it is the acidic hydrogen, and in ethanol it does almost nothing. A base needs something that can take a proton, such as a lone pair on nitrogen.
A worked example
In sodium hydroxide the OH is a hydroxide ion, OH-, which is a separate negative ion in water. It carries a full negative charge and is very good at grabbing a hydrogen ion, which makes it a strong base.
In an organic molecule, the O-H is attached to a carbon by a covalent bond and the molecule stays in one piece. That O-H can behave in quite different ways. In acetic acid, CH3COOH, the O-H is next to a C=O, and the leftover ion, acetate, is stable because the charge is shared between two oxygens. So the hydrogen comes off fairly easily and the molecule is an acid, with a pKa of 4.76. In ethanol, CH3CH2OH, the leftover would have its charge stuck on one oxygen, so the hydrogen barely leaves: ethanol is a very weak acid, with a pKa of about 16, and close to neutral.
To spot a base, look for a lone pair that can pick up a proton. The nitrogen in ammonia, NH3, or in an amine such as methylamine, has one. Ask 'where is the loose hydrogen?' for an acid and 'where is the lone pair?' for a base, and ignore whether the word OH appears.
How to catch yourself
- You label molecules with OH as bases.
- You call acetic acid a base because its formula ends in OH.
- You look at the formula's letters instead of its structure.
What to hold instead
A base is something that can take a proton, such as the hydroxide ion or a lone pair on nitrogen. An O-H bonded in a molecule is not hydroxide: in acetic acid it is the acidic hydrogen, and in ethanol it is nearly inert.
Where you’ll meet it
- Reading an organic structure for the first time.
- Predicting whether an unfamiliar molecule is acidic or basic.
- Explaining why vinegar is acidic although its formula ends in OH.
Questions people ask
- Does an OH group make a molecule basic?
- No. Only the free hydroxide ion, OH-, is a strong base. An OH bonded inside a molecule can be acidic, as in acetic acid, or almost neutral, as in ethanol.
- How can you tell whether a molecule is basic?
- Look for a lone pair of electrons that can accept a proton, for example on nitrogen in ammonia and amines. The more available the lone pair, the stronger the base.