Hydroxyl
Definition and meaning of Hydroxyl in chemistry.
Hydroxyl is a functional group consisting of one oxygen atom covalently bonded to one hydrogen atom (-OH). It is typically attached to a larger carbon-containing organic molecule. This important functional group defines alcohols and phenols and appears widely in carbohydrates.
In more detail
The oxygen atom in this group is far more electronegative than the hydrogen atom. This significant difference in electronegativity makes the covalent O-H bond highly polar. The hydroxyl group gains a partial negative charge on the oxygen atom.
It simultaneously carries a partial positive charge on the attached hydrogen atom. This distinct polarity allows hydroxyl groups to form strong hydrogen bonds with water. These groups can also form hydrogen bonds with other nearby hydroxyl groups.
These hydrogen bonds explain why small alcohols and simple sugars dissolve readily in water. They also explain why alcohols have much higher boiling points than similarly sized nonpolar hydrocarbons. Extra thermal energy is required to break these strong intermolecular hydrogen bonds before boiling.
The hydroxyl group is completely neutral and covalently bound to its carbon skeleton. Students very commonly confuse the covalently bonded hydroxyl group with the hydroxide ion. The hydroxide ion (OH-) is a separate and negatively charged chemical species.
Hydroxide ions are typically found in strong ionic bases like sodium hydroxide. A hydroxyl group does not easily detach to form an ion under normal conditions.
Key facts
| Formula | -OH |
|---|---|
| Bond type | Polar covalent (O-H) |
| Field | Organic Chemistry |
| Primary characteristic | Can form hydrogen bonds |
| Found in | Alcohols, phenols, carbohydrates |
| Charge | Neutral (unlike hydroxide ion) |
Ethanol (CH3CH2OH) contains a single hydroxyl group bonded to its terminal carbon atom. This specific terminal carbon is designated as C1 in the formal IUPAC naming system. This defining -OH group is exactly what classifies the ethanol molecule as an alcohol. The polar group allows liquid ethanol to mix completely with water in all proportions. Without this hydroxyl group, the molecule would simply be the insoluble gas ethane.
Frequently asked questions
Is a hydroxyl group exactly the same as a hydroxide ion?
No. A hydroxyl group (-OH) is completely neutral and covalently attached to a carbon skeleton. A hydroxide ion (OH-) is a separate negatively charged chemical species found in ionic bases.
Why do hydroxyl groups make organic compounds more water-soluble?
The polar O-H bond can easily form strong hydrogen bonds with surrounding water molecules. Compounds with hydroxyl groups mix with water much more readily than nonpolar hydrocarbons do.
How does a hydroxyl group affect the boiling point of a substance?
Hydroxyl groups allow molecules to form hydrogen bonds with each other. This creates strong intermolecular attractions that require significantly more thermal energy to break apart during boiling.
Related terms
Sources & references
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