Difference Between Hydroxyl and Hydroxide

Main Difference – Hydroxyl vs Hydroxide

Although the two terms hydroxyl and hydroxide sound similar and have similar chemical structures, they are different from each other in several ways. The term hydroxyl is the name given to a group of atoms (one oxygen atom and one hydrogen atom) that is bonded to a molecule through a covalent bond. The term hydroxide refers to an anion composed of one oxygen atom and one hydrogen atom. According to IUPAC rules hydroxyl is the OH radical and –OH group is given the name hydroxy. However, in common usage, we use the term hydroxyl group to refer to the –OH functional group. The main difference between Hydroxyl and Hydroxid is that hydroxyl is not available in its free form whereas hydroxide can be found in its free form as an anion.

Key Areas Covered

1. What is Hydroxyl
      – Definition, Examples and Reactions
2. What is Hydroxide
      – Definition, Examples and Reactions
3. What is the Difference Between Hydroxyl and Hydroxide
      – Comparison of Key Differences

Key Terms: Anion, Covalent Bond, Hydroxide, Hydroxyl

What is Hydroxyl

Hydroxyl is a group of atoms that is bonded to a molecule. Therefore, it is always found as a part of a molecule. There are no existing free forms of hydroxyl groups since the free form of hydroxyl is an unstable radical and radicals react with another molecule in order to become stabilized. Hydroxyl groups can be found in both organic and inorganic molecules.

Figure 1: Hydroxyl group is in blue color. “R” indicates the rest of the molecule.

Hydroxyl groups always form covalent bonds. Therefore, hydroxyl groups are found only in covalent compounds. Most common organic compounds containing hydroxyl groups are alcohols and carboxylic acids. Here, the hydroxyl group acts as the functional group of that molecule. The chemical and physical properties of the rest of the molecule are determined by the hydroxyl group attached to it.

Hydroxyl groups are found in inorganic compounds as well. For example, some acids such as sulfuric acid, and phosphoric acid are composed of hydroxyl groups. Hydroxyl groups also contribute to the chemical properties of the compound. Hydroxyl groups can form strong hydrogen bonds.

What is Hydroxide

Hydroxide is an inorganic anion. It is composed of one oxygen atom bonded to a hydrogen atom through a covalent bond. This anion is -1 charged. This electrical charge is on the oxygen atom due to the presence of an extra electron on the oxygen atom. The hydroxide anion is a stable compound. The chemical symbol for the hydroxide anion is OH^–.

Figure 2: Hydroxide Anion

Hydroxide can be found in ionic compounds. It acts as the negative ion for the formation of the ionic compound. Hydroxide ions are the chemical species responsible for the basicity of a system. The basicity of a system can be determined by measuring the amount of hydroxide ions present in that system. In basic compounds, the basicity of that compound is indicated by the presence of hydroxide groups that can be removed.

For example, sodium hydroxide (NaOH) is a highly basic compound and is composed of a Na⁺ cation and OH^– anion. Many metals can form complexes with hydroxide ions. They are called metal hydroxides. Hydroxide ions can act as nucleophiles. Here, these hydroxide ions can attack carbon atoms with a partial positive charge; sometimes, hydroxide ions also act as catalysts for reactons. These are called base catalysts.

Difference Between Hydroxyl and Hydroxide

Definition

Hydroxyl: Hydroxyl is a group of atoms that is bonded to a molecule.

Hydroxide:  Hydroxide is an inorganic anion.

Nature

Hydroxyl:  Hydroxyl groups are always found as a group of atoms bonded to a molecule.

Hydroxide:  Hydroxide can be found in its free form as an anion.

Chemical Bonding

Hydroxyl:  Hydroxyl groups can form covalent bonds.

Hydroxide:  Hydroxide groups can form ionic bonds.

Electrical Charge

Hydroxyl:  Hydroxyl group does not have a fixed electrical charge.

Hydroxide:  Hydroxides are anions having -1 electrical charge.

Reactivity

Hydroxyl:  Hydroxyl groups act as functional groups for molecules.

Hydroxide: Hydroxide ions react with cations in order to form ionic compounds.

Conclusion

Although the terms hydroxyl groups and hydroxide groups are used interchangeably, there are several differences between these terms. The main difference between hydroxyl and hydroxide is that hydroxyl is not available in its free form whereas hydroxide can be found in its free form as an anion.

References:

1. “Hydroxide Ion: Definition & Formula.” Study.com, Available here. Accessed 29 Aug. 2017.
2. “Hydroxy group.” Wikipedia, Wikimedia Foundation, 11 Aug. 2017, Available here. Accessed 29 Aug. 2017.

Image Courtesy:

1. “Hydroxy Group Structural Formulae” By Jü – Own work (Public Domain) via Commons Wikimedia
2. “Hydroxide lone pairs-2D” By DoSiDo – Own work (CC BY-SA 3.0) via Commons Wikimedia