Hydroxyl
Definition and meaning of Hydroxyl in chemistry.
Hydroxyl is a functional group made of one oxygen atom covalently bonded to one hydrogen atom (-OH), attached to a larger carbon-containing molecule. It is the defining group of alcohols and phenols and also appears in carbohydrates and carboxylic acids.
In more detail
Because oxygen is far more electronegative than hydrogen, the O-H bond is highly polar, giving the hydroxyl group a partial negative charge on oxygen and a partial positive charge on hydrogen. This polarity lets hydroxyl groups form hydrogen bonds with water and with each other, which explains why small alcohols and sugars dissolve readily in water and why alcohols have higher boiling points than similarly sized hydrocarbons. The group is neutral and covalently bound, which distinguishes it from the hydroxide ion (OH-), a separate, negatively charged species found in ionic bases.
Key facts
| Formula | -OH |
|---|---|
| Bond type | Covalent (O-H) |
| Field | Organic Chemistry |
| Found in | Alcohols, phenols, carbohydrates, carboxylic acids |
Ethanol (CH3CH2OH) contains a hydroxyl group bonded to its terminal carbon (C1 in the IUPAC name ethan-1-ol); this -OH group is what classifies ethanol as an alcohol and allows it to mix completely with water in all proportions.
Frequently asked questions
Is a hydroxyl group the same as a hydroxide ion?
No. A hydroxyl group (-OH) is neutral and covalently attached to a carbon skeleton, as in alcohols. A hydroxide ion (OH-) is a separate, negatively charged species, such as the ion released by NaOH in water.
Why do hydroxyl groups make compounds more water-soluble?
The polar O-H bond can form hydrogen bonds with water molecules, so compounds with hydroxyl groups, like glucose or ethanol, mix with water more readily than nonpolar hydrocarbons do.