What is the difference between hydroxyl, hydroxide and hydroxy?

Question

Is there any difference between hydroxide, hydroxyl, and hydroxy used in chemistry or are they all the same?

Answer 1

1. Hydroxide (OH⁻)

1. Definition: A negatively charged polyatomic ion made of one oxygen and one hydrogen atom, with a -1 charge.
2. Where it appears: Found in bases (like NaOH, KOH, Ca(OH)₂).
3. Role: It’s what makes a solution basic/alkaline because OH⁻ can accept protons (H⁺).

Example: Sodium hydroxide (NaOH) dissolves in water to give Na⁺ and OH⁻ ions.

2. Hydroxyl (-OH group)

1. Definition: A neutral functional group consisting of oxygen and hydrogen, bonded covalently to a carbon (or another atom).
2. Where it appears: In alcohols (R–OH), phenols, carbohydrates, etc.
3. Role: Hydroxyl groups make molecules polar and able to form hydrogen bonds.

Example: In ethanol (CH₃CH₂OH), the –OH is a hydroxyl group.

3. Hydroxy (prefix in naming)

1. Definition: A prefix used in chemical nomenclature (IUPAC naming) to show that a molecule has a hydroxyl group (–OH) attached.
2. Where it appears: In systematic names of compounds when the –OH group isn’t the main functional group.
3. Role: It’s a naming convention, not a separate species.

Example: 2-hydroxybenzoic acid (salicylic acid) → has an –OH group on the benzene ring.

In summary:

1. Hydroxide = OH⁻ ion (charged, makes bases).
2. Hydroxyl = –OH functional group (neutral, in alcohols/phenols).
3. Hydroxy = prefix in chemical names to indicate an –OH group.

Answer 2

Hydroxyl or hydroxy group refers to the OH pair itself. If it carries a negative charge, it's referred to as hydroxide.